Digestion in Intestine
Small intestine
The small intestine (or small bowel) is the part of the gastrointestinal tract following the stomach and followed by the large intestine, and is where much of the digestion and absorption of food takes place. It receives bile juice and pancreatic juice through the hepatopancreatic duct, controlled by Sphincter of oddi. In invertebrates such as worms, the terms “gastrointestinal tract” and “large intestine” are often used to describe the entire intestine. This article is primarily about the human gut, though the information about its processes is directly applicable to most placental mammals. The primary function of the small intestine is the absorption of nutrients and minerals found in food. (A major exception to this is cows; for information about digestion in cows and other similar mammals, see ruminants.)
The small intestine is where most chemical digestion takes place. Most of the digestive enzymes that act in the small intestine are secreted by the pancreas and enter the small intestine via thepancreatic
duct
. Enzymes enter the small intestine in response to the hormone cholecystokinin, which is produced in the small intestine in response to the presence of nutrients. The hormonesecretin
also causes bicarbonate to be released into the small intestine from the pancreas in order to neutralize the potentially harmful acid coming from the stomach.
The three major classes of nutrients that undergo digestion are proteins, lipids (fats) and carbohydrates:
· Proteins are degraded into small peptides and amino acids before absorption. Chemical breakdown begins in the stomach and continues in the small intestine. Proteolyticenzymes, including trypsin and chymotrypsin, are secreted by the pancreas and cleave proteins into smaller peptides. Carboxypeptidase, which is a pancreatic brush border enzyme, splits one amino acid at a time. Aminopeptidase and dipeptidase free the end amino acid products.
· Lipids (fats) are degraded into fatty acids and glycerol. Pancreatic lipase breaks down triglycerides into free fatty acids and monoglycerides. Pancreatic lipase works with the help of the salts from the bile secreted by the liver and the gall bladder. Bile salts attach to triglycerides to help emulsify them, which aids access by pancreatic lipase. This occurs because the lipase is water-soluble but the fatty triglycerides are hydrophobic and tend to orient towards each other and away from the watery intestinal surroundings. The bile salts emulsify the triglycerides in the watery surroundings until the lipase can break them into the smaller components that are able to enter the villi for absorption.
· Some carbohydrates are degraded into simple sugars, or monosaccharides (e.g., glucose). Pancreatic amylase breaks down some carbohydrates (notably starch) into oligosaccharides. Other carbohydrates pass undigested into the large intestine and further handling by intestinal bacteria. Brush border enzymes take over from there. The most important brush border enzymes are dextrinase and glucoamylase which further break down oligosaccharides. Other brush border enzymes are maltase, sucrase and lactase. Lactase is absent in most adult humans and for them lactose, like most poly-saccharides, are not digested in the small intestine. Some carbohydrates, such as cellulose, are not digested at all, despite being made of multiple glucose units. This is because the cellulose is made out of beta-glucose, making the inter-monosaccharidal bindings different from the ones present in starch, which consists of alpha-glucose. Humans lack the enzyme for splitting the beta-glucose-bonds, something reserved for herbivores and bacteria from the large 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 pylorus is the region of the stomach that connects to the duodenum (the beginning of the small intestines). It is divided into two parts:
the pyloric antrum, which connects to the body of the stomach.
the pyloric canal, which connects to the duodenum.
The pyloric sphincter, or valve, is a strong ring of smooth muscle at the end of the pyloric canal which lets food pass from the stomach to the duodenum. It receives sympathetic innervation from the celiac ganglionThe majority of digestion and absorption occurs here after the milky chyme enters theduodenum
.
Chyme is the semifluid mass of partly digested food expelled by the stomach into the duodenum.
Also known as “chymus“, it is the liquid substance found in the stomach before passing through the pyloric valve and entering the duodenum. It results from the mechanical and chemical breakdown of a bolus and consists of partially digested food, water, hydrochloric acid, and various digestive enzymes. Chyme slowly passes through the pyloric sphincter and into the duodenum, where the extraction of nutrients begins. Depending on the quantity and contents of the meal, the stomach will digest the food into chyme in anywhere between 40 minutes to a few hours.
With a pH of around 2, chyme emerging from the stomach is very acidic. To raise its pH, the duodenum secretes a hormone, cholecystokinin (CCK), which causes the gall bladder to contract, releasing alkaline bile into the duodenum. The duodenum also produces the hormone secretin to stimulate the pancreatic secretion of large amounts of sodium bicarbonate, which raises the chyme’s pH to 7 before it reaches the jejunum. As it is protected by a thick layer of mucus and utilizes the neutralizing actions of the sodium bicarbonate and bile, the duodenum is not as sensitive to highly acidic chyme as the rest of the small intestine.
At a pH of 7, the enzymes that were present from the stomach are no longer active. This then leads into the further breakdown of the nutrients still present by anaerobic bacteria which at the same time help to package the remains. These bacteria also help synthesize vitamin B and vitamin K.
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 thegallbladder where it will be released to the small intestine via the bile duct. The bile in the gallbladder is much more concentrated.
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).
Bile acts to some extent as a surfactant, helping to emulsify the fats in food. Bile salt anions are hydrophilic on one side and hydrophobic on the other side; consequently, they tend to aggregate around droplets of fat (triglycerides and phospholipids) to form micelles, with the hydrophobic sides towards the fat and hydrophilic sides facing outwards. The hydrophilic sides are negatively charged, and this charge prevents fat droplets coated with bile from re-aggregating into larger fat particles. Ordinarily, the micelles in the duodenum have a diameter of around 14–33 μm.
The dispersion of food fat into micelles thus provides a greatly increased surface area for the action of the enzyme pancreatic lipase, which actually digests the triglycerides, and is able to reach the fatty core through gaps between the bile salts. A triglyceride is broken down into two fatty acids and a monoglyceride, which are absorbed by the villi on the intestine walls. After being transferred across the intestinal membrane, the fatty acids reform into triglycerides, before being absorbed into the lymphatic system through lacteals. Without bile salts, most of the lipids in food would be excreted in feces, undigested.
Since bile increases the absorption of fats, it is an important part of the absorption of the fat-soluble substances, such as the vitamins D, E, K and A.
Besides its digestive function, bile serves also as the route of excretion for bilirubin, a byproduct of red blood cells recycled by the liver. Bilirubin derives from hemoglobin byglucuronidation.
Bile is alkaline and also has the function of neutralizing any excess stomach acid before it enters the duodenum, the first section of the small intestine. Bile salts also act as bactericides, destroying many of the microbes that may be present in the food.
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 theliver 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.
Pancreatic juice is a liquid secreted by the pancreas, which contains a variety of enzymes, including trypsinogen, chymotrypsinogen, elastase, carboxypeptidase, pancreatic lipase, nucleases and amylase.
Pancreatic juice is alkaline iature due to the high concentration of bicarbonate ions. This is useful ieutralizing the acidic gastric acid, allowing for effective enzymic action.
Pancreatic juice secretion is regulated by the hormones secretin and cholecystokinin, which is produced by the walls of the duodenum upon detection of acid food, proteins and fats, vitamins. Pancreatic secretion consists of an aqueous bicarbonate component from the duct cells and enzymatic component from the acinar cells. A clear alkaline secretion of the pancreas containing enzymes that aid in the digestion of proteins, carbohydrates, and fats.
Amylase is an enzyme that catalyses the hydrolysis of starch into sugars. Amylase is present in the saliva of humans and some other mammals, where it begins the chemical process of digestion. Foods that contain much starch but little sugar, such as rice and potato, taste slightly sweet as they are chewed because amylase turns some of their starch into sugar in the mouth. The pancreas also makes amylase (alpha amylase) to hydrolyse dietary starch into disaccharides and trisaccharides which are converted by other enzymes to glucose to supply the body with energy. Plants and some bacteria also produce amylase. As diastase, amylase was the first enzyme to be discovered and isolated. Specific amylase proteins are designated by different Greek letters. All amylases are glycoside hydrolases and act on α-1,4-glycosidic bonds.
Lipase is an enzyme that catalyzes the breakdown of’ hydrolysis of fats (lipids). Lipases are a subclass of the esterases.
Lipases perform essential roles in the digestion, transport and processing of dietary lipids (e.g. triglycerides, fats, oils) in most, if not all, living organisms. Genes encoding lipases are even present in certain viruses.
Most lipases act at a specific position on the glycerol backbone of lipid substrate (A1, A2 or A3)(small intestine). For example, human pancreatic lipase (HPL), which is the main enzyme that breaks down dietary fats in the human digestive system, converts triglyceride substrates found in ingested oils to monoglycerides and two fatty acids.
Several other types of lipase activities exist in nature, such as phospholipases [5] and sphingomyelinases,[6] however these are usually treated separately from “conventional” lipases.
Some lipases are expressed secreted by pathogenic organisms during the infection. In particular, Candida albicans has a large number of different lipases, possibly reflecting broadlipolytic activity, which may contribute to the persistence and virulence of C. albicans in human tissue.
Trypsinogen is the precursor form or zymogen of the pancreatic enzyme trypsin. It is found in pancreatic juice, along with amylase, lipase, and chymotrypsinogen. It is activated by enteropeptidase, which is found in the intestinal mucosa, to form trypsin. Once activated, the trypsin can activate more trypsinogen into trypsin. Trypsin cleaves the peptide bond on the carboxyl side of basic amino acids such as arginine and lysine.
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 thatexsogenic cells of pancreas and ducts cells produce pancreatic juice. It helps to hydrolyzed protein to peptides and amino acids, carbohydrates to monosaccharides, lipids to thefat 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 – 1,5-2,0 L. Reaction of it – pH =8,0-8,5. It has a big quantity of hydrocarbonates. It has near 10 % of protein – enzymes, which are act on protein, lipids and carbohydrates. Proteolytic enzymes secreted in form, which are not active, for example, trypsinogen,chymotrypsinogen. Trypsinogen activated by enzymes enterokinase (produced by the cells of mucous of duodenum) and after that it has another name – trypsin. It activateschymotripsinogen to chymotrypsin. In pancreatic juice presents another proteolytic enzymes – elastase, nuclease etc. They hydrolyzed protein to peptides and amino acids.Lipolytic enzymes – lipase and phospholipase – hydrolyzed lipids to the fat acids and glycerine. Amilolytic enzyme alpha-amilase hydrolyzed starch and glikogen to oligo-, di– andmonosaccharides.)
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 (200 g per day). pH of juice – 7,5-8,0; production per day – near 1,8 L. Functions: ending hydrolyses of all nutritive substances; protective of mucus wall; support of chymus in fluid condition; formed of base reaction of intestine contents.)
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.)
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 1.5 meters long, with three parts: the cecum at the junction with the small intestine, the colon, and the rectum. The colon itself has four parts: the ascending colon, thetransverse colon, the descending colon, and the sigmoid colon. The large intestine absorbs water from the chyme and stores feces until it can be egested.Food products that cannot go through the villi, such as cellulose (dietary fiber), are mixed with other waste products from the body and become hard and concentrated feces. Thefeces is stored in the rectum for a certain period and then the stored feces is eliminated from the body due to the contraction and relaxation through the anus. The exit of this waste material is regulated by the anal sphincter.
Protein digestion occurs in the stomach and duodenum in which 3 main enzymes, pepsin secreted by the stomach and trypsin and chymotrypsin secreted by the pancreas, break down food proteins into polypeptides that are then broken down by various exopeptidases and dipeptidases into amino acids. The digestive enzymes however are mostly secreted as their inactive precursors, the zymogens. For example, trypsin is secreted by pancreas in the form of trypsinogen, which is activated in the duodenum by enterokinase to form trypsin. Trypsin then cleavesproteins to smaller polypeptides.
Digestion of some fats can begin in the mouth where lingual lipase breaks down some short chain lipids into diglycerides. However fats are mainly digested in the small intestine. The presence of fat in the small intestine produces hormones that stimulate the release of pancreatic lipase from the pancreas and bile from the liver which helps in the emulsification of fats for absorption fatty acids. Complete digestion of one molecule of fat (a triglyceride) results in 3 fatty acid molecules and one glycerol molecule.
In humans, dietary starches are composed of glucose units arranged in long chains called amylose, a polysaccharide. During digestion, bonds between glucose molecules are broken by salivary and pancreatic amylase, resulting in progressively smaller chains of glucose. This results in simple sugars glucose and maltose (2 glucose molecules) that can be absorbed by the small intestine.
Lactase is an enzyme that breaks down the disaccharide lactose to its component parts, glucose and galactose. Glucose and galactose can be absorbed by the small intestine. Approximately half of the adult population produce only small amounts of lactase and are unable to eat milk-based foods. This is commonly known as lactose intolerance.
Sucrase is an enzyme that breaks down the disaccharide sucrose, commonly known as table sugar, cane sugar, or beet sugar. Sucrose digestion yields the sugars fructose and glucose which are readily absorbed by the small intestine.
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.)
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.
