1. PHYSIOLOGY
OF DIGESTION.
2. Digestion
in the oral cavity, STOMACH.
Digestion in intestine and
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.
Schematic drawing of bacterial conjugation. 1- Donor cell
produces pilus. 2- Pilus attaches to recipient cell, bringing the two cells
together. 3- The mobile plasmid is nicked and a single strand of DNA is
transferred to the recipient cell. 4- Both cells recircularize their plasmids,
synthesize second strands, and reproduce pili; both cells are now viable
donors.
Secretion systems
Main article: Secretion
Bacteria use several systems to obtain nutrients
from other organisms in the environments.
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.
Release of outer membrane vesicles
In addition to the use of the multiprotein
complexes listed above, Gram-negative bacteria possess another method for
release of material: the formation of outer membrane vesicles.Portions of the
outer membrane pinch off, forming spherical structures made of a lipid bilayer
enclosing periplasmic materials. Vesicles from a number of bacterial species
have been found to contain virulence factors, some have immunomodulatory
effects, and some can directly adhere to and intoxicate host cells. While
release of vesicles has been demonstrated as a general response to stress
conditions, the process of loading cargo proteins seems to be selective.
Gastrovascular cavity
The gastrovascular cavity functions as a stomach
in both digestion and the distribution of nutrients to all parts of the body.
Extracellular digestion takes place within this central cavity, which is lined
with the gastrodermis, the internal layer of epithelium. This cavity has only
one opening to the outside that functions as both a mouth and an anus: waste
and undigested matter is excreted through the mouth/anus, which can be
described as an incomplete gut.
In a plant such as the Venus Flytrap that can
make its own food through photosynthesis, it does not eat and digest its prey
for the traditional objectives of harvesting energy and carbon, but mines prey
primarily for essential nutrients (nitrogen and phosphorus in particular) that
are in short supply in its boggy, acidic habitat.[10]
Trophozoites of Entamoeba histolytica with
ingested erythrocytes
Phagosome
A phagosome is a vacuole formed around a particle
absorbed by phagocytosis. The vacuole is formed by the fusion of the cell
membrane around the particle. A phagosome is a cellular compartment in which
pathogenic microorganisms can be killed and digested. Phagosomes fuse with
lysosomes in their maturation process, forming phagolysosomes. In humans,
Entamoeba histolytica can phagocytose red blood cells.
Specialised organs and behaviours
To aid in the digestion of their food animals
evolved organs such as beaks, tongues, teeth, a crop, gizzard, and others.
Tongue
The tongue is skeletal muscle on the floor of the
mouth that manipulates food for chewing (mastication) and swallowing
(deglutition). It is sensitive and kept moist by saliva. The underside of the
tongue is covered with a smooth mucous membrane. The tongue also has a touch
sense for locating and positioning food particles that require further chewing.
The tongue is utilized to roll food particles into a bolus before being
transported down the esophagus through peristalsis.
The sublingual region underneath the front of the
tongue is a location where the oral mucosa is very thin, and underlain by a
plexus of veins. This is an ideal location for introducing certain medications
to the body. The sublingual route takes advantage of the highly vascular
quality of the oral cavity, and allows for the speedy application of medication
into the cardiovascular system, bypassing the gastrointestinal tract.
The tongue is a muscular hydrostat on the floors
of the mouths of most vertebrates which manipulates food for mastication. It is
the primary organ of taste (gustation), as much of the upper surface of the
tongue is covered in papillae and taste buds. It is sensitive and kept moist by
saliva, and is richly supplied with nerves and blood vessels. In humans a
secondary function of the tongue is phonetic articulation. The tongue also
serves as a natural means of cleaning one's teeth. The ability to perceive
different tastes is not localised in different parts of the tongue, as is
widely believed. This error arose because of misinterpretation of some
19th-century research (see tongue map).
The word tongue derives from the Old English
tunge, which comes from Proto-Germanic tungōn.It has cognates in other
Germanic languages — for example tonge in West Frisian, tong in
Dutch/Afrikaans, Zunge in German, tunge in Danish/Norwegian and tunga in
Icelandic/Faroese/Swedish. The ue ending of the word seems to be a
fourteenth-century attempt to show "proper pronunciation", but it is
"neither etymological nor phonetic". Some used the spelling tunge and
tonge as late as the sixteenth century.
It can be used as a metonym for language, as in
the phrase mother tongue. Many languages have the same word for
"tongue" and "language".
Figures of speech
A common temporary failure in word retrieval from
memory is referred to as the tip-of-the-tongue phenomenon. The expression
tongue in cheek refers to a statement that is not to be taken entirely
seriously – something said or done with subtle ironic or sarcastic humour. A
tongue twister is a phrase made specifically to be very difficult to pronounce.
Aside from being a medical condition, "tongue-tied" means being
unable to say what you want to due to confusion or restriction. The phrase
"cat got your tongue" refers to when a person is speechless. To
"bite one's tongue" is a phrase which describes holding back an
opinion to avoid causing offence. A "slip of the tongue" refers to an
unintentional utterance, such as a Freudian slip. Speaking in tongues is a common
phrase used to describe glossolalia, which is to make smooth,
language-resembling sounds that is no true spoken language itself. A deceptive
person is said to have a forked tongue, and a smooth-talking person said to
have a silver tongue.
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.
The teeth of carnivores are shaped to kill and
tear meat, using specially shaped canine teeth. Herbivores' teeth are made for
grinding food materials, in this case, plant parts.
Crop
A crop, or croup, is a thin-walled expanded
portion of the alimentary tract used for the storage of food prior to
digestion. In some birds it is an expanded, muscular pouch near the gullet or
throat. In adult doves and pigeons, the crop can produce crop milk to feed
newly hatched birds.
Certain insects may have a crop or enlarged
esophagus.
Rough
illustration of a ruminant digestive system
Abomasum
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.
Swallowing transports the chewed food into the
esophagus, passing through the oropharynx and hypopharynx. The mechanism for
swallowing is coordinated by the swallowing center in the medulla oblongata and
pons. The reflex is initiated by touch receptors in the pharynx as the bolus of
food is pushed to the back of the mouth.
Coordination
and control
Eating and swallowing are complex neuromuscular activities
consisting essentially of three phases, an oral, pharyngeal and esophageal
phase. Each phase is controlled by a different neurological mechanism. The oral
phase, which is entirely voluntary, is mainly controlled by the medial temporal
lobes and limbic system of the cerebral cortex with contributions from the
motor cortex and other cortical areas. The pharyngeal swallow is started by the
oral phase and subsequently is co-ordinated by the swallowing center in the
medulla oblongata and pons. The reflex is initiated by touch receptors in the
pharynx as a bolus of food is pushed to the back of the mouth by the tongue, or
by stimulation of the palate (palatal reflex).
Phases
Oral
phase
Prior to the following stages of the oral phase,
the mandible depresses and the lips abduct to allow food or liquid to enter the
oral cavity. Upon entering the oral cavity, the mandible elevates and the lips
adduct to assist in oral containment of the food and liquid. The following
stages describe the normal and necessary actions to form the bolus, which is
defined as the state of the food in which it is ready to be swallowed.
1) Moistening
Food is moistened by saliva from the salivary
glands (parasympathetic).
2) Mastication
Food is mechanically broken down by the action of
the teeth controlled by the muscles of mastication (Vc)
acting on the temporomandibular joint. This results in a bolus which is moved
from one side of the oral cavity to the other by the tongue. Buccinator (VII)
helps to contain the food against the occlusal surfaces of the teeth. The bolus
is ready for swallowing when it is held together by (largely mucus) saliva
(VII—chorda tympani and IX—lesser petrosal), sensed by the lingual nerve of the
tongue (Vc). Any food that is too dry to form a bolus will not be swallowed.
3) Trough formation
A trough is then formed at the back of the tongue
by the intrinsic muscles (XII). The trough obliterates against the hard palate
from front to back, forcing the bolus to the back of the tongue. The intrinsic
muscles of the tongue (XII) contract to make a trough (a longitudinal concave
fold) at the back of the tongue. The tongue is then elevated to the roof of the
mouth (by the mylohyoid (mylohyoid nerve—Vc), genioglossus, styloglossus and
hyoglossus (the rest XII)) such that the tongue slopes downwards posteriorly.
The contraction of the genioglossus and styloglossus (both XII) also
contributes to the formation of the central trough.
4) Movement of the bolus posteriorly
At the end of the oral preparatory phase, the
food bolus has been formed and is ready to be propelled posteriorly into the
pharynx. In order for anterior to posterior transit of the bolus to occur,
orbicularis oris contracts and adducts the lips to form a tight seal of the
oral cavity. Next, the superior longitudinal muscle elevates the apex of the
tongue to make contact with the hard palate and the bolus is propelled to the
posterior portion of the oral cavity. Once the bolus reaches the palatoglossal
arch of the oropharynx, the pharyngeal phase, which is reflex and involuntary,
then begins. Receptors initiating this reflex are proprioceptive (afferent limb
of reflex is IX and efferent limb is the pharyngeal plexus- IX and X). They are
scattered over the base of the tongue, the palatoglossal and palatopharyngeal
arches, the tonsillar fossa, uvula and posterior pharyngeal wall. Stimuli from
the receptors of this phase then provoke the pharyngeal phase. In fact, it has
been shown that the swallowing reflex can be initiated entirely by peripheral
stimulation of the internal branch of the superior laryngeal nerve. This phase
is voluntary and involves important cranial nerves: V (trigeminal), VII
(facial) and XII (hypoglossal).
Pharyngeal
phase
For the pharyngeal phase to work properly all
other egress from the pharynx must be occluded—this includes the nasopharynx
and the larynx. When the pharyngeal phase begins, other activities such as
chewing, breathing, coughing and vomiting are concomitantly inhibited.
5) Closure of the nasopharynx
The soft palate is tensed by tensor palatini (Vc), and then elevated by levator palatini (pharyngeal
plexus—IX, X) to close the nasopharynx. There is also the simultaneous
approximation of the walls of the pharynx to the posterior free border of the
soft palate, which is carried out by the palatopharyngeus (pharyngeal
plexus—IX, X) and the upper part of the superior constrictor (pharyngeal
plexus—IX, X).
6) The pharynx prepares to receive the bolus
The pharynx is pulled upwards and forwards by the
suprahyoid and longitudinal pharyngeal muscles – stylopharyngeus (IX),
salpingopharyngeus (pharyngeal plexus—IX, X) and palatopharyngeus (pharyngeal
plexus—IX, X) to receive the bolus. The palatopharyngeal folds on each side of
the pharynx are brought close together through the superior constrictor
muscles, so that only a small bolus can pass.
7) Opening of the auditory tube
The actions of the levator palatini (pharyngeal
plexus—IX, X), tensor palatini (Vc) and salpingopharyngeus (pharyngeal
plexus—IX, X) in the closure of the nasopharynx and elevation of the pharynx
opens the auditory tube, which equalises the pressure between the nasopharynx
and the middle ear. This does not contribute to swallowing, but happens as a
consequence of it.
8) Closure of the oropharynx
The oropharynx is kept closed by palatoglossus
(pharyngeal plexus—IX, X), the intrinsic muscles of tongue (XII) and
styloglossus (XII).
9) Laryngeal closure
It is true vocal fold closure that is the primary
laryngopharyngeal protective mechanism to prevent aspiration during swallowing.
The adduction of the vocal cords are effected by the contraction of the lateral
cricoarytenoids and the oblique and transverse arytenoids (all recurrent
laryngeal nerve of vagus). Since the true vocal folds adduct during the
swallow, a finite period of apnea (swallowing apnea) must necessarily take
place with each swallow. When relating swallowing to respiration, it has been
demonstrated that swallowing occurs most often during expiration, even at full
expiration a fine air jet is expired probably to clear the upper larynx from
food remnants or liquid. The clinical significance of this finding is that
patients with a baseline of compromised lung function will, over a period of
time, develop respiratory distress as a meal progresses. Subsequently, false
vocal fold adduction, adduction of the aryepiglottic folds and retroversion of
the epiglottis take place. The aryepiglotticus (recurrent laryngeal nerve of
vagus) contracts, causing the arytenoids to appose each other (closes the
laryngeal aditus by bringing the aryepiglottic folds together), and draws the
epiglottis down to bring its lower half into contact with arytenoids, thus
closing the aditus. Retroversion of the epiglottis, while not the primary
mechanism of protecting the airway from laryngeal penetration and aspiration,
acts to anatomically direct the food bolus laterally towards the piriform
fossa. Additionally, the larynx is pulled up with the pharynx under the tongue
by stylopharyngeus (IX), salpingopharyngeus (pharyngeal plexus—IX, X),
palatopharyngeus (pharyngeal plexus—IX, X) and inferior constrictor (pharyngeal
plexus—IX, X).This phase is passively controlled reflexively and involves
cranial nerves V, X (vagus), XI (accessory) and XII (hypoglossal). The respiratory
center of the medulla is directly inhibited by the swallowing center for the
very brief time that it takes to swallow. This means that it is briefly
impossible to breathe during this phase of swallowing and the moment where
breathing is prevented is known as deglutition apnea.
10) Hyoid elevation
The hyoid is elevated by digastric (V & VII)
and stylohyoid (VII), lifting the pharynx and larynx up even further.
11) Bolus transits pharynx
The bolus moves down towards the esophagus by
pharyngeal peristalsis which takes place by sequential contraction of the
superior, middle and inferior pharyngeal constrictor muscles (pharyngeal
plexus—IX, X). The lower part of the inferior constrictor (cricopharyngeus) is
normally closed and only opens for the advancing bolus. Gravity plays only a
small part in the upright position—in fact, it is possible to swallow solid
food even when standing on one’s head. The velocity through the pharynx depends
on a number of factors such as viscosity and volume of the bolus. In one study,
bolus velocity in healthy adults was measured to be approximately 30–40 cm/s.
Esophageal
phase
12) Esophageal peristalsis
Like the pharyngeal phase of swallowing, the
esophageal phase of swallowing is under involuntary neuromuscular control.
However, propagation of the food bolus is significantly slower than in the
pharynx. The bolus enters the esophagus and is propelled downwards first by
striated muscle (recurrent laryngeal, X) then by the smooth muscle (X) at a
rate of 3–5 cm/s. The upper esophageal sphincter relaxes to let food pass,
after which various striated constrictor muscles of the pharynx as well as
peristalsis and relaxation of the lower esophageal sphincter sequentially push
the bolus of food through the esophagus into the stomach.
13) Relaxation phase
Finally the larynx and pharynx move down with the
hyoid mostly by elastic recoil. Then the larynx and pharynx move down from the
hyoid to their relaxed positions by elastic recoil. Swallowing therefore
depends on coordinated interplay between many various muscles, and although the
initial part of swallowing is under voluntary control, once the deglutition
process is started, it is quite hard to stop it.
Esophagus
Main article: Esophagus
The esophagus is a narrow muscular tube about 20-
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.
Sections[edit]
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.
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 in newborn 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.
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.
The act of
swallowing food, which at this place in the system is called a bolus, brings
many organs into action. As the top of your tongue presses up against the hard
palate , the roof of your mouth, food is shoved to the back of the mouth.
This action in turn brings the soft palate and ursula
(the place at the very back of the mouth where there is a teardrop shape
located) into action. They keep the food from being misguided toward the nose.
Once past the soft palate, the food is in the pharynx, a train station
with two tracks, one leading to the trachea (windpipe), the other to the
esophagus (food tube). The epiglottis
projects out from the trachea side and helps to admit free movement of air
as it is swallowed and at the same time restricts entrance to the esophagus. The larynx -hyper link, provides the
epiglottis with most of its muscle for movement. It applies an upward force
that helps to relax some tension on the esophagus, so
that food enters where it is meant to go, down the esophagus
and not down the windpipe. Many people have experienced at some time or another
when the swallowing action did not go as it was supposed to.
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.
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.[clarification needed]
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.
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.
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
than normal 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.
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 respiration not 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.
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.
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