1. Digestion in the oral cavity, STOMACH.
2. Digestion in intestine and
3. Absorption in gastrointestinal tract. Motility of gastrointestinal tract
Digestion is the mechanical and chemical breakdown of food into smaller components that are more easily absorbed into a blood stream, for instance. Digestion is a form of catabolism: a breakdown of large food molecules to smaller ones.
When food enters the mouth, digestion of the food starts by the action of mastication, a form of mechanical digestion, and the wetting contact of saliva. Saliva, a liquid secreted by the salivary glands, contains salivary amylase, an enzyme which starts the digestion of starch in the food. After undergoing mastication and starch digestion, the food will be in the form of a small, round slurry mass called a bolus. It will then travel down the esophagus and into the stomach by the action of peristalsis. Gastric juice in the stomach starts protein digestion. Gastric juice mainly contains hydrochloric acid and pepsin. As these two chemicals may damage the stomach wall, mucus is secreted by the stomach, providing a slimy layer that acts as a shield against the damaging effects of the chemicals. At the same time protein digestion is occurring, mechanical mixing occurs by peristalsis, which is waves of muscular contractions that move along the stomach wall. This allows the mass of food to further mix with the digestive enzymes. After some time (typically 1-2 hours in humans, 4–6 hours in dogs, 3-4 hours in house cats)[citation needed], the resulting thick liquid is called chyme. When the pyloric sphincter valve opens, chyme enters the duodenum where it mixes with digestive enzymes from the pancreas, and then passes through the small intestine, in which digestion continues. When the chyme is fully digested, it is absorbed into the blood. 95% of absorption of nutrients occurs in the small intestine. Water and minerals are reabsorbed back into the blood in the colon (large intestine) where the pH is slightly acidic about 5.6 ~ 6.9. Some vitamins, such as biotin and vitamin K (K2MK7) produced by bacteria in the colon are also absorbed into the blood in the colon. Waste material is eliminated from the rectum during defecation.
Digestive systems
Digestive systems take many forms. There is a fundamental distinction between internal and external digestion. External digestion is more primitive, and most fungi still rely on it.In this process, enzymes are secreted into the environment surrounding the organism, where they break down an organic material, and some of the products diffuse back to the organism. Later, animals form a tube in which internal digestion occurs, which is more efficient because more of the broken down products can be captured, and the internal chemical environment can be more efficiently controlled.
Some organisms, including nearly all spiders, simply secrete biotoxins and digestive chemicals (e.g., enzymes) into the extracellular environment prior to ingestion of the consequent “soup”. In others, once potential nutrients or food is inside the organism, digestion can be conducted to a vesicle or a sac-like structure, through a tube, or through several specialized organs aimed at making the absorption of nutrients more efficient.
Channel transport system
In a channel transupport system, several proteins form a contiguous channel traversing the inner and outer membranes of the bacteria. It is a simple system, which consists of only three protein subunits: the ABC protein, membrane fusion protein (MFP), and outer membrane protein (OMP)[specify]. This secretion system transports various molecules, from ions, drugs, to proteins of various sizes (20 – 900 kDa). The molecules secreted vary in size from the small Escherichia coli peptide colicin V, (10 kDa) to the Pseudomonas fluorescens cell adhesion protein LapA of 900 kDa.
Molecular syringe
One molecular syringe is used through which a bacterium (e.g. certain types of Salmonella, Shigella, Yersinia) can inject nutrients into protist cells. One such mechanism was first discovered in Y. pestis and showed that toxins could be injected directly from the bacterial cytoplasm into the cytoplasm of its host’s cells rather than simply be secreted into the extracellular medium.
Conjugation machinery
The conjugation machinery of some bacteria (and archaeal flagella) is capable of transporting both DNA and proteins. It was discovered in Agrobacterium tumefaciens, which uses this system to introduce the Ti plasmid and proteins into the host, which develops the crown gall (tumor).[6] The VirB complex of Agrobacterium tumefaciens is the prototypic system.
The nitrogen fixing Rhizobia are an interesting case, wherein conjugative elements naturally engage in inter-kingdom conjugation. Such elements as the Agrobacterium Ti or Ri plasmids contain elements that can transfer to plant cells. Transferred genes enter the plant cell nucleus and effectively transform the plant cells into factories for the production of opines, which the bacteria use as carbon and energy sources. Infected plant cells form crown gall or root tumors. The Ti and Ri plasmids are thus endosymbionts of the bacteria, which are in turn endosymbionts (or parasites) of the infected plant.
The Ti and Ri plasmids are themselves conjugative. Ti and Ri transfer between bacteria uses an independent system (the tra, or transfer, operon) from that for inter-kingdom transfer (the vir, or virulence, operon). Such transfer creates virulent strains from previously avirulent Agrobacteria.
Teeth
Teeth (singular tooth) are small whitish structures found in the jaws (or mouths) of many vertebrates that are used to tear, scrape, milk and chew food. Teeth are not made of bone, but rather of tissues of varying density and hardness, such as enamel, dentine and cementum. Human teeth have a blood and nerve supply which enables proprioception. This is the ability of sensation when chewing, for example if you were to bite into something too hard for our teeth, such as a nectarine pip, our teeth tell our brain it cannot be chewed, stop. The shape of an animal’s teeth is related to its diet. An example of this is plant matter is difficult to digest, so herbivores have many molars for grinding.
Main article: Abomasum
Herbivores have evolved cecums (or an abomasum in the case of ruminants). Ruminants have a fore-stomach with four chambers. These are the rumen, reticulum, omasum, and abomasum. In the first two chambers, the rumen and the reticulum, the food is mixed with saliva and separates into layers of solid and liquid material. Solids clump together to form the cud (or bolus). The cud is then regurgitated, chewed slowly to completely mix it with saliva and to break down the particle size.
Fibre, especially cellulose and hemi-cellulose, is primarily broken down into the volatile fatty acids, acetic acid, propionic acid and butyric acid in these chambers (the reticulo-rumen) by microbes: (bacteria, protozoa, and fungi). In the omasum, water and many of the inorganic mineral elements are absorbed into the blood stream.
The abomasum is the fourth and final stomach compartment in ruminants. It is a close equivalent of a monogastric stomach (e.g., those in humans or pigs), and digesta is processed here in much the same way. It serves primarily as a site for acid hydrolysis of microbial and dietary protein, preparing these protein sources for further digestion and absorption in the small intestine. Digesta is finally moved into the small intestine, where the digestion and absorption of nutrients occurs. Microbes produced in the reticulo-rumen are also digested in the small intestine
In earthworms
An earthworm’s digestive system consists of a mouth, pharynx, esophagus, crop, gizzard, and intestine. The mouth is surrounded by strong lips, which act like a hand to grab pieces of dead grass, leaves, and weeds, with bits of soil to help chew. The lips break the food down into smaller pieces. In the pharynx, the food is lubricated by mucus secretions for easier passage. The esophagus adds calcium carbonate to neutralize the acids formed by food matter decay. Temporary storage occurs in the crop where food and calcium carbonate are mixed. The powerful muscles of the gizzard churn and mix the mass of food and dirt. When the churning is complete, the glands in the walls of the gizzard add enzymes to the thick paste, which helps chemically breakdown the organic matter. By peristalsis, the mixture is sent to the intestine where friendly bacteria continue chemical breakdown. This releases carbohydrates, protein, fat, and various vitamins and minerals for absorption into the body.
Overview of vertebrate digestion
In most vertebrates, digestion is a multistage process in the digestive system, starting from ingestion of raw materials, most often other organisms. Ingestion usually involves some type of mechanical and chemical processing. Digestion is separated into four steps:
Ingestion: placing food into the mouth (entry of food in the digestive system),
Mechanical and chemical breakdown: mastication and the mixing of the resulting bolus with water, acids, bile and enzymes in the stomach and intestine to break down complex molecules into simple structures,
Absorption: of nutrients from the digestive system to the circulatory and lymphatic capillaries through osmosis, active transport, and diffusion, and
Egestion (Excretion): Removal of undigested materials from the digestive tract through defecation.
Underlying the process is muscle movement throughout the system through swallowing and peristalsis. Each step in digestion requires energy, and thus imposes an “overhead charge” on the energy made available from absorbed substances. Differences in that overhead cost are important influences on lifestyle, behavior, and even physical structures. Examples may be seen in humans, who differ considerably from other hominids (lack of hair, smaller jaws and musculature, different dentition, length of intestines, cooking, etc.).
The major part of digestion takes place in the small intestine. The large intestine primarily serves as a site for fermentation of indigestible matter by gut bacteria and for resorption of water from digests before excretion.
In mammals, preparation for digestion begins with the cephalic phase in which saliva is produced in the mouth and digestive enzymes are produced in the stomach. Mechanical and chemical digestion begin in the mouth where food is chewed, and mixed with saliva to begin enzymatic processing of starches. The stomach continues to break food down mechanically and chemically through churning and mixing with both acids and enzymes. Absorption occurs in the stomach and gastrointestinal tract, and the process finishes with defecation.
Human digestion process
Main article: Human gastrointestinal tract
Upper and Lower human gastrointestinal tract
The whole digestive system is around
Phases of gastric secretion
Cephalic phase – This phase occurs before food enters the stomach and involves preparation of the body for eating and digestion. Sight and thought stimulate the cerebral cortex. Taste and smell stimulus is sent to the hypothalamus and medulla oblongata. After this it is routed through the vagus nerve and release of acetylcholine. Gastric secretion at this phase rises to 40% of maximum rate. Acidity in the stomach is not buffered by food at this point and thus acts to inhibit parietal (secretes acid) and G cell (secretes gastrin) activity via D cell secretion of somatostatin.
Gastric phase – This phase takes 3 to 4 hours. It is stimulated by distension of the stomach, presence of food in stomach and decrease in pH. Distention activates long and myenteric reflexes. This activates the release of acetylcholine, which stimulates the release of more gastric juices. As protein enters the stomach, it binds to hydrogen ions, which raises the pH of the stomach. Inhibition of gastrin and gastric acid secretion is lifted. This triggers G cells to release gastrin, which in turn stimulates parietal cells to secrete gastric acid. Gastric acid is about 0.5% hydrochloric acid (HCl), which lowers the pH to the desired pH of 1-3. Acid release is also triggered by acetylcholine and histamine.
Intestinal phase – This phase has 2 parts, the excitatory and the inhibitory. Partially digested food fills the duodenum. This triggers intestinal gastrin to be released. Enterogastric reflex inhibits vagal nuclei, activating sympathetic fibers causing the pyloric sphincter to tighten to prevent more food from entering, and inhibits local reflexes.
Oral cavity
Main article: Mouth (human)
In humans, digestion begins in the Mouth, otherwise known as the “Buccal Cavity”, where food is chewed. Saliva is secreted in large amounts (1-1.5 litres/day) by three pairs of exocrine salivary glands (parotid, submandibular, and sublingual) in the oral cavity, and is mixed with the chewed food by the tongue. Saliva cleans the oral cavity, moistens the food, and contains digestive enzymes such as salivary amylase, which aids in the chemical breakdown of polysaccharides such as starch into disaccharides such as maltose. It also contains mucus, a glycoprotein that helps soften the food and form it into a bolus. An additional enzyme, lingual lipase, hydrolyzes long-chain triglycerides into partial glycerides and free fatty acids.
Stomach
Main article: Stomach
The stomach is a small, ‘J’-shaped pouch with walls made of thick, distensible muscles, which stores and helps break down food. Food reduced to very small particles is more likely to be fully digested in the small intestine, and stomach churning has the effect of assisting the physical disassembly begun in the mouth. Ruminants, who are able to digest fibrous material (primarily cellulose), use fore-stomachs and repeated chewing to further the disassembly. Rabbits and some other animals pass some material through their entire digestive systems twice. Most birds ingest small stones to assist in mechanical processing in gizzards.
Food enters the stomach through the cardiac orifice where it is further broken apart and thoroughly mixed with gastric acid, pepsin and other digestive enzymes to break down proteins. The enzymes in the stomach also have an optimum conditions, meaning that they work at a specific pH and temperature better than any others. The acid itself does not break down food molecules, rather it provides an optimum pH for the reaction of the enzyme pepsin and kills many microorganisms that are ingested with the food. It can also denature proteins. This is the process of reducing polypeptide bonds and disrupting salt bridges, which in turn causes a loss of secondary, tertiary, or quaternary protein structure. The parietal cells of the stomach also secrete a glycoprotein called intrinsic factor, which enables the absorption of vitamin B-12. Mucus neck cells are present in the gastric glands of the stomach. They secrete mucus, which along with gastric juice plays an important role in lubrication and protection of the mucosal epithelium from excoriation by the highly concentrated hydrochloric acid. Other small molecules such as alcohol are absorbed in the stomach, passing through the membrane of the stomach and entering the circulatory system directly. Food in the stomach is in semi-liquid form, which upon completion is known as chyme.
After consumption of food, digestive “tonic” and peristaltic contractions begin, which helps break down the food and move it onward.When the chyme reaches the opening to the duodenum known as the pylorus, contractions “squirt” the food back into the stomach through a process called retropulsion, which exerts additional force and further grinds down food into smaller particles. Gastric emptying is the release of food from the stomach into the duodenum; the process is tightly controlled with liquids being emptied much more quickly than solids.Gastric emptying has attracted medical interest as rapid gastric emptying is related to obesity and delayed gastric emptying syndrome is associated with diabetes mellitus, aging, and gastroesophageal reflux.
The transverse section of the alimentary canal reveals four (or five, see description under mucosa) distinct and well developed layers within the stomach:
Serous membrane, a thin layer of mesothelial cells that is the outermost wall of the stomach.
Muscular coat, a well-developed layer of muscles used to mix ingested food, composed of three sets running in three different alignments. The outermost layer runs parallel to the vertical axis of the stomach (from top to bottom), the middle is concentric to the axis (horizontally circling the stomach cavity) and the innermost oblique layer, which is responsible for mixing and breaking down ingested food, runs diagonal to the longitudinal axis. The inner layer is unique to the stomach, all other parts of the digestive tract have only the first two layers.
Submucosa, composed of connective tissue that links the inner muscular layer to the mucosa and contains the nerves, blood and lymph vessels.
Mucosa is the extensively folded innermost layer. It can be divided into the epithelium, lamina propria, and the muscularis mucosae, though some consider the outermost muscularis mucosae to be a distinct layer, as it develops from the mesoderm rather than the endoderm (thus making a total of five layers). The epithelium and lamina are filled with connective tissue and covered in gastric glands that may be simple or branched tubular, and secrete mucus, hydrochloric acid, pepsinogen and rennin. The mucus lubricates the food and also prevents hydrochloric acid from acting on the walls of the stomach.
The stomach is divided into four sections, each of which has different cells and functions. The sections are:
Cardia Where the contents of the esophagus empty into the stomach.
Fundus Formed by the upper curvature of the organ.
Body or Corpus The main, central region.
Pylorus The lower section of the organ that facilitates emptying the contents into the small intestine
In humans, gastrin is a peptide hormone that stimulates secretion of gastric acid (HCl) by the parietal cells of the stomach and aids in gastric motility. It is released by G cells in the antrum of the stomach (the portion of the stomach adjacent the pyloric valve), duodenum, and the pancreas.
Gastrin binds to cholecystokinin B receptors to stimulate the release of histamines in enterochromaffin-like cells, and it induces the insertion of K+/H+ ATPase pumps into the apical membrane of parietal cells (which in turn increases H+ release into the stomach cavity). Its release is stimulated by peptides in the lumen of the stomach.
Gastrin is released in response to certain stimuli. These include:
stomach distension
vagal stimulation (mediated by the neurocrine bombesin, or GRP in humans)
the presence of partially digested proteins especially amino acids
hypercalcemia
Gastrin release is inhibited by:
The presence of acid (primarily the secreted HCl) in the stomach (a case of negative feedback).
Somatostatin also inhibits the release of gastrin, along with secretin, GIP (gastroinhibitory peptide), VIP (vasoactive intestinal peptide), glucagon and calcitonin.
Function
G cell is visible near bottom left, and gastrin is labeled as the two black arrows leading from it. Note: this diagram does not illustrate gastrin’s stimulatory effect on ECL cells.The presence of gastrin stimulates parietal cells of the stomach to secrete hydrochloric acid (HCl)/gastric acid. This is done both directly on the parietal cell and indirectly via binding onto CCK2/gastrin receptors on ECL cells in the stomach, which then responds by releasing histamine, which in turn acts in a paracrine manner on parietal cells stimulating them to secrete H+ ions. This is the major stimulus for acid secretion by parietal cells.
Along with the above mentioned function, gastrin has been shown to have additional functions as well:
Stimulates parietal cell maturation and fundal growth.
Causes chief cells to secrete pepsinogen, the zymogen (inactive) form of the digestive enzyme pepsin.
Increases antral muscle mobility and promotes stomach contractions.
Strengthens antral contractions against the pylorus, and relaxes the pyloric sphincter, which stimulates gastric emptying.
Plays a role in the relaxation of the ileocecal valve.
Induces pancreatic secretions and gallbladder emptying.
Impacts lower esophageal sphincter (LES) tone, causing it to contract.
Cholecystokinin (CCK or CCK-PZ; from Greek chole, “bile”; cysto, “sac”; kinin, “move”; hence, move the bile-sac (gallbladder)) is a peptide hormone of the gastrointestinal system responsible for stimulating the digestion of fat and protein. Cholecystokinin, previously called pancreozymin, is synthesized by I-cells in the mucosal epithelium of the small intestine and secreted in the duodenum, the first segment of the small intestine, and causes the release of digestive enzymes and bile from the pancreas and gallbladder, respectively. It also acts as a hunger suppressant. Recent evidence has suggested that it also plays a major role in inducing drug tolerance to opioids like morphine and heroin, and is partly implicated in experiences of pain hypersensitivity during opioid withdrawal.
CCK mediates a number of physiological processes, including digestion and satiety. It is released by I cells located in the mucosal epithelium of the small intestine (mostly in the duodenum and jejunum), neurons of the enteric nervous system and neurons in the brain. Release of CCK is stimulated by monitor peptide released by pancreatic acinar cells as well as CCK-releasing protein, a paracrine factor secreted by enterocytes in the gastrointestinal mucosa. In addition, release of acetylcholine by the parasympathetic nerve fibers of the vagus nerve also stimulate its secretion. The presence of fatty acids and/or certain amino acids in the chyme entering the duodenum is the greatest stimulator of CCK release.
CCK mediates digestion in the small intestine by inhibiting gastric emptying and gastric acid secretion. It stimulates the acinar cells of the pancreas to release digestive enzymes and stimulates the secretion of a juice rich in pancreatic digestive enzymes, hence the old name pancreozymin. Together these enzymes catalyze the digestion of fat, protein, and carbohydrates. Thus, as the levels of the substances that stimulated the release of CCK drop, the concentration of the hormone drops as well. The release of CCK is also inhibited by somatostatin. Trypsin, a protease released by pancreatic acinar cells hydrolyzes CCK-releasing peptide and monitor peptide effectively turning off the additional signals to secrete CCK.
CCK also causes the increased production of hepatic bile, and stimulates the contraction of the gall bladder and the relaxation of the Sphincter of Oddi (Glisson’s sphincter), resulting in the delivery of bile into the duodenal part of the small intestine. Bile salts form amphipathic micelles that emulsify fats, aiding in their digestion and absorption.
Secretin is a hormone that both controls the environment in the duodenum by regulating secretions of the stomach and pancreas, and regulates water homeostasis throughout the body. It is produced in the S cells of the duodenum, which are located in the crypts of Lieberkühn. In humans, the secretin peptide is encoded by the SCT gene. Secretin was also the first hormone to be identified.
Secretin helps in regulating the pH within the duodenum by inhibiting gastric acid secretion by the parietal cells of the stomach, and by stimulating bicarbonate production by the centroacinar cells and intercalated ducts of the pancreas.
In 2007, secretin was discovered to play a role in osmoregulation by acting on the hypothalamus, pituitary, and kidney.
Gastric inhibitory polypeptide (GIP), also known as the glucose-dependent insulinotropic peptide is a member of the secretin family of hormones
It has traditionally been called gastrointestinal inhibitory peptide or gastric inhibitory peptide and was believed to neutralize stomach acid[citatioeeded] to protect the small intestine from acid damage, reduce the rate at which food is transferred through the stomach, and inhibit the GI motility and secretion of acid. However, this is incorrect, as it was discovered that these effects are achieved only with higher-than-normal physiological level, and that these results naturally occur in the body through a similar hormone, secretin.
It is now believed that the function of GIP is to induce insulin secretion, which is stimulated primarily by hyperosmolarity of glucose in the duodenum.After this discovery, some researchers prefer the new name of glucose-dependent insulinotropic peptide, while retaining the acronym “GIP.” The amount of insulin secreted is greater when glucose is administered orally than intravenously.
GIP is also thought to have significant effects on fatty acid metabolism through stimulation of lipoprotein lipase activity in adipocytes. GIP release has been demonstrated in the ruminant animal and may play a role iutrient partitioning in milk production (lipid metabolism). GIP is secreted in response to the first maternal feed (colostrum) in goat kids—GIP being measured via umbilical vein before its closure. For ethical reasons, GIP secretion has been demonstrated in humans only at approx 10 days of age. With respect to the role of GIP in lipid metabolism, supraphysiological levels have shown a lipogenic action, however the action of collagenase in experimental protocols is known to degrade GIP/ GIP receptors. GIP is part of the diffuse endocrine system and, as a consequence, difficult to demonstrate physiological or clinical effects. In comparison to insulin its effects are very subtle.
GIP recently appeared as a major player in bone remodelling. Researchers at Universities of Angers and
Enteroglucagon is a peptide hormone derived from preproglucagon. It is a gastrointestinal hormone, secreted from mucosal cells primarily of the colon and terminal ileum. It has 37 amino acids. Enteroglucagon is released following ingestion of a mixed meal, and delays gastric emptying.
Mechanical Digestion takes place in the mouth, where the the saliva, teeth, and tongue all play an important role in this digestive process.
Salivary Glands and Saliva
Saliva is produced in and secreted from salivary glands. The basic secretory units of salivary glands are clusters of cells called an acini. These cells secrete a fluid that contains water, electrolytes, mucus and enzymes, all of which flow out of the acinus into collecting ducts.
Within the ducts, the composition of the secretion is altered. Much of the sodium is actively reabsorbed, potassium is secreted, and large quantities of bicarbonate ion are secreted. Bicarbonate secretion is of tremendous importance to ruminants because it, along with phosphate, provides a critical buffer that neutralizes the massive quantities of acid produced in the forestomachs. Small collecting ducts within salivary glands lead into larger ducts, eventually forming a single large duct that empties into the oral cavity.
Most animals have three major pairs of salivary glands that differ in the type of secretion they produce:
parotid glands – produce a serous, watery secretion.
submaxillary (mandibular) glands – produce a mixed serous and mucous secretion.
sublingual glands – secrete a saliva that is predominantly mucous in character.
The basis for different glands secreting saliva of differing composition can be seen by examining salivary glands histologically. Two basic types of acinar epithelial cells exist:
serous cells, which secrete a watery fluid, essentially devoid of mucus.
mucous cells, which produce a very mucus-rich secretion.
Acini in the parotid glands are almost exclusively of the serous type, while those in the sublingual glands are predominantly mucous cells. In the submaxillary glands, it is common to observe acini composed of both serous and mucous epithelial cells.
Secretion of saliva is under control of the autonomic nervous system, which controls both the volume and type of saliva secreted. This is actually fairly interesting: a dog fed dry dog food produces saliva that is predominantly serous, while dogs on a meat diet secrete saliva with much more mucus. Parasympathetic stimulation from the brain, as was well demonstrated by Ivan Pavlov, results in greatly enhanced secretion, as well as increased blood flow to the salivary glands.
Potent stimuli for increased salivation include the presence of food or irritating substances in the mouth, and thoughts of or the smell of food. Knowing that salivation is controlled by the brain will also help explain why many psychic stimuli also induce excessive salivation – for example, why some dogs salivate all over the house when it’s thundering.
Functions of Saliva
What then are the important functions of saliva? Actually, saliva serves many roles, some of which are important to all species, and others to only a few:
Lubrication and binding: the mucus in saliva is extremely effective in binding masticated food into a slippery bolus that (usually) slides easily through the esophagus without inflicting damage to the mucosa. Saliva also coats the oral cavity and esophagus, and food basically never directly touches the epithelial cells of those tissues.
Solubilises dry food: in order to be tasted, the molecules in food must be solubilised.
Oral hygiene: The oral cavity is almost constantly flushed with saliva, which floats away food debris and keeps the mouth relatively clean. Flow of saliva diminishes considerably during sleep, allow populations of bacteria to build up in the mouth — the result is dragon breath in the morning. Saliva also contains lysozyme, an enzyme that lyses many bacteria and prevents overgrowth of oral microbial populations.
Initiates starch digestion: in most species, the serous acinar cells secrete an alpha-amylase which can begin to digest dietary starch into maltose. Amylase does not occur in the saliva of carnivores or cattle.Provides alkaline buffering and fluid: this is of great importance in ruminants, which have non-secretory forestomachs.Evaporative cooling: clearly of importance in dogs, which have very poorly developed sweat glands – look at a dog panting after a long run and this function will be clear. Diseases of the salivary glands and ducts are not uncommon in animals and man, and excessive salivation is a symptom of almost any lesion in the oral cavity. The dripping of saliva seen in rabid animals is not actually a result of excessive salivation, but due to pharyngeal paralysis, which prevents saliva from being swallowed.
Saliva is a watery substance located in the mouths of organisms, secreted by the salivary glands. Human saliva is 99.5% water, while the other 0.5% consists of electrolytes, mucus, glycoproteins, enzymes, and antibacterial compounds such as secretory IgA and lysozyme. The enzymes found in saliva are essential in beginning the process of digestion of dietary starches and fats. These enzymes also play a role in breaking down food particles entrapped within dental crevices, protecting teeth from bacterial decay.Furthermore, saliva serves a lubricative function, wetting food and permitting the initiation of swallowing, and protecting the mucosal surfaces of the oral cavity from desiccation.
Role of saliva in vitality of human
¨ 1. Moisten of solid food;
¨ 2. Dissolving of substances;
¨ 3. Moisten of mouth;
¨ 4. Cover food;
¨ 5. To help of swallowing;
¨ 6. Primary hydrolyzing of carbohydrates;
¨ 7. Antibacterial properties;
¨ 8. Neutralized the stomach juice.
Digestion
The digestive functions of saliva include moistening food and helping to create a food bolus. This lubricative function of saliva allows the food bolus to be passed easily from the mouth into the esophagus. Saliva contains the enzyme amylase, also called ptyalin, which is capable of breaking down starch into simpler sugars that can be later absorbed or further broken down in the small intestine. Salivary glands also secrete salivary lipase (a more potent form of lipase) to begin fat digestion. Salivary lipase plays a large role in fat digestion iewborn infants as their pancreatic lipase still needs some time to develop.[5] It also has a protective function, helping to prevent bacterial build-up on the teeth and washing away adhered food particles.
Stimulation
The production of saliva is stimulated both by the sympathetic nervous system and the parasympathetic.[11]
The saliva stimulated by sympathetic innervation is thicker, and saliva stimulated parasympathetically is more watery.
Sympathetic stimulation of saliva is to facilitate respiration, whereas parasympathetic stimulation is to facilitate digestion.
Parasympathetic stimulation leads to acetylcholine (ACh) release onto the salivary acinar cells. ACh binds to muscarinic receptors, specifically M3, and causes an increased intracellular calcium ion concentration (through the IP3/DAG second messenger system). Increased calcium causes vesicles within the cells to fuse with the apical cell membrane leading to secretion. ACh also causes the salivary gland to release kallikrein, an enzyme that converts kininogen to lysyl-bradykinin. Lysyl-bradykinin acts upons blood vessels and capillaries of the salivary gland to generate vasodilation and increased capillary permeability respectively. The resulting increased blood flow to the acini allows production of more saliva. In addition, Substance P can bind to Tachykinin NK-1 receptors leading to increased intracellular calcium concentrations and subsequently increased saliva secretion. Lastly, both parasympathetic and sympathetic nervous stimulation can lead to myoepitheilium contraction which causes the expulsion of secretions from the secretory acinus into the ducts and eventually to the oral cavity.
Sympathetic stimulation results in the release of norepinephrine. Norepinephrine binding to α-adrenergic receptors will cause an increase in intracellular calcium levels leading to more fluid vs. protein secretion. If norepinephrine binds β-adrenergic receptors, it will result in more protein or enzyme secretion vs. fluid secretion. Stimulation by norepinephrine initially decreases blood flow to the salivary glands due to constriction of blood vessels but this effect is overtaken by vasodilation caused by various local vasodilators.
Saliva production may also be pharmacologically stimulated by so-called sialagogues. It can also be suppressed by so-called antisialagogues.
Daily salivary output
There is much debate about the amount of saliva that is produced in a healthy person per day; estimates range from 0.75 to 1.5 liters per day while it is generally accepted that during sleep the amount drops to almost zero. In humans, the submandibular gland contributes around 70–75% of secretion, while the parotid gland secretes about 20–25% and small amounts are secreted from the other salivary glands.
Contents
Produced in salivary glands, human saliva is 99.5% water, but it contains many important substances, including electrolytes, mucus, antibacterial compounds and various enzymes.Atomar saliva
Latin saliva atomaris
Gives rise to molecular saliva
Molecular saliva
Latin saliva molecularis
Precursor atomar saliva
Gives rise to normal saliva
Normal saliva
Latin saliva normalis
Precursor molecular saliva
It is a fluid containing:
Water
Electrolytes:
2–21 mmol/L sodium (lower than blood plasma)
10–36 mmol/L potassium (higher than plasma)
1.2–2.8 mmol/L calcium (similar to plasma)
0.08–0.5 mmol/L magnesium
5–40 mmol/L chloride (lower than plasma)
25 mmol/L bicarbonate (higher than plasma)
1.4–39 mmol/L phosphate
Iodine (mmol/L usually higher than plasma, but dependent variable according to dietary iodine intake)
Mucus. Mucus in saliva mainly consists of mucopolysaccharides and glycoproteins;
Antibacterial compounds (thiocyanate, hydrogen peroxide, and secretory immunoglobulin A)
Epidermal growth factor or EGF
Various enzymes. There are three major enzymes found in saliva.
α-amylase (EC3.2.1.1). α-Amylase, or ptyalin, secreted by the acinar cells of the parotid and submandibular glands, starts the digestion of starch before the food is even swallowed. It has a pH optima of 7.4.
Lingual lipase. Lingual lipase, which is secreted by the acinar cells of the sublingual gland, has a pH optimum ~4.0 so it is not activated until entering the acidic environment of the stomach.
Kallikrein. Kallikrein is an enzyme that proteolytically cleaves high-molecular-weight kininogen to produce bradykinin, which is a vasodilator. It is secreted by the acinar cells of all three major salivary glands.
Antimicrobial enzymes that kill bacteria.
Lysozyme
Salivary lactoperoxidase
Lactoferrin
Immunoglobulin A
Proline-rich proteins (function in enamel formation, Ca2+-binding, microbe killing and lubrication)
Minor enzymes include salivary acid phosphatases A+B, N-acetylmuramoyl-L-alanine amidase, NAD(P)H dehydrogenase (quinone), superoxide dismutase, glutathione transferase, class 3 aldehyde dehydrogenase, glucose-6-phosphate isomerase, and tissue kallikrein (function unknown).
Cells: Possibly as many as 8 million human and 500 million bacterial cells per mL. The presence of bacterial products (small organic acids, amines, and thiols) causes saliva to sometimes exhibit foul odor.
Opiorphin, a newly researched pain-killing substance found in human saliva.
Haptocorrin, a protein which binds to Vitamin B12 to protect it against degradation in the stomach, before it binds to Intrinsic Factor
A building being renovated in the Carrollton section of New Orleans
Spitting
Spitting, or expectoration, is the act of forcibly ejecting saliva or other substances from the mouth. It is often considered rude and a social taboo in many parts of the world, including Western countries, where it is frequently forbidden by local laws (as it was thought to facilitate the spread of disease). These laws are generally not strictly enforced. In Singapore
Various species have special uses for saliva that go beyond predigestion. Some swifts use their gummy saliva to build nests. Aerodramus nests are prized for use in bird’s nest soup.Cobras, vipers, and certain other members of the venom clade hunt with venomous saliva injected by fangs. Some arthropods, such as spiders and caterpillars, create thread from salivary glands. The salivary glands in mammals are exocrine glands, glands with ducts, that produce saliva. They also secrete amylase, an enzyme that breaks down starch into maltose.
Salivary glands: #1 is Parotid gland, #2 is Submandibular gland, #3 is Sublingual gland
Latin Glandulae salivariae
Teeth
The aftermath of the action of the teeth in digestion results in two outcomes: havoc and devastation. The teeth are gears to demolish chunks of food by a series of actions such as clamping, slashing, piercing, grinding and crushing. The teeth do the first drastic destruction to food in the digestive system.
Tongue
The tongue consists of four types of taste buds–salty, sweet, sour, and bitter–and is a very maneuverable and pliable arrangement of muscle. It helps to remove, and dislocate food particles in the teeth and shifts food around in the mouth in order to assist with the all important act of swallowing.
Peristalsis
This mechanical action has to do with sets of muscles that cooperate to move both liquid and solid food along the digestive tract. In other word, it pushes food along your esophagus, stomach, and intestine.. Gravitational pull is lessened in a sense when food enters the esophagus because of peristalsis. Peristalsis helps a person to swallow lying down or even standing on their head. Peristalsis has another essential task besides assisting in the movement of food through the body. It also helps to knead, agitate, and pound the solid residue that is left after the teeth or those without teeth, the gums, have done their best. Digestive Sphincters
Many people spend a third of their time consciously trying to control how to get food into their digestive tracts and another third thinking about how that food is doing when it gets into their digestive tracts and another third of their time consciously trying to control how to get their food intake out of their digestive tracts. However, once food is swallowed, the conscious ability to control the passage of food is almost completely lost. When the food reaches the point of elimination some conscious control is again reestablished in the digestive system. The gastrointestinal track or as people call it, the digestive system, has the main purpose of break down food, both solid and fluid into sustenance for the various tissues and systems in the body. A normal digestive tract squeezes the utmost benefit from what it eats. Feces are the products left over when the body has selected everything that is of use from the food that has been eaten.The digestive system distance ranges from the mouth to the bottom of the trunk, which when we look at it, seems like no more than two or three feet, but is really about
From the moment the three main types of food-carbohydrates, fats and proteins-enter the mouth, they are exposed to chemical and mechanical actions that begin to break them apart so that they can be absorbed through the intestinal walls into the circulatory system.
Taste Receptors
A very large number of molecules elicit taste sensations through a rather small number of taste receptors. Furthermore, it appears that individual taste receptor cells bear receptors for one type of taste. In other words, within a taste bud, some taste receptor cells sense sweet, while others have receptors for bitter, sour, salty and umami tastes. Much of this understanding of taste receptors has derived from behavioral studies with mice engineered to lack one or more taste receptors.
The pleasant tastes (sweet and umami) are mediated by a family of three T1R receptors that assemble in pairs. Diverse molecules that lead to a sensation of sweet bind to a receptor formed from T1R2 and T1R3 subunits. Cats have a deletion in the gene for T1R2, explaining their non-responsiveness to sweet tastes. Also, mice engineered to express the human T1R2 protein have a human-like response to different sweet tastes. The receptor formed as a complex of T1R1 and T1R3 binds L-glutamate and L-amino acids, resulting the umami taste.
The bitter taste results from binding of diverse molecules to a family of about 30 T2R receptors. Sour tasting itself involves activation of a type of TRP (transient receptor potential) channel. Surprisingly, the molecular mechanisms of salt taste reception are poorly characterized relative to the other tastes.
Taste is a chemical sense which is detected by special structures called taste buds, of which we all have about 10,000, mainly on the tongue with a few at the back of the throat and on the palate. Taste buds surround pores within the protuberances on the tongue’s surface and elsewhere. There are four types of taste buds: these are sensitive to sweet, salty, sour and bitter chemicals. All tastes are formed from a mixture of these basic elements.
Many different tastes can be distinguished because of the combination of taste and the more discriminating sense of smell. The sense of smell is estimated to be about 10,000 times more sensitive than the sense of taste. The two senses are very closely related. It is usually correct to say that one smells more flavours than one tastes. When the nose fails, from a bad cold for instance, 80% of the taste ability is lost. Loss of taste without loss of smell is pretty rare, but “dry mouth” can contribute because taste buds can only detect flavour when food is dissolved in saliva. Taste can also be lost as a result of damage to the taste buds themselves or damage to the cranial nerves that carry taste sensations to the brain. Full sensory appreciation of food also involves its appearance, its consistency, and its temperature.
The picture of the tongue shows the areas where different types of taste are detected.
|
Green represents the area where sweet taste is interpreted.
Blue interprets salty tastes.
Red detects sour tastes and yellow picks out the bitter tastes.
Observation of the Tongue
Observation of the tongue, also known as tongue diagnosis, is an important procedure in diagnosis by inspection. It provides primary information for the Chinese physicians to make diagnosis.
Physiology of the tongue. The tongue directly or indirectly connects with many zang – fu organs through the meridians and collaterals. The deep branch of Heart Meridian of Hand – Shaoyin goes to the root of the tongue ; the Spleen Meridian of Foot – Taiyin traverses the root of the tongue and spreads over its lower surface ; the Kidney Meridian of Foot – Shaoyin terminates at the root of the tongue. So the essential qi of the zang – fu organs can go upward to nourish the tongue, and pathological changes of the zang – fu organs can be reflected by changes in tongue conditions. This is why the observation of the tongue can determine the pathological changes of the internal organs.
1. Heart and lung
2. Spleen and Stomach
3. Kidney
4. Liver and gallbladder
Observation of the tongue includes the tongue proper and its coating. The tongue proper refers to the muscular tissue of the tongue, which is also known as the tongue body. The tongue coating refers to a layer of ” moss ” over the tongue surface, which is produced by the stomach qi. A normal tongue is of proper size, soft in quality, free in motion, slightly red in color and with a thin layer of white coating which is neither dry nor over moist.
The tongue is divided into four areas, namely, tip, central part, root and border. The tip of the tongue often reveals the pathological changes of the heart and lung ; its border reveals those of the liver and gallbladder ; its central part reveals those of the spleen and stomach ; and its root reveals those of the kidney. This method of diagnosing the pathological changes of the zang – fu organs by dividing the tongue into corresponding areas is clinically significant.
Stomach
Experimental method of studying of stomach secretion (Method of Basov – during the operation on dogs put the fistula in stomach. It connect stomach with the external environment. During eating the stomach juice go out through this fistula, but it has food and saliva. Method of Pavlov – method of “imaging eating” – during the operation on dogs put 2 fistulas: in esophagus and stomach. During eating the food go out through the esophagus fistula, that is way we have only juice. Method of Geydengine – a little stomach – to apart a little part of stomach, in which cut n.vagus. In this case we may to study humoral stimulation. Method of Pavlov – to separate little stomach from whole organ by 2 layers of mucous. In this case presents all regulatory mechanisms.)
c) Clinical method of stomach investigation (Gastroscopy, stomach sound, ultrasonic investigation, electrogastrography, pH-metry, determine helycobacter pylory.)
About
To stimulate production of duodinum gormon – secretin.)
Phases of stomach secretion
a) Cephalic phase (This phase caused by nervous system. It has conditional and unconditional reflexes. Conditional reactions caused by appearance of food, it smell and other stimulus, which are connect with food. Unconditional influences have parasympathetic. Parasympathetic components of unconditional influences beginning from receptors of tongue and other receptors of the oral cavity. From these receptors impulses pass through the fibers of nervus trigeminus, nervus facialis, nervus glossopharyngeus, nervus vagus to the medulla oblongata. Impulses return to stomach by nervus vagus. Except neuron influences this phase has humoral influences – brunch of nervus vagus produce gormon gastrin. These phase is very shortly.)
b) Stomach phase (These phase depend from quantity of food, which are present in stomach. It has vago-vagal reflexes (by mean of central nerves system) and local – peripheral reflexes, which are closed in stomach walls. Duration of these phase is longer and quantity of juice is much. It has humoral mechanisms too (production of gastrin and histamin).
c) Intestine phase (Presence of food in the upper portion of small intestine can cause the stomach to secrete small amount of gastric juice. This probably results of gastrin are also released by the duodenal mucosa in response to distension or chemical stimuli of the same type as those that stimulate the stomach gastrin mechanism.)
Value of gastric juice secretion
In norm gastric juice secretion must be
|
NN |
Indexes |
Empty stomach |
Basal secretion |
Stimulated secretion |
|
1. |
pH |
to 3,5 |
1,5-2 |
1,3-1,4 |
|
2. |
Production of common HCl, mmol/L |
10-35 |
40-60 |
80-100 |
|
3. |
Production of free HCl, mmol/L |
0-20 |
20-40 |
65-85 |
|
4. |
Debit of common HCl, mmol/hour |
to 1,5 |
1,5-5,5 |
8-14 |
|
5. |
Debit of free HCl, mmol/hour |
to 1 |
1-4 |
6,5-12 |
1. Digestion in the small intestine
Small intestine
It has three parts: the Duodenum, Jejunum, and Ileum.
After being processed in the stomach, food is passed to the small intestine via the pyloric sphincter. The majority of digestion and absorption occurs here after the milky chyme enters the duodenum. Here it is further mixed with three different liquids:
Bile, which emulsifies fats to allow absorption, neutralizes the chyme and is used to excrete waste products such as bilin and bile acids. Bile is produced by the liver and then stored in the gallbladder where it will be released to the small intestine via the bile duct. The bile in the gallbladder is much more concentrated.[clarificatioeeded]
Pancreatic juice made by the pancreas, which secretes enzymes such as pancreatic amylase, pancreatic lipase, and trypsinogen (inactive form of protease).
Intestinal juice secreted by the intestinal glands in the small intestine. It contains enzymes such as enteropeptidase, erepsin, trypsin, chymotrypsin, maltase, lactase and sucrase (all three of which process only sugars).
The pH level increases in the small intestine as all three fluids are alkaline. A more basic environment causes more helpful enzymes to activate and begin to help in the breakdown of molecules such as fat globules. Small, finger-like structures called villi, and their epithelial cells is covered with numerous microvilli to improve the absorption of nutrients by increasing the surface area of the intestine and enhancing speed at which nutrients are absorbed. Blood containing the absorbed nutrients is carried away from the small intestine via the hepatic portal vein and goes to the liver for filtering, removal of toxins, and nutrient processing.
The small intestine and remainder of the digestive tract undergoes peristalsis to transport food from the stomach to the rectum and allow food to be mixed with the digestive juices and absorbed. The circular muscles and longitudinal muscles are antagonistic muscles, with one contracting as the other relaxes. When the circular muscles contract, the lumen becomes narrower and longer and the food is squeezed and pushed forward. When the longitudinal muscles contract, the circular muscles relax and the gut dilates to become wider and shorter to allow food to enter.
a) Role of duodenum in the digestive system (There are two secretor functions of pancreas – external and internal. The external secretor function of pancreas means that exsogenic cells of pancreas and ducts cells produce pancreatic juice. It helps to hydrolyzed protein to peptides and amino acids, carbohydrates to monosaccharides, lipids to the fat acids and glycerine. It neutralizes acidic chymus, which come from stomach.)
b) External secretor function of pancreas (The external secretor function of pancreas means that exsogenic cells of pancreas and ducts cells produce pancreatic juice).
c) Composition and property of pancreas juice (Quantity of pancreatic juice per day – 1,5-
d) Regulation of pancreas secretion (Regulation act by complex of neuro-humoral mechanisms. There are three phases of pancreatic secretion: cephalic, stomach and intestine. The first stage caused by act of nervous influences. Nervus vagus realizes this effect by means of conditioned and unconditioned reflexes. Secretion begins after 1-2 minutes of food. This juice consists of enzymes, small quantity of water and ions. Sympathetic influences have a trophic role. During the second phase there are two kinds of influences: nervous and humoral, for example, gastrin from stomach. The third phase caused by chymus contents. The main is humoral factors. In that time secrete 2 hormons – secretin and cholecystokinin-pancreasemin. Secretin stimulates production of a big quantity of juice with a high concentration of hydro carbonates and a small quantity of enzymes in ducts cells. Cholecystokinin-pancreasemin stimulates production of a less quantity of juice with a big concentration of enzymes in acinars cells.)
e) Bile production and bile secrete (Secretion of bile occur all time and increase by influences of bile acids, cholecystokinin-pancreasemin, secretin. Bile secretion in the duodenum depends from take food. It depends of nervus vagus and humoral influences – concentration of cholecystokinin-pancreasemin, secretin, fats.)
f) Composition of bile, their role in digestive processes (Composition: bilirubin, bile acids, cholesterol, leukocytes, some epitheliocytes, cristalls of bilirubin, calcium, cholesterol. The role of bile: 1. Neutrolyze the stomach acid; 2. Inhibit he act of stomach proteases; 3. Increase the activity of pancreatic lipase; 4. Emulgate the lipids; 5. Increase the absorption of fat acids, vitamins K, D, E; 6. Increase tone and motor function of intestines; 7. Decrease the activity of intestine microflora.)
g) Composition and properties of intestine juice (Composition of intestine juice: mucus, enzymes – peptidase, saccharase, maltase, lactase, lipase, immunoglobulins, leukocytes; epitheliocytes (
h) Cavity and membrane hydrolyses of substances (On the glicocalix of micro fibers present enzymes, which are adsorbed and digest small molecules of nutritive substances – membrane hydrolyses of substances. Cavity hydrolyses of substances provide by enzymes, which are in intestine space.)
Digestion in the large intestine
After the food has been passed through the small intestine, the food enters the large intestine. Within it, digestion is retained long enough to allow fermentation due to the action of gut bacteria, which breaks down some of the substances that remain after processing in the small intestine; some of the breakdown products are absorbed. In humans, these include most complex saccharides (at most three disaccharides are digestible in humans). In addition, in many vertebrates, the large intestine reabsorbs fluid; in a few, with desert lifestyles, this reabsorbtion makes continued existence possible.
In general, the large intestine is less vigorous in absorptive activity. It produces sacculation, renews epithelial cells, and provides protective mucus and mucosal immunity. In humans, the large intestine is roughly
a) Composition of intestine juice and their properties (Composition of intestine juice: mucus, epithelial cells. Functions: protective from mechanical, chemical irritations; formed of base reaction of intestine contents.)
b) Role of the micro flora of big intestine (1. Ending decompose of all nutritive substances, which are do not digestive; synthesis of some vitamins – of B group, vitamin K; take place in metabolic processes.)
Small intestine
The small intestine is the longest organ of the digestive tract. It is divided up indiscriminately into three sections: the duodenum, the jejunum, and the ilium.
Duodenum
This is the place where the ultimate destruction of food digestion reaches its completion and where the acidity of chyme is nullified. The nutrients in the food eaten many hours ago have almost been diminished to molecules small enough to be absorbed through the intestinal walls into the bloodstream. Carbohydrates are diminished into simpler sugars; proteins to amino acids; and fats to fatty acids and glycerol. Enzymes are secreted by the walls of the duodenum and unite with the bile (essential for the digestion and absorption of tenacious fatty materials) and pancreatic enzymes in the duodenum.
Jejunum
Peristalsis pushes the nutrient liquid out of the duodenum into the first reaches of the jejunum. A greater number of villi , microscopic, hair like structures, begin to absorb amino acids , sugars, fatty acids and glycerol from the digested contents of the small intestine, and starts them on their way to other parts of the body. This part of the small intestine executes a digestive operation so that what is passed on to the large intestine is a thin watery substance almost completely devoid of nutrients.
Ilium
This is the place which is about a third of the small intestine. The greatest number of the estimated five or six million villi in the small intestine are found along the ilium making it the main absorption locale of the gastrointestinal tract. The villi here are always in a fretful movement: oscillating, pulsating, lengthening, shortening, growing narrower then wider, extorting every particle of nutrient.
The Liver, Gallbladder, and Pancreas
Legitimately, these three organs lie outside of the gastrointestinal tract. Nevertheless, digestive fluids from all three meet like intersections of a railway track at the common bile duct, and their movement from there into the duodenum is controlled by a sphincter muscle.
The pancreas is a producer of digestive enzymes. The gallbladder is a small reservoir for bile. The liver reproduces nutrients so that they can be used for cell-rebuilding and energy.
Large Intestine
There is a merger between the illium and the cecum, the first section of the large intestine. Any solid substances that flow into the large intestine through the ileocecal valve (which prevents back flow into the small intestine) are as a rule indigestible, or are bile constituents. What the cecum primarily inherits is water.
What the large intestine essentially does, other than act as a passageway for removal of body wastes, is to act as a provisional reservoir for water. There are no villi in the large intestine and peristalsis is much less forceful than in the small intestine. As water is absorbed, the contents of the large intestine change from a watery liquid and are compressed into semisolid feces. Nerve endings in the large intestine signal the brain that it is time for a bowel movement. The fecal material moves through the colon down to several remaining inches known as the rectum and out through the anus an opening controlled by the outlet valves of the large intestine.
Enzymes
|
Site of Enzyme Origin |
Enzyme |
Nutrient It Breacks Down |
Product Of Enzyme Action |
Place of Enzyme Action |
|
Salivary Glands |
Salivary Almalase |
Carbohydrates-sugars |
Simple Sugars |
Mouth |
|
Gastric glands |
Pepsin |
Proteins |
Amino Acids |
Stomach |
|
Liver |
Bile |
Fats/Lipids |
Emulsifide Fats |
Small Intestine |
|
Small Intestine |
Maltase, Lactase, Sucrase |
Carbohydrates |
Simple sugars |
Small Intestine |
|
Pancrease |
Trypsin, Lipase, Amylase |
Proteins, Fats/Lipids, Carbohydrates |
Amino acids, Glycerol/Fatty Acids, Simple Sugars |
Small Intestine |
The gastrointestinal tract is supplied with a number of muscular valves. These control and direct the quantity of food that goes through the digestive tract and inhibits the back movement of partially digested food.
Digestive hormones
Action of the major digestive hormones
There are at least five hormones that aid and regulate the digestive system in mammals. There are variations across the vertebrates, as for instance in birds. Arrangements are complex and additional details are regularly discovered. For instance, more connections to metabolic control (largely the glucose-insulin system) have been uncovered in recent years.
Gastrin – is in the stomach and stimulates the gastric glands to secrete pepsinogen (an inactive form of the enzyme pepsin) and hydrochloric acid. Secretion of gastrin is stimulated by food arriving in stomach. The secretion is inhibited by low pH .
Secretin – is in the duodenum and signals the secretion of sodium bicarbonate in the pancreas and it stimulates the bile secretion in the liver. This hormone responds to the acidity of the chyme.
Cholecystokinin (CCK) – is in the duodenum and stimulates the release of digestive enzymes in the pancreas and stimulates the emptying of bile in the gall bladder. This hormone is secreted in response to fat in chyme.
Gastric inhibitory peptide (GIP) – is in the duodenum and decreases the stomach churning in turn slowing the emptying in the stomach. Another function is to induce insulin secretion.
Motilin – is in the duodenum and increases the migrating myoelectric complex component of gastrointestinal motility and stimulates the production of pepsin.
Significance of pH in digestion
Digestion is a complex process controlled by several factors. pH plays a crucial role in a normally functioning digestive tract. In the mouth, pharynx, and esophagus, pH is typically about 6.8, very weakly acidic. Saliva controls pH in this region of the digestive tract. Salivary amylase is contained in saliva and starts the breakdown of carbohydrates into monosaccharides. Most digestive enzymes are sensitive to pH and will denature in a high or low pH environment.
The stomach’s high acidity inhibits the breakdown of carbohydrates within it. This acidity confers two benefits: it denatures proteins for further digestion in the small intestines, and provides non-specific immunity, damaging or eliminating various pathogens.[citation needed]
In the small intestines, the duodenum provides critical pH balancing to activate digestive enzymes. The liver secretes bile into the duodenum to neutralize the acidic conditions from the stomach, and the pancreatic duct empties into the duodenum, adding bicarbonate to neutralize the acidic chyme, thus creating a neutral environment. The mucosal tissue of the small intestines is alkaline with a pH of about 8.5.
1. Common characteristic of absorption process
a)Determine of notion “absorption”
Absorption is a complex of processes, which are provide transport of substances from digestive tract into internal surroundings of organism (blood, lymph, intercellular substances).
Main types of transport of nutritive substances in internal surroundings of organism are: passive and active.
b)
1.Passive transport include diffusion and osmosis. This transport do not need presents of energy. In this case substances transport through the mucus shell by help of concentrative gradient. This way of transport have water, water disolved vitamins (C, B6, B2).
2.Active transport include pinocytosis and active transport by help of protein and energy. Active transport need energy of ATP. This way characteristic of amino acids, monosaccharids, vitamin B12, ions of calcium, enzymes. Pinocytosis – by help of pynocytic bulb, where secreted enzymes for proteins hydrolysis. Products of hydrolysis adsorbed by cell.
1. ANATOMICAL BASIS OF ABSORPTION
The total quantity of fluid that must be absorbed each day is equal to the ingested fluid (about
The stomach is a poor absorptive area of the gastrointestinal tract because it lacks the typical villus type of absorptive membrane and also because the junctions between the epithelial cells are tight junctions. Only a few highly lipid-soluble substances, such as alcohol and some drugs like aspirin, can be absorbed in small quantities.
The Absorptive Surface of the Intestinal Mucosa – The Villi.
The absorptive surface of the intestinal mucosa, showing many folds called valvulae conniventes (or folds of Kerckring), which increase the surface area of the absorptive mucosa about threefold. These folds extend circularly most of the way around the intestine and are especially well developed in the duodenum and jejunum, where they often protrude as much as
Located over the entire surface of the small intestine, from approximately the point at which the common bile duct empties into the duodenum down to the ileocecal valve, are literally millions of small villi, which project about
The intestinal epithelial cells are characterized by a brush border, consisting of about 600 microvilli 1 μm in length and 0,1 μm in diameter protruding from each cell. This increases the surface area exposed to the intestinal materials another 20-fold. Thus, the combination of the folds of Kerckring, the villi, and the microvilli increases the absorptive area of the mucosa about 600-fold, making a tremendous total area of about
The general organization of a villus, emphasizing especially the advantageous arrangement of the vascular system for absorption of fluid and dissolved material into the portal blood, and the arrangement of the central lacteal for absorption into the lymph. Many small pinocytic vesicles, which are pinched-off portions of infolded epithelium surrounding extracellular materials that have been entrapped inside the cells. Small amounts of substances are absorbed by this physical process of pinocytosis, though, as noted later in the chapter, most absorption occurs by means of single molecular transfer. Located near the brush border of the epithelial cell are many mitochondria, which supply the cell with oxidative energy needed for active transport of materials through the intestinal epithelium. Also, extending linearly into each microvillus of the brush border are multiple actin filaments that are believed to contract and cause continual movement of the microvilli, keeping them constantly exposed to new quantities of intestinal fluid.
BASIC MECHANISMS OF ABSORPTION
Absorption through the gastrointestinal mucosa occurs by active transport and by diffusion, as is also true for other membranes.
Briefly, active transport imparts energy to the substance as it is being transported for the purpose of concentrating it on the other side of the membrane or for moving it against an electrical potential On the other hand, the term diffusion means simply transport of substances through the membrane as a result of molecular movement along, rather than against, an electrochemical gradient.
c)Absorption in the mouth cavity and stomach
In the mouth cavity absorbed water, water soluble medicines (for example, validol, nitroglycerin, adelphan, furosemid, corinfar and others). In our oral cavity, under the tongue present a big quantity of vessels. That is why all water soluble substances absorbed in this place. They go to the bloodstream, and have immediately action on our receptors. They do not go through the liver, and do not desintoxicated, that is why may be toxic effect of some substances, for example products of food, drugs.
In esophagus do not absorbed nutritive substances as a rule.
In stomach absorbed alcohol, water and small quantity of other substances.
ABSORPTION IN THE SMALL INTESTINE
d) Absorption in intestines
Virtually all nutrients from the diet are absorbed into blood across the mucosa of the small intestine. In addition, the intestine absorbs water and electrolytes, thus playing a critical role in maintenance of body water and acid-base balance.
It’s probably fair to say that the single most important process that takes place in the small gut to make such absorption possible is establishment of an electrochemical gradient of sodium across the epithelial cell boundary of the lumen. This is a critical concept and actually quite interesting. Also, as we will see, understanding this process has undeniably resulted in the saving of millions of lives.
To remain viable, all cells are required to maintain a low intracellular concentration of sodium. In polarized epithelial cells like enterocytes, low intracellular sodium is maintained by a large number of Na+/K+ ATPases – so-called sodium pumps – embedded in the basolateral membrane. These pumps export 3 sodium ions from the cell in exchange for 2 potassium ions, thus establishing a gradient of both charge and sodium concentration across the basolateral membrane.
In rats, as a model of all mammals, there are about 150,000 sodium pumps per small intestinal enterocyte which collectively allow each cell to transport about 4.5 billion sodium ions out of each cell per minute (J Membr Biol 53:119-128, 1980). Pretty impressive! This flow and accumulation of sodium is ultimately responsible for absorption of water, amino acids and carbohydrates.
Aside from the electrochemical gradient of sodium just discussed, several other concepts are required to understand absorption in the small intestine. Also, dietary sources of protein, carbohydrate and fat must all undergo the final stages of chemical digestion just prior to absorption of, for example, amino acids, glucose and fatty acids.
At this point, its easiest to talk separately about absorption of each of the major food groups, recognizing that all of these processes take place simultaneously.
Water and electrolytes
Carbohydrates, after digestion to monosaccharides
Proteins, after digestion to small peptides and amino acids
Neutral fat, after digestion to monoglyceride and free fatty acids
Absorption in the Small Intestine:
Normally, absorption from the small intestine each day consists of several hundred grams of carbohydrates, 100 or more grams of fat, 50 to
ABSORPTION IN THE LARGE INTESTINE
Approximately 1500 ml of chyme pass through the ileocecal valve into the large intestine each day. Most of the water and electrolytes in this are absorbed in the colon, usually leaving less than 100 ml of fluid to be excreted in the feces. Also, essentially all the ions are also absorbed, leaving only about 1 mEq each of sodium and chloride ions to be lost in the feces.
Most of the absorption in the large intestine occurs in the proximal half of the colon, giving this portion the name absorbing colon, whereas the distal colon functions principally for storage and is therefore called the storage colon.
Absorption and Secretion of Electrolytes and Water.
The mucosa of the large intestine, like that of the small intestine, has a high capability for active absorption of sodium, and the electrical potential created by the absorption of the sodium causes chloride absorption as well. The „tight junctions“ between the epithelial cells of the large intestinal epithelium are much tighter than those of the small intestine. This prevents significant amounts of back-diffusion of ions through these junctions, thus allowing the large intestinal mucosa to absorb sodium ions far more completely – that is, against a much higher concentration gradient – than can occur in the small intestine.
In addition, as in the distal portion of the small intestine, the mucosa of the large intestine actively secretes bicarbonate ions while it simultaneously actively absorbs an equal amount of chloride ions in an exchange transport process. The bicarbonate helps neutralize the acidic end-products of bacterial action in the colon.
The absorption of sodium and chloride ions creates an osmotic gradient across the large intestinal mucosa, which in turn causes absorption of water.
Bacterial Action in the
Numerous bacteria, especially colon bacilli, are present in the absorbing colon. These are capable of digesting small amounts of cellulose, in this way providing a few calories of nutrition to the body each day. In herbivorous animals this source of energy is very significant, though it is of negligible importance in the human being. Other substances formed as a result of bacterial activity are vitamin K, vitamin B12, thiamin, riboflavin, and various gases that contribute to flatus in the colon – especially carbon dioxide, hydrogen gas, and methane. Vitamin K is especially important, for the amount of this vitamin in the ingested foods is normally insufficient to maintain adequate blood coagulation.
Composition of the Feces. The feces normally are about three-fourths water and one-fourth solid matter composed of about 30 per cent dead bacteria, 10 to 20 per cent fat, 10 to 20 per cent inorganic matter, 2 to 3 per cent protein, and 30 per cent undigested roughage of the food and dried constituents of digestive juices, such as bile pigment and sloughed epithelial cells. The large amount of fat derives mainly from fat formed by bacteria and fat in the sloughed epithelial cells.
The brown color of feces is caused by stercobilin and urobilin, which are derivatives of bilirubin. The odor is caused principally by the products of bacterial action; these vary from one person to another, depending on each person’s colonic bacterial flora and on the type of food eaten. The actual odoriferous products include indole, skatole, mercaptans, and hydrogen sulfide.
ABSORPTION OF WATER
Isosmotic Absorption. Water is transported through the intestinal membrane entirely by the process of diffusion. Furthermore, this diffusion obeys the usual laws of osmosis. Therefore, when the chyme is dilute, water is absorbed through the intestinal mucosa into the blood of the villi by osmosis.
On the other hand, water can also be transported in the opposite direction, from the plasma into the chyme. This occurs especially when hyperosmotic solutions are discharged from the stomach into the duodenum Usually within minutes, sufficient water is transferred by osmosis to make the chyme isosmotic with the plasma Thereafter, the chyme remains almost exactly isosmotic throughout its total passage through the small and large intestines.
As dissolved substances are absorbed from the lumen of the gut into the blood the absorption tends to decrease the osmotic pressure of the chyme, but water diffuses so readily through the intestinal membrane (because of large 7 to
e) Methods of absorptions’ investigation
1. Angiostoma (experimental method). Surgeon put stoma, aperture, on one of the gastrointestinal vessels in which absorbed nutritive substances. He add it by help of catheter of body surface. In this case he investigate absorption processes in anybody part of intestines.
In the case of angiostoma he may investigate each stage of digestion in different organs – oral cavity, esophagus, stomach, small and large intestines. He may determining the speed of absorption; quantity of different substances, which are absorbed in different part of digestive tract; components of food, which can absorbed in different part of gastro-intestinal tract; speed of bloodstream in the different part of gastro-intestinal tract, which help to absorbed some substances; mechanism of absorption in different part of gastrointestinal tract.
2. X-ray investigation (experimental or clinical method). In this case by help of different substances, for example, suspension of barium for determining motor function of gastrointestinal tract and other water-soluble substances to determining absorption. Doctor do X-ray investigation and determining place of absorption, place of increase or decrease speed of absorption, part of digestive tract, where present decrease of absorption. This method may be act on animal too, for example, if we need to determining absorption of new substances.
3. Biochemical method of investigation (experimental or clinical method). In this case laboratory assistant investigate blood, urine, saliva to content of different substances – glucose, amino acids, fat acids, lactose, mannose, sugar and others. For example, to determining pathology of carbohydrates absorption in intestines doctor laboratory assistant investigate quantity of glucose, or galactose, or lactose, or mannose in blood and urine and if he know the quantity of glucose which are coming into organism, he may value absorption of glucose, or galactose, or lactose, or mannose in digestive tract. For example, to determining pathology of sodium or potassium absorption in digestive tract doctor laboratory assistant investigate concentration of sodium or potassium in saliva, blood, urine and after that doctor may value their absorption.
4. Radioisotopic investigation (clinical method). Nurses inject intravenously radioisotop, which absorbed in digestive tract, into the organism of patient. After some time, which is necessary for investigation, doctor scan the places, where this isotop must absorbed. Then he determining the absorptive function of intestines, as he see the speed of absorption, quantity of radioisotop, which are absorbed and place of absorption of radioisotop.
Water and mineral salts
Active Transport of Sodium. Twenty to
The principles of sodium absorption from the intestine are also essentially the same as those for absorption of sodium from the renal tubules. The motive power for the sodium absorption is provided by active transport of sodium from inside the epithelial cells through the side walls of these cells into the intercellular spaces. This active transport obeys the usual laws of active transport it requires energy, and it is catalyzed by appropriate ATPase carrier enzymes in the cell membrane. Part of the sodium is transported along with chloride ions that are passively „dragged“ along by the positive electrical charges of the sodium ion. However, other sodium ions are absorbed while either potassium or hydrogen ions are transported into the lumen of the gut in exchange for the sodium ions. In the membrane of the brush border are special transport proteins that facilitate these exchanges between sodium and potassium or sodium and hydrogen.
The active transport of sodium reduces its concentration in the cell to a low value (about 50 mEq/liter). Since the sodium concentration in the chyme is normally about 142 mEq/liter (that is, approximately equal to that in the plasma), sodium moves by passive absorption from the chyme through the brush border of the epithelial cell into the epithelial cell cytoplasm. This replaces the sodium that is actively transported out of the epithelial cells into the intercellular spaces.
The next step in the transport process is osmosis of water into the intercellular spaces. This movement is caused by the osmotic gradient created by the elevated concentration of ions in the intercellular space. Most of this osmosis occurs through the „tight junctions“ between the apical borders of the epithelial cells, as discussed earlier, but a smaller proportion occurs through the cells themselves. The osmotic movement of water creates a flow of fluid into the intercellular space, then through the basement membrane of the epithelium, and finally into the circulating blood of the villi.
Absorption of Chloride Ions in the Duodenum and Jejunum. In the upper part of the small intestine chloride absorption is mainly by passive diffusion. The absorption of sodium ions through the epithelium creates electronegativity in the chyme and electropositivity on the basal side of the epithelial cells. Then chloride ions move along this electrical gradient to „follow“ the sodium ions.
„Active“ Absorption of Bicarbonate Ions in the Duodenum and Jejunum. Often, large quantities of bicarbonate ions must be reabsorbed from the upper small intestine because of the large amounts of bicarbonate ions in both the pancreatic secretion and bile. However, the bicarbonate ion is absorbed in an indirect way as follows: When sodium ions are absorbed, moderate amounts of hydrogen ions are secreted into the lumen of the gut in exchange for some of the sodium, as explained earlier. These hydrogen ions in turn combine with the bicarbonate ion to form carbonic acid (H2CO3), and this then dissociates to form H2O and CO3. The water remains part of the chyme in the intestines, but the carbon dioxide is readily absorbed into the blood and subsequently expired through the lungs. Thus, this is the so-called „active“ absorption of bicarbonate ions. It is the same mechanism that occurs in the tubules of the kidneys.
Active Absorption of Chloride Ions and Active Secretion of Bicarbonate Ions in the Ileum and Large Intestine. The epithelial cells of the ileum and of the large intestine have the special capability of actively absorbing chloride ions by means of a tightly coupled transport mechanism in which an equivalent number of bicarbonate ions are secreted. The functional role of this mechanism is to provide bicarbonate ions for neutralization of acidic products formed by bacteria – especially in the large intestine.
Various bacterial toxins, particularly those of cholera, colon bacilli, and staphylococci, can strongly stimulate this chloride-bicarbonate exchange mechanism.
Absorption of Other Ions. Calcium ions are actively absorbed, especially from the duodenum, and calcium ion absorption is exactly controlled in relation to the need of the body for calcium. One important factor controlling calcium absorption is parathyroid hormone secreted by the parathyroid glands, and another is vitamin D. The parathyroid hormone activates vitamin D in the kidneys, and the activated vitamin D in turn greatly enhances calcium absorption.
Iron ions are also actively absorbed from the small intestine. The principles of iron absorption and the regulation of its absorption in proportion to the body’s need for iron.
Potassium, magnesium, phosphate, and probably still other ions can also be actively absorbed through the mucosa. In general, the monovalent ions are absorbed with ease and in great quantities. On the other hand, the bivalent ions are normally absorbed in only small amounts; for instance, the maximum absorption of calcium ions is only 1/50 as great as the normal absorption of sodium ions. Fortunately, only small quantities of the divalent ions are normally needed by the body.
b) Products of proteins hydrolyses
Absorption of Proteins
Most proteins are absorbed in the form of amino acids. However, small quantities of dipeptides and even tripeptides are also absorbed, and extremely minute quantities of whole proteins can at times be absorbed by the process of pinocytosis, though not by the usual absorptive mechanisms.
The absorption of amino acids also obeys the principles listed above for active absorption of glucose; that is, the different types of amino acids are absorbed selectively and certain ones interfere with the absorption of others, illustrating that common carrier systems exist. Finally, metabolic poisons block the absorption of amino acids in the same way that they block the absorption of glucose.
Absorption of amino acids through the intestinal mucosa can occur far more rapidly than can protein digestion in the lumen of the intestine. As a result, the normal rate of absorption is determined not by the rate at which they can be absorbed but by the rate at which they can be released from the proteins during digestion. For these reasons, essentially no free ammo acids can be found in the intestine during digestion – that is, they are absorbed as rapidly as they are formed. Since most protein digestion occurs in the upper small intestine, most protein absorption occurs in the duodenum and jejunum.
Basic Mechanisms of Amino Acid Transport. As is true for monosaccharide absorption, very little is known about the basic mechanisms of amino acid transport. However, at least four different carrier systems transport different amino acids – one transports neutral amino acids, a second transports basic amino acids, a third transports acidic amino acids, and a fourth has specificity for the two imino acids proline and hydroxyproline. Also, the transport mechanisms have far greater affinity for transporting L-stereoisomers of amino acids than D-stereoisomers.
Amino acid transport (at least for most of the amino acids), like glucose transport, occurs only in the presence of simultaneous sodium transport. Furthermore, the carrier systems for amino acid transport, like those for glucose transport, are in the brush border of the epithelial cell. It is believed that amino acids are transported by the same sodium cotransport mechanism as that explained above for glucose transport. That is, the theory postulates that the carrier has receptor sites for both an amino acid molecule and a sodium ion. Only when both of the sites are filled will the carrier move both the sodium and the amino acid to the interior of the cell at the same time. Because of the sodium gradient across the brush border, the sodium diffusion to the cell interior pulls the amino acid to the interior where the amino acid becomes trapped. Therefore, amino acid concentration increases within the cell, and it then diffuses through the sides or base of the cell into the portal blood, probably by a facilitated diffusion process.
c)Products of carbohydrates hydrolyses
Essentially all the carbohydrates are absorbed in the form of monosaccharides, only a small fraction of a per cent being absorbed as disaccharides and almost none as larger carbohydrate compounds. Furthermore, little carbohydrate absorption results from simple diffusion, for the pores of the mucosa through which diffusion occurs are essentially impermeable to water-soluble solutes with molecular weights greater than 100.
That the transport of most monosaccharides through the intestinal membrane is an active process is demonstrated by several important experimental observations:
1. Transport of most of them, especially glucose and galactose, can be blocked by metabolic inhibitors, such as iodoacetic acid, cyanides, and phlorhizin.
2. The transport is selective, specifically transporting certain monosaccharides without transporting others. The order of preference for transporting different monosaccharides and their relative rates of transport in comparison with glucose are:
3. There is a maximum rate of transport for each type of monosaccharide. The most rapidly transported monosaccharide is galactose, with glucose running a close second. Fructose, which is also one of the three important monosaccharides for nutrition, is absorbed less than half as rapidly as either galactose or glucose; also, its mechanism of absorption is different, as will be explained below.
4. There is competition between certain sugars for the respective carrier system. For instance, if large amounts of galactose are being transported, the amount of glucose that can be transported simultaneously is considerably reduced.
Mechanism of Glucose and Galactose Absorption.
Glucose and galactose transport either ceases or is greatly reduced wherever active sodium transport is blocked. Therefore, it is assumed that the energy required for transport of these two monosaccharides is actually provided by the sodium transport system. A theory that attempts to explain this is the following: It is known that the carrier protein for transport of glucose (which is the carrier for galactose as well) is present in the brush border of the epithelial cell. However, this carrier will not transport the glucose in the absence of sodium transport. Therefore, it is believed that the carrier protein has receptor sites for both a glucose molecule and a sodium ion, and that it will not transport either of these to the interior of the epithelial cell until both receptor sites are simultaneously filled. The energy to cause movement of the carrier from the exterior of the membrane to the interior is derived from the difference in sodium concentration between the outside and inside. That is, as sodium diffuses to the inside of the cell it „drags“ the glucose along with it, thus providing the energy for transport of the glucose. For obvious reasons, this explanation is called the sodium cotransport theory for glucose transport; it is also called secondary active transport of glucose. This sodium cotransport of glucose obviously moves the glucose only to the interior of the cell. However, this increases the intracellular glucose concentration to a higher thaormal level, and the glucose then diffuses, probably by facilitated diffusion, through the basolateral membrane of the epithelial cell into the extracellular fluid.
Subsequently, we will see that sodium transport is also required for transport of many if not all amino acids, suggesting a similar „carrier-drag“ mechanism for ammo acid transport.
Absorption of Fructose.
Transport of fructose is slightly different from that of most other monosaccharides. It is not blocked by some of the same metabolic poisons – specifically, phlorhizin – and it does not require metabolic energy for transport, even though it does require a specific carrier. Therefore, it is transported by facilitated diffusion rather than active transport. Also, it is mainly converted into glucose inside the epithelial cell before entering the portal blood, the fructose first becoming phosphorylated, then converted to glucose, and finally released from the epithelial cell into the blood.
Products of fats hydrolyses
As fats are digested to form monoglycerides and free fatty acids, both of these digestive end-products become dissolved in the lipid portion of the bile acid micelles. Because of the molecular dimensions of these micelles, only 2,5 nanometers in diameter, and also because of their highly charged exterior, they are soluble in the chyme. In this form the monoglycerides and the fatty acids are transported to the surfaces of the brush border microvilli, even penetrating into the recesses among the moving, agitating microvilli. On coming in contact with these surfaces, both the monoglycerides and the fatty acids immediately diffuse through the epithelial membrane, because they are equally as soluble in this membrane as in the micelles. This leaves the bile acid micelles still in the chyme. The micelles then diffuse back through the chyme and absorb still more monoglycerides and fatty acids, and similarly transport these also to the epithelial cells. Thus, the bile acids perform a „ferrying“ function, which is highly important for fat absorption. In the presence of an abundance of bile acids, approximately 97 per cent of the fat is absorbed; in the absence of bile acids, only 50 to 60 per cent is normally absorbed.
The mechanism for absorption of the monoglycerides and fatty acids through the brush border is based entirely on the fact that both these substances are highly lipid-soluble. Therefore, they become dissolved in the membrane and simply diffuse to the interior of the cell.
The undigested triglycerides and the diglycerides are both also highly soluble in the lipid membrane of the epithelial cell. However, only small quantities of these are normally absorbed because the bile acid micelles will not dissolve either triglycerides or diglycerides and therefore will not ferry them to the epithelial membrane.
After entering the epithelial cell, the fatty acids and monoglycerides are taken up by the smooth endoplasmic reticulum, and here they are mainly recombined to form new triglycerides. However, a few of the monoglycerides are further digested into glycerol and fatty acids by an epithelial cell lipase. Then, the free fatty acids are reconstituted by the smooth endoplasmic reticulum into triglycerides. Most of the glycerol that is utilized for this purpose is synthesized de novo from alpha-glycerophosphate, this synthesis requiring both energy from ATP and a complex of enzymes to catalyze the reactions.
Once formed, the triglycerides aggregate within the endoplasmic reticulum into globules along with absorbed cholesterol, absorbed phospholipids, and small amounts of newly synthesized cholesterol and phospholipids. The phospholipids arrange themselves in these globules with the fatty portion of the phospholipid toward the center and the polar portions located on the surface. This provides an electrically charged surface that makes these globules miscible with the fluids of the cell. In addition, small amounts of beta-lipoprotein, also synthesized by the endoplasmic reticulum, coat part of the surface of each globule. In this form the globule diffuses to the side of the epithelial cell and is excreted by the process of cellular exocytosis into the space between the cells; from there it passes into the lymph in the central lacteal of the villus. These globules are then called chylomicrons.
The beta-lipoprotein is essential for cellular exocytosis of the chylomicrons to occur, because this protein provides a means for attaching the fatty globule to the cell membrane before it is extruded. In persons who have a genetic inability to form this (3-lipoprotein, the epithelial cells become engorged with fatty products that cannot proceed the rest of the way to be absorbed.
Transport of the Chylomicrons in the Lymph.
From the sides of the epithelial cells the chylomicrons wend their way into the central lacteals of the villi and from here are propelled, along with the lymph, by the lymphatic pump upward through the thoracic duct to be emptied into the great veins of the neck. Between 80 and 90 per cent of all fat absorbed from the gut is absorbed in this manner and is transported to the blood by way of the thoracic lymph in the form of chylomicrons.
Direct Absorption of Fatty Acids into the Portal Blood.
Small quantities of short chain fatty acids, such as those from butterfat, are absorbed directly into the portal blood rather than being converted into triglycerides and absorbed into the lymphatics. The cause of this difference between short and long chain fatty acid absorption is that the shorter chain fatty acids are more water-soluble and are not reconverted into triglycerides by the endoplasmic reticulum. This allows direct diffusion of these fatty acids from the epithelial cells into the capillary blood of the villus.
FUNCTIONAL TYPES OF MOVEMENTS IN THE GASTROINTESTINAL TRACT.
Two basic types of movements occur in the gastrointestinal tract: (1) mixing movements, which keep the intestinal contents thoroughly mixed at all times, and (2) propulsive movements, which cause food to move forward along the tract at an appropriate rate for digestion and absorption.
The mixing movements in most parts of the alimentary tract are caused by either peristaltic contractions or local constrictive contractions of small segments of the gut wall. These movements are modified in different parts of the gastrointestinal tract for proper performance of the respective activities of each part.
The basic propulsive movement of the gastrointestinal tract is peristalsis. A contractile ring appears around the gut and then moves forward; this is analogous to putting one’s fingers around a thin distended tube, then constricting the fingers and moving forward along the tube. Obviously, any material in front of the contractile ring is moved forward.
The usual stimulus for peristalsis is distension. That is, if a large amount of food collects at any point in the gut, the distension stimulates the gut wall 2 to
Function of the Myenteric Plexus in Peristalsis. Peristalsis occurs only weakly, if at all, in portions of the gastrointestinal tract that have congenital absence of the my enteric plexus. It is greatly depressed or completely blocked in the entire gut when the person is treated with atropme to paralyze the cholinergic nerve endings of the myenteric plexus. Therefore, effectual peristalsis requires an active myenteric plexus.
Analward Peristaltic Movements. Peristalsis, theoretically, can occur in either direction from a stimulated point, but it normally dies out rapidly in the orad direction while continuing for a considerable distance analward. The exact cause of this directional transmission of peristalsis has never been ascertained, though it probably results mainly from the fact that the myenteric plexus itself is “polarized” in the anal direction, which can be explained as follows:
The Peristaltic Reflex and the „Law of the Gut“. When a segment of the intestinal tract is excited by distension and thereby initiates peristalsis, the contractile ring for causing the peristalsis begins slightly on the oral side of the distended segment; then it moves toward the distended segment, thus pushing the intestinal contents in the anal direction. At the same time, the gut sometimes relaxes several centimeters downstream toward the anus, which is called „receptive relaxation“, thus allowing the food to be propelled more easily analward than in the oral direction.
This complex pattern, consisting of contraction of the gut above the point of distension and relaxation below, does not occur in the absence of the myenteric plexus. Therefore, the complex is frequently called the myenteric reflex, or it is also called simply the peristaltic reflex. And the peristaltic reflex plus the analward direction of movement of the peristalsis is called the „law of the gut“.
Peristalsis In the Fasting Human Being – The Migrating Myoelectric Motor Complex
After a person eats a meal, the nature of the gastrointestinal motor functions is determined mainly by the stimulating effects of the food in the gastrointestinal tract itself. Late at night, many hours after a meal, or at other times when the human being is fasting, a distinct pattern of activity occurs in the stomach and small intestine called the migrating myoelectric motor complex. The most important feature of this migrating complex is that it causes peristaltic waves to sweep slowly and rhythmically downward along the stomach and small intestine approximately once every two hours, sweeping the excess digestive secretions into the colon and therefore preventing their accumulation in the upper gastrointestinal tract.
The migrating complex begins in the body of the stomach and spreads all the way through the ileum. At any one time only about
The active phase of the migrating complex in any given segment of the intestine usually lasts for 10 to 15 minutes. During this time, about 50 peristaltic waves move through this portion of the intestine, that is, through this distance of about
INGESTION OF FOOD (actual mechanical aspects of food ingestion including mastication and swallowing).
MASTICATION (CHEWING).
The teeth are admirably designed for chewing, the anterior teeth (incisors) providing a strong cutting action and the posterior teeth (molars) a grinding action. All the jaw muscles working together can close the teeth with a force as great as
Most of the muscles of chewing are innervated by the motor branch of the 5th cranial nerve, and the chewing process is controlled by nuclei in the hindbrain. Stimulation of the reticular formation near the hindbrain centers for taste can cause continual rhythmic chewing movements. Also, stimulation of areas in the hypothalamus, amygdala, and even in the cerebral cortex near the sensory areas for taste and smell can cause chewing.
Much of the chewing process is caused by the chewing reflex, which may be explained as follows: The presence of a bolus of food in the mouth causes reflex inhibition of the muscles of mastication, which allows the lower jaw to drop. The drop in turn initiates a stretch reflex of the jaw muscles that leads to rebound contraction. This automatically raises the jaw to cause closure of the teeth, but it also compresses the bolus again against the linings of the mouth, which inhibits the jaw muscles once again, allowing the jaw to drop and rebound another time, and this is repeated again and again.
Chewing of the food is important for digestion of all foods, but it is especially important for most fruits and raw vegetables, because these have undigestible cellulose membranes around their nutrient portions which must be broken before the food can be utilized. Chewing aids in the digestion of food for the following simple reason: Since the digestive enzymes act only on the surfaces of food particles, the rate of digestion is highly dependent on the total surface area exposed to the intestinal secretions. Also, grinding the food to a very fine particulate consistency prevents excoriation of the gastrointestinal tract and increases the ease with which food is emptied from the stomach into the small intestine and thence into all succeeding segments of the gut.
SWALLOWING (DEGLUTITION). Swallowing is a complicated mechanism, principally because the pharynx most of the time subserves several other functions besides swallowing and is converted for only a few seconds at a time into a tract for propulsion of food. Especially is it important that respiratioot be seriously compromised during swallowing.
In general, swallowing can be divided into (1) the voluntary stage, which initiates the swallowing process, (2) the pharyngeal stage, which is involuntary and constitutes the passage of food through the pharynx into the esophagus, and (3) the esophageal stage, another involuntary phase which promotes passage of food from the pharynx to the stomach.
Voluntary Stage of Swallowing. When the food is ready for swallowing, it is „voluntarily“ squeezed or rolled posteriorly in the mouth by pressure of the tongue upward and backward against the palate. Thus, the tongue forces the bolus of food into the pharynx. From here on, the process of swallowing becomes entirely, or almost entirely, automatic and ordinarily cannot be stopped.
Pharyngeal Stage of Swallowing. When the bolus of food is pushed backward in the mouth, it stimulates swallowing receptor areas all around the opening of the pharynx, especially on the tonsillar pillars, and impulses from these pass to the brain stem to initiate a series of automatic pharyngeal muscular contractions as follows:
1. The soft palate is pulled upward to close the posterior nares, in this way preventing reflux of food into the nasal cavities.
2. The palatopharyngeal folds on either side of the pharynx are pulled medialward to approximate each other. In this way these folds form a sagittal slit through which the food must pass into the posterior pharynx. This slit performs a selective action, allowing food that has been masticated properly to pass with ease while impeding the passage of large objects. Since this stage of swallowing lasts less than 1 second, any large object is usually impeded too much to pass through the pharynx into the esophagus.
3. The vocal cords of the larynx are strongly approximated, and the hyoid bone and larynx are pulled upward and anteriorly by the neck muscles, causing the epiglottis to swing backward over the superior opening of the larynx. Both these effects prevent passage of food into the trachea. Especially important is the approximation of the vocal cords, but the epiglottis helps to prevent food from ever getting as far as the vocal cords. Destruction of the vocal cords or of the muscles that approximate them can cause strangulation. On the other hand, removal of the epiglottis usually does not cause serious debility in swallowing.
4. The upward movement of the larynx also stretches the opening of the esophagus. At the same time, the upper 3 to
5. At the same time that the larynx is raised and the pharyngoesophageal sphincter is relaxed, the superior constrictor muscle of the pharynx contracts, giving rise to a rapid peristaltic wave passing downward over the middle and inferior pharyngeal muscles and into the esophagus, which also propels the food into the esophagus.
To summarize the mechanics of the pharyngeal stage of swallowing – the trachea is closed, the esophagus is opened, and a fast peristaltic wave originating in the pharynx then forces the bolus of food into the upper esophagus, the entire process occurring in 1 to 2 seconds.
Nervous Control of the Pharyngeal Stage of Swallowing. The most sensitive tactile areas of the pharynx for initiation of the pharyngeal stage of swallowing lie in a ring around the pharyngeal opening, with greatest sensitivity in the tonsillar pillars. Impulses are transmitted from these areas through the sensory portions of the trigeminal and glossopharyngeal nerves into a region of the medulla oblongata closely associated with the tractus solitarius which receives essentially all sensory impulses from the mouth.
The successive stages of the swallowing process are then automatically controlled in orderly sequence by neuronal areas distributed throughout the reticular substance of the medulla and lower portion of the pons. The sequence of the swallowing reflex is the same from one swallow to the next, and the timing of the entire cycle also remains constant from one swallow to the next. The areas in the medulla and lower pons that control swallowing are collectively called the deglutition or swallowing center.
The motor impulses from the swallowing center to the pharynx and upper esophagus that cause swallowing are transmitted by the 5th, 9th, 10th, and 12th cranial nerves and even a few of the superior cervical nerves.
In summary, the pharyngeal stage of swallowing is principally a reflex act. It is almost never initiated by direct stimuli to the swallowing center from higher regions of the central nervous system. Instead, it is almost always initiated by voluntary movement of food into the back of the mouth, which, in turn, elicits the swallowing reflex.
Effect of the Pharyngeal Stage of Swallowing on Respiration. The entire pharyngeal stage of swallowing occurs in less than 1 to 2 seconds, thereby interrupting respiration for only a fraction of a usual respiratory cycle. The swallowing center specifically inhibits the respiratory center of the medulla during this time, halting respiration at any point in its cycle to allow swallowing to proceed. Yet, even while a person is talking, swallowing interrupts respiration for such a short time that it is hardly noticeable.
Esophageal Stage of Swallowing. The esophagus functions primarily to conduct food from the pharynx to the stomach, and its movements are organized specifically for this function.
Normally the esophagus exhibits two types of peristaltic movements – primary peristalsis and secondary peristalsis. Primary peristalsis is simply a continuation of the peristaltic wave that begins in the pharynx and spreads into the esophagus during the pharyngeal stage of swallowing. This wave passes all the way from the pharynx to the stomach in approximately 8 to 10 seconds. However, food swallowed by a person who is in the upright position is usually transmitted to the lower end of the esophagus even more rapidly than the peristaltic wave itself, in about 5 to 8 seconds, because of the additional effect of gravity pulling the food downward. If the primary peristaltic wave fails to move all the food that has entered the esophagus into the stomach, secondary peristaltic waves, generated by the enteric nervous system of the esophagus, result from distension of the esophagus by the retained food. These waves are essentially the same as the primary peristaltic waves, except that they originate in the esophagus itself rather than in the pharynx. Secondary peristaltic waves continue to be initiated until all the food has emptied into the stomach.
The peristaltic waves of the esophagus are initiated by vagal reflexes that are part of the overall swallowing mechanism. These reflexes are transmitted through vagal afferent fibers from the esophagus to the medulla and then back again to the esophagus through vagal afferent fibers.
The musculature of the pharynx and the upper quarter of the esophagus is striated muscle, and, therefore, the peristaltic waves in these regions are controlled only by skeletal nerve impulses in the glossopharyngeal and vagus nerves. In the lower two thirds of the esophagus, the musculature is smooth, but this portion of the esophagus is also strongly controlled by the vagus nerves acting through their connections with the enteric nervous system. However, when the vagus nerves to the esophagus are sectioned, the myenteric nerve plexus of the esophagus becomes excitable enough after several days to cause secondary peristaltic waves even without support from the vagal reflexes. Therefore, following paralysis of the swallowing reflex, food forced into the upper esophagus and then pulled by gravity to the lower esophagus still passes readily into the stomach.
Receptive Relaxation of the Stomach. As the esophageal peristaltic wave passes toward the stomach, a wave of relaxation, transmitted through myenteric inhibitory neurons, precedes the constriction. Furthermore, the entire stomach and, to a lesser extent, even the duodenum become relaxed as this wave reaches the lower end of the esophagus. Especially important, also, is relaxation of the gastroesophageal sphincter at the juncture between the esophagus and the stomach. In other words, the constrictor and the stomach are prepared ahead of time to receive food being propelled down the esophagus during the swallowing act.
FUNCTION OF THE LOWER ESOPHAGEAL SPHINCTER (GASTROESOPHAGEAL SPHINCTER)
At the lower end of the esophagus, extending from about 2 to
A principal function of the lower esophageal sphincter is to prevent reflux of stomach contents into the esophagus. The stomach contents are highly acidic and contain many proteolytic enzymes The esophageal mucosa, except in the lower eighth of the esophagus, is not capable of resisting for long the digestive action of gastric secretions. Fortunately, the tonic constriction of the lower esophageal sphincter prevents significant reflux of stomach contents into the esophagus except under abnormal conditions. Indeed, increased intragastric pressure, except during retching or vomiting, causes a vagal reflex that further constricts the sphincter to add extra insurance against reflux.
Prevention of Reflux by Flutter-Valve Closure of the Distal End of the Esophagus. Another factor that prevents reflux is a valvelike mechanism of that portion of the esophagus that lies immediately beneath the diaphragm. Greatly increased intra-abdommal pressure caves the esophagus inward at this point at the same time that the abdominal pressure also increases the mtragastnc pressure. This flutter-valve closure of the lower esophagus therefore prevents the high pressure in the stomach from forcing stomach contents into the esophagus. Otherwise, every time we should walk, cough, or breath hard, we might expel acid into the esophagus.
MOTOR FUNCTIONS OF THE STOMACH
The motor functions of the stomach are three-fold. (1) storage of large quantities of food until it can be accommodated in the lower portion of the gastrointestmal tract, (2) mixing of this food with gastric secretions until it forms a semifluid mixture called chyme, and (3) slow emptying of the food from the stomach into the small intestine at a rate suitable for proper digestion and absorption by the small intestine.
Physiologically, the stomach can be divided into two major parts: (1) the corpus, or body, and (2) the antrum. The fundus, located at the upper end of the body of the stomach, is considered by some anatomists to be a separate entity from the body, but physiologically the fundus functions mainly as part of the body.
STORAGE FUNCTION OF THE STOMACH
As food enters the stomach, it forms concentric circles in the body and fundus of the stomach, the newest food lying closest to the esophageal opening and the oldest food lying nearest the wall of the stomach. Normally, when food enters the stomach, a vagal reflex greatly reduces the tone in the muscular wall of the body of the stomach so that the wall can bulge progressively outward, accommodating greater and greater quantities of food up to a limit of about
EMPTYING OF THE STOMACH
Basically, stomach emptying is opposed by resistance of the pylorus to the passage of food, and it is promoted by peristaltic waves in the antrum of the stomach.
Role of the Pylorus in Stomach Emptying.
The pylorus normally remains almost, but not completely, closed because of tonic contraction of the pyloric muscle. The closing force is weak enough that water and other fluids empty from the stomach with ease. On the other hand, it is great enough to prevent movement of semi-solid chyme into the duodenum except when a strong antral peristaltic wave forces the chyme through. However, the degree of constriction of the pyloric sphincter can increase or decrease under the influence of signals both from the stomach and from the duodenum, as we shall discuss subsequently. In this way, the pylorus plays an important role in the control of stomach emptying.
Role of Antral Peristalsis in Stomach Emptying – The Pyloric Pump. The intensity of antral peristalsis changes markedly under different conditions, especially in response to signals both from the stomach and from the duodenum. Most of the time the antral peristaltic contractions are weak and function mainly to cause mixing of the food and gastric secretions, thus increasing the fluidity of the chyme. However, about 20 per cent of the time while food is in the stomach, these peristaltic contractions become very intense at the incisura angularis of the stomach and spread through the antrum no longer as weak mixing waves, but instead as strong peristaltic, ringlike constrictions. As the stomach becomes progressively more and more empty, these constrictions begin farther and farther up the body of the stomach, gradually pinching off the lowermost portions of the stored food and adding this food to the chyme in the antrum. These intense peristaltic waves often create as much as 50 to
When pyloric tone is normal, each strong antral peristaltic wave forces several milliliters of chyme into the duodenum. Thus, the peristaltic waves provide a pumping action that is frequently called the „pyloric pump“.
Regulation of Stomach Emptying. The rate at which the stomach empties is regulated by signals both from the stomach and from the duodenum. The stomach signals are mainly twofold: (1) nervous signals caused by distension of the stomach by food, and (2) the hormone gastrin released from the antral mucosa in response to the presence of certain types of food in the stomach. Both these signals increase pyloric pumping force and at the same time inhibit the pylorus, thus promoting stomach emptying.
On the other hand, signals from the duodenum depress the pyloric pump and usually increase pyloric tone at the same time. In general, when an excess volume of chyme or excesses of certain types of chyme enter the duodenum, strong negative feedback signals, both nervous and hormonal, depress the pyloric pump and enhance pyloric sphincter tone. Obviously, these feedback signals allow chyme to enter the duodenum only as rapidly as it can be processed by the small intestine.
Effect of Gastric Food Volume on Rate of Emptying. It is very easy to see how increased food volume in the stomach could promote increased emptying from the stomach. However, this increased emptying does not occur for the reasons that one would expect. It is not increased pressure in the stomach that causes the increased emptying because, in the usual normal range of volume, the increase in volume does not increase the pressure significantly. On the other hand, stretch of the stomach wall does elicit vagal and local myenteric reflexes in the wall that increase the activity of the pyloric pump and at the same time inhibit the pylorus. In general, the rate of food emptying from the stomach is approximately proportional to the square root of the volume of food remaining in the stomach at any given time.
Effect of the Hormone Gastrin on Stomach Emptying. In the following chapter we shall see that stretch, as well as the presence of certain types of foods in the stomach –particularly meat – elicits release of a hormone called gastrin from the antral mucosa, and this has potent effects on causing secretion of highly acidic gastric juice by the stomach fundic glands. However, gastrin also has moderate stimulatory effects on motor functions of the stomach as well. Most important, it enhances the activity of the pyloric pump while at the same time relaxing the pylorus. Thus, it is an important influence for promoting stomach emptying. It also has a slight constrictor effect on the gastroesophageal sphincter at the lower end of the esophagus for preventing reflux of gastric contents into the esophagus during the enhanced gastric activity.
The Inhibitory Effect of the Enterogastric Reflex from the Duodenum on Pyloric Activity. Reflex nervous signals are transmitted from the duodenum back to the stomach most of the time when the stomach is emptying food into the duodenum These signals play an especially important role in controlling both the pyloric pump and the pylorus and, therefore, also in determining the rate of emptying of the stomach. The nervous reflexes are mediated partly through the extrinsic nerves, some going to the prevertebral sympathetic ganglia and then returning through inhibitory sympathetic nerve fibers to the stomach, and others transmitted in the vagus nerves to the brain stem and then back through the same vagi to the stomach. However, local signals are probably also transmitted from the upper portion of the duodenum to the pylorus directly through the enteric nervous system within the gut wall itself.
The types of factors that are continually monitored in the duodenum and that can elicit the enterogastric reflex include:
1 The degree of distension of the duodenum.
2. The presence of any degree of irritation of the duodenal mucosa.
3. The degree of acidity of the duodenal chyme.
4. The degree ofosmolality of the chyme.
5. The presence of certain breakdown products in the chyme, especially breakdown products of proteins and perhaps to a lesser extent of fats.
The enterogastric reflex is especially sensitive to the presence of irritants and acids in the duodenal chyme. For instance, whenever the pH of the chyme in the duodenum falls below approximately 3.5 to 4, this reflex is immediately elicited, which inhibits the pyloric pump and increases pyloric constriction, thus reducing or even blocking further release of acidic stomach contents into the duodenum until the duodenal chyme can be neutralized by pancreatic and other secretions.
Breakdown products of protein digestion will also elicit this reflex; by slowing the rate of stomach emptying, sufficient time is insured for adequate protein digestion in the upper portion of the small intestine.
Finally, either hypo- or hypertonic fluids (especially hypertonic) will elicit the enterogastric reflex. This effect prevents too rapid flow of nonisotonic fluids into the small intestine, thereby preventing rapid changes in electrolyte balance of the body fluids during absorption of the intestinal contents.
Hormonal Feedback from the Duodenum in Inhibiting Gastric Emptying – Role of Fats. Not only do nervous reflexes from the duodenum to the stomach inhibit stomach emptying, but hormones released from the upper intestine do so as well. The stimulus for producing the hormones is mainly fats entering the duodenum, though other types of foods can stimulate the hormones to a lesser degree. On entering the duodenum, the fats extract several different hormones from the duodenal andjejunal epithelium, and these in turn are carried by way of the blood to the stomach where they (a) inhibit the activity of the pyloric pump and at the same time (b) increase slightly the strength of contraction of the pyloric sphincter. These effects are important because fats are much slower to be digested than are most other foods; this inhibitory feedback mechanism allows the necessary time before the fats pass deeper into the intestinal tract where they are to be absorbed.
Unfortunately, the precise hormones that cause the hormonal feedback inhibition of the stomach are not fully clear. In the past, this mixture of hormones has been called enterogastrone, but such a single hormone has never been identified as a specific entity. On the other hand, several different hormones released by the mucosa of the upper small intestine are known to inhibit stomach emptying. One of these is cholecystokinin, which is released from the mucosa of the jejunum in response to fatty substances in the chyme. This hormone acts as a competitive inhibitor to block the increased stomach motility caused by gastrin. Another is the hormone secretin, which is released mainly from the duodenal mucosa in response to gastric acid released from the stomach through the pylorus. This hormone has the general but only weak effect of decreasing gastrointestinal motility. Finally, a hormone called gastric inhibitory peptide, which is released from the upper small intestine in response mainly to fat in the chyme but to carbohydrates as well, is known also to inhibit gastric motility under some conditions. (However, its effect at physiological concentrations is probably mainly to stimulate the secretion of insulin by the pancreas.) All these hormones will be discussed at greater length elsewhere in this text, especially in the following chapter where both cholecystokinin and secretin will be discussed in detail.
In summary, several different hormones are known that could serve as hormonal mechanisms for inhibiting gastric emptying when excess quantities of chyme, especially acidic or fatty chyme, enter the duodenum from the stomach.
Summary of the Control of Stomach Emptying
Emptying of the stomach is controlled to a moderate degree by stomach factors, such as the degree of filling in the stomach and the excitatory effect of gastrin on antral peristalsis. However, probably the more important control of stomach emptying resides in feedback signals from the duodenum, including both the enterogastric feedback reflex and hormonal feedback. These two feedback inhibitory signals work together to slow the rate of emptying when (a) too much chyme is already in the small intestine or (b) the chyme is excessively acid, contains too much protein or fat, is hypotonic or hypertonic, or is irritating. In this way the rate of stomach emptying is limited to that amount of chyme that the small intestine can process.
MOVEMENTS OF THE SMALL INTESTINE
The movements of the small intestine, as elsewhere in the gastrointestinal tract, can be divided into the mixing contractions and the propulsive contractions. However, to a great extent this separation is artificial because essentially all movements of the small intestine cause at least some degree of both mixing and propulsion. Yet, the usual classification of these processes is the following:
MIXING CONTRACTIONS (SEGMENTATION CONTRACTIONS)
When a portion of the small intestine becomes distended with chyme, the stretch of the intestinal wall elicits localized concentric contractions spaced at intervals along the intestine. The longitudinal length of each one of the contractions is only about
The maximum frequency of the segmentation contractions in the small intestine is determined by the frequency of the slow waves in the intestinal wall, which is the basic electrical rhythm (BER) as explained earlier. Since this frequency is about 12 per minute in the duodenum, the maximum frequency of the segmentation contractions in the duodenum is also about 12 per minute. However, in the ileum, the maximum frequency is usually 8 to 9 contractions per minute.
The segmentation contractions are exceedingly weak when the excitatory activity of the enteric nervous system is blocked by atropine. Therefore, even though it is the slow waves in the smooth muscle itself that control the segmentation contractions, these contractions are not really effective without background excitation by the enteric nervous system, especially by the myenteric plexus.
PROPULSIVE MOVEMENTS. Peristalsis in the Small Intestine. Chyme is propelled through the small intestine by by peristaltic waves. These can occur in any part of the small intestine, and they move analward at a velocity of 0.5 to
Peristaltic activity of the small intestine is greatly increased after a meal This is caused partly by the beginning entry of chyme into the duodenum but also by the so-called gastroenteric reflex that is initiated by distension of the stomach and conducted principally through the myenteric plexus from the stomach down along the wall of the small intestine. This reflex increases the overall degree of excitability of the small intestine, including both increased motility and secretion.
The function of the peristaltic waves in the small intestine is not only to cause progression of the chyme toward the ileocecal valve but also to spread out the chyme along the intestinal mucosa. As the chyme enters the intestine from the stomach and causes initial distension of the proximal intestine, the elicited peristaltic waves begin immediately to spread the chyme along the intestine, and this process intensifies as additional chyme enters the intestine. On reaching the ileocecal valve the chyme is sometimes blocked for several hours until the person eats another meal, when a new gastroenteric (also called gastroileal) reflex intensifies the peristalsis in the ileum and forces the remaining chyme through the ileocecal valve into the cecum.
The Propulsive Effect of the Segmentation Movements. The segmentation movements, though they last for only a few seconds, also travel in the analward direction and help propel the food down the intestine. Therefore, the difference between the segmentation and the peristaltic movements is not as great as might be implied by their separation into these two classifications.
The Peristaltic Rush. Though peristalsis in the small intestine is normally very weak, intense irritation of the intestinal mucosa, as occurs in some severe cases of infectious diarrhea, can cause both very powerful and rapid peristalsis called the peristaltic rush. This is initiated mainly by extrinsic nervous reflexes to the brain stem and back again to the gut. The powerful peristaltic contractions then travel long distances in the small intestine within minutes, sweeping the contents of the intestine into the colon and thereby relieving the small intestine of either irritative chyme or excessive distension.
Movements Caused by the Muscularis Mucosae and Muscle Fibers of the Villi
The muscularis mucosae, which is stimulated by local nervous reflexes in the submucosal plexus, can cause short or long folds to appear in the intestinal mucosa and also cause the folds to move to progressively newer areas of mucosa. Individual fibers from this muscle extend into the intestinal villi and cause them to contract intermittently. The mucosal folds increase the surface area exposed to the chyme, thereby increasing the rate of absorption. The contractions of the villi – shortening, elongating, and shortening again – „milk“ the villi so that lymph flows freely from the central lacteals into the lymphatic system. Both these types of contraction also agitate the fluids surrounding the villi so that progressively new areas of fluid become exposed to absorption.
These mucosal and villous contractions are initiated by local nervous reflexes that occur in response to chyme in the small intestine.
FUNCTION OF THE ILEOCECAL VALVE
A principal function of the ileocecal valve is to prevent backflow of fecal contents from the colon into the small intestine. The lips of the ileocecal valve protrude into the lumen of the cecum and therefore are forcefully closed when the cecum fills. Usually the valve can resist reverse pressure of as much as 50 to
The wall of the ileum for several centimeters immediately preceding the ileocecal valve has a thickened muscular coat called the ileocecal sphincter. This normally remains mildly constricted and slows the emptying of ileal contents into the cecum except immediately following a meal when a gastroileal reflex (described earlier) intensifies the peristalsis in the ileum. Also, the hormone gastrin, which is liberated from the stomach mucosa in response to food in the stomach, has a relaxant effect on the ileocecal sphincter, thus allowing increased emptying. Even so, only about 1500 ml of chyme empty into the cecum each day. The resistance to emptying at the ileocecal valve prolongs the stay of chyme in the ileum and, therefore, facilitates absorption.
Feedback Control of the Ileocecal Sphincter. The degree of contraction of the ileocecal sphincter, as well as the intensity of peristalsis in the terminal ileum, is also controlled strongly by reflexes from the cecum. Whenever the cecum is distended, the degree of contraction of the ileocecal sphincter is intensified while ileal peristalsis is inhibited, which greatly delays emptying of additional chyme from the ileum. Also, any irritant in the cecum delays emptying. For instance, when a person has an inflamed appendix, the irritation of this vestigial remnant of the cecum can cause such intense spasm of the ileocecal sphincter and paralysis of the ileum that this completely blocks emptying of the ileum. These reflexes from the cecum to the ileocecal sphincter and ileum are mediated both by way of the myenteric plexus and through extrinsic nerves, especially reflexes by way of the prevertebral sympathetic ganglia.
MOVEMENTS OF THE
OTHER AUTONOMIC REFLEXES AFFECTING BOWEL ACTIVITY
Aside from the duodenocolic, gastrocolic, gastroileal, enterogastric, and defecation reflexes, several other important nervous reflexes can affect the overall degree of bowel activity. These are the peritoneointestinal reflex, reno-intestinal reflex, vesicointestinal reflex, and somato-intestinal reflex. All these reflexes are initiated by sensory signals that pass to the prevertebral sympathetic ganglia or to the spinal cord and then are transmitted through the sympathetic nervous system back to the gut. And they all inhibit gastrointestinal activity.
The peritoneointestinal reflex results from irritation of the peritoneum, it strongly inhibits the excitatory enteric nerves and thereby causes intestinal paralysis. The renointestinal and vesicointestinal reflexes inhibit intestinal activity as a result of kidney or bladder irritation. Finally, the somato-intestinal reflex causes intestinal inhibition when the skin over the abdomen is irritatingly stimulated.
References:
1. Review of Medical Physiology // W.F.Ganong. – 24th edition, 2012.
2. Textbook of Medical Physiology // A.C.Guyton, J.E.Hall. – Eleventh edition, 2005.
3. Physiology // V.M.Moroz, O.A. Shandra. – Vinnytsia. – Nova khyha Publishers, 2011
