Processes occurring in the small intestine

Digestion in the small intestine

From the stomach chyme enters the small intestine. In ensuring the intestinal digestion of the most important processes occurring in the duodenum. Here, food masses are exposed to intestinal juice, bile and pancreatic juice. The length of the duodenum is small, so the food is not delayed here, and the main processes of digestion occur in the underlying intestine. Intestinal juice, formed by the glands of the duodenal mucosa, contains a large amount of mucus and the peptidase enzyme that breaks down proteins. This juice has a weaker effect on fats and starch. It also contains the enzyme enterokinase, which activates trypsinogen pancreatic juice. Cells of the duodenum produce two hormones - secretin and cholecystokinin - pancreoimin, which increases the secretion of the pancreas.

Acidic contents of the stomach when passing into the duodenum acquires an alkaline reaction under the influence of bile, intestinal and pancreatic juice. In humans, the pH of the duodenal content ranges from 4.0 to 8.0.In the hydrolysis of nutrients carried out in the duodenum, pancreatic juice plays an especially important role. The bulk of pancreatic tissue produces digestive juice( pancreatic), which is excreted through the duct into the cavity of the duodenum. In an adult, normally, 1.5–2.0 L of pancreatic juice is released per day, which is a clear liquid with an alkaline reaction( pH = 7.8–8.5).Pancreatic juice is rich in enzymes that break down proteins, fats and carbohydrates. Amylase, lactase, nuclease and lipase are secreted by the pancreas in an active state and break down starch, milk sugar, nucleic acids and fats, respectively. Nucleases( trypsin and chymotrypsin) are formed by the gland cells in the inactive state in the form of trypsinogen and chymotrypsinogen. Trypsinogen in the duodenum under the influence of its enzyme enterokinase turns into trypsin. In turn, trypsin converts chymotrypsinogen into active chymotrypsin. Under the influence of trypsin and chymotrypsin, proteins and high-molecular polypeptides are cleaved to low-molecular peptides and free amino acids.

The role of the liver in digestion is also significant. Liver cells continuously excrete bile, which is one of the most important digestive juices. In humans, about 500-1200 ml of bile is produced per day. The process of formation of bile is continuous, and its entry into the duodenum - periodically, mainly in connection with food intake. On an empty stomach, bile does not enter the intestine, it goes to the gallbladder and accumulates there. In composition, cystic bile differs from hepatic bile.

Bile contains bile acids, bile pigments and other substances. Bile acids are involved in the process of digesting fat. The bile pigment bilirubin is formed both by the cells of the liver and from hemoglobin during the destruction of the red blood cells there. The dark color of bile is due to the presence of this pigment in it.

Bile increases the activity of enzymes of pancreatic and intestinal juices, especially lipases. It emulsifies fats and dissolves the products of their hydrolysis, which contributes to their absorption. Creating an alkaline reaction in the duodenum, bile prevents the destruction of trypsin by pepsin. It also performs a regulatory role, stimulating bile formation, biliary excretion, motor and secretory activity of the small intestine. Bile also has bacteriostatic properties. It delays the putrefactive processes in the intestines. The role of bile in the absorption of fat-soluble vitamins, cholesterol, amino acids and calcium salts from the intestine is great.

Liver, forming bile, performs not only secretory, but also excretory( excretory) function. The main organic excreta of the liver are bile salts, bilirubin, cholesterol, fatty acids and lecithin, as well as calcium, sodium, chlorine, bicarbonates. Getting into the intestine with bile, all these substances are removed from the body.

The food masses( chyme) from the duodenum are transferred to the small intestine, where their digestion by the digestive juices that have been released into the duodenum continues. At the same time, its own intestinal juice, produced by the liberkunov and brunner glands of the small intestinal mucosa, begins to act here. Intestinal juice contains enterokinase, as well as a complete set of enzymes that break down proteins, fats and carbohydrates. These enzymes are involved only in parietal digestion, since they are not released into the intestinal cavity. Abdominal digestion in the small intestine is carried out by enzymes that come with food chyme. Abdominal digestion is most effective for the hydrolysis of coarse-molecular substances.

Pristenochnoe( membrane) digestion, opened by Academician A.M.Ugolov in the 1950s and 1960s, occurs on the surface of the microvilli of the small intestine. It completes the intermediate and final stages of digestion by hydrolysis of the intermediate cleavage products. Microvilli are cylindrical processes of the intestinal epithelium with a height of 1-2 microns. Their number is huge - from 50 to 200 million per 1 mm 2 of the intestinal surface, which increases the inner surface of the small intestine by 300-500 times. The large total surface of the microvilli improves the suction processes. The products of intermediate hydrolysis fall into the zone of the so-called brush border formed by microvilli, where the final stage of hydrolysis occurs and the transition to absorption occurs. The main enzymes involved in parietal digestion are amylase, lipase and protease. Thanks to this digestion, 80-90% of peptide and glycolysis bonds and 55-60% of triglycerides are split.

Pristenochnaya digestion closely interacts with abdominal. Abdominal digestion prepares the original food substrates for parietal digestion, which reduces the amount of chyme processed in abdominal digestion by transferring the products of partial hydrolysis to the brush border of the intestinal mucosa. These processes contribute to the most complete digestion of all components of food and prepare them for absorption.

The motor activity of the small intestine provides mixing of the chyme with digestive secretions and its promotion along the intestine due to the reduction of the circular and longitudinal muscles. The duration of the periods of contraction and relaxation of sections of the intestine with pendulum movements is 4-6 s. Such a periodicity of contractions is due to the automatism of the smooth muscles of the intestines - the ability of the muscles to contract and relax periodically without external influences. Contractions of the circular muscles of the intestine cause peristaltic movements that promote the movement of food forward. Along the length of the intestine several peristaltic waves move simultaneously.

The contraction of the longitudinal and circular muscles is regulated by the vagus and sympathetic nerves. The vagus nerve stimulates intestinal motor function. The brake signals are transmitted along the sympathetic nerve, which reduce muscle tone and inhibit the mechanical movements of the intestine. The humoral factors also influence the motor function of the intestine: serotonin, choline and enterokinin stimulate bowel movement.

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Biology and Medicine

Functions of the small intestine

Thanks to the movements of the small intestine, the acidic chyme.coming from the stomach, mixed with alkaline pancreatic juices.liver and intestinal glands.moreover, the intestinal contents are constantly in contact with the mucous membrane of the small intestine. During the day, a person secretes up to 2.5 liters of intestinal juice. His numerous enzymes, which break down proteins, fats, carbohydrates, come from the destroyed, desquamated epithelial cells of the intestinal mucosa. As a result of the continuous process of cell regeneration, it is restored. Chemical processing of food and the absorption of products, as well as mechanical mixing and its promotion in the direction of the colon, continue in the small intestine. Endocrine cells produce various hormones and biologically active substances. In humans, the suction surface of the small intestine due to the presence of folds of the mucous membrane, villi and microvilli of intestinal epithelium cells reaches 200 m2.During digestion, the secretion of bile and its secretion into the intestinal lumen increases dramatically.

Digestion of food and absorption of its components takes place in the small intestine. At the same time, in the intestinal cavity under the influence of the enzymes of the intestinal and pancreatic juices, bile occurs only the splitting of nutrient molecules into individual fragments. The final cleavage occurs due to membrane digestion, which is carried out on the surface of the microvilli cells of the intestinal epithelium. They found a large number of active enzymes involved in the breakdown and absorption of food. A.M.Ugolev( 1967) discovered parietal digestion, which, unlike cavitary, occurring in the intestinal lumen, takes place on the surface of microvilli. The latter produce a number of their own digestive enzymes, adsorb on their surface some enzymes from the intestinal lumen and food substances that are most intensively broken down and absorbed. As a result of protein breakdown, amino acids are formed, fats - glycerin and fatty acids, carbohydrates - monosaccharides. With the collapse of food substances, many of their properties are lost, including harmful ones. This prevents the entry of foreign protein into the body.

Rhythmic contractions of the villi promote the absorption of substances, which is carried out from the outer surface of the epithelial cells( facing the intestinal lumen) to the internal( facing the blood and lymphatic capillaries).Amino acids and monosaccharides are absorbed into the blood;water, mineral salts, vitamins, fatty acids and glycerin - in the lymph.

Active digestion and absorption are promoted by high blood flow in the small intestine, which is 400 ml / min with food and 750 to 800 ml / min at the height of digestion. At the beginning of the 20th century, I.P.Pavlov showed that in each section of the digestive system various enzymes are produced that are involved in the breakdown of proteins, fats and carbohydrates. He studied their interaction and the regulation of excretion, the joint activity of the digestive organs and the influence of one department on another. In 1904, Pavlov was awarded the Nobel Prize in Physiology and Medicine for his work in the physiology of digestion, thanks to which a clearer understanding of the vital aspects of this issue was formed.


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Abstracts »Nature» General characteristics of digestion in the small intestine.

Concept of Membrane Digestion. The total length of the small intestine in humans is about 2.8 m( 12-scalp intestine # 8212; 30 cm, empty bowel 120 cm).

In the small intestine, the main processes of hydrolytic cleavage of proteins, fats and carbohydrates are completed and they are absorbed into the blood. Here, water, mineral salts, vitamins and other biologically active substances are also absorbed. In the mucous membrane of the small intestine hormones are secreted, which regulate motility and secretion of the gastrointestinal tract.

Villi are located on the inner surface of the ileum and ileum, the number of which is 22-40 per 1 mm2 in the empty and 18-30 per 1 mm2 in the ileum. The villi are composed of smooth muscle fibers, due to which they rhythmically reduce. These cuts improve the contact of the villi with the chyme and, in addition, contribute to the extrusion of the lymph from the blind ends of the lymphatic capillaries, which in turn promotes the absorption of fat. The villi are covered with enteric epithelium, whose cells have numerous outgrowths( microvilli) in an amount of up to 4,000 in one cell. Due to this, the total surface of the small intestine in an adult reaches 200 m2.Such structural features of the mucous membrane of the small intestine are important not only for the absorption of nutrients from the chyme into the blood, but also give this shell the properties of a porous catalyst. The existence of submicroscopic pores in the intestinal mucosa was established by electron microscopy. Thus, it is estimated that from 1 to 2 mm of the surface of the intestinal epithelium from 50 to 200 million microvilli are located. The diameter of the pores between them is 10-20 nm, which roughly corresponds to the size of the molecules of nutrients.

General characteristics of digestion in the small intestine.

Modern studies of the digestive processes in the small intestine have shown that two interrelated types of digestion are carried out here # 8212;cavity and membrane. Abdominal digestion, which occurs mainly in the proximal small intestine due to pancreatic juice and bile, provides the primary hydrolysis of nutrients in relatively large molecular fragments. Membrane( parietal) digestion takes place on the surface of the small intestine microvilli. It completes the hydrolysis of cavity digestion intermediate products. These processes occur on cellular membranes in the zone of the so-called glycocalyx, covering the surface of microvilli, and adsorb part of the enzymes from the chyme( protease, lipase, amylase, pancreatic juice).Another part of the enzymes is synthesized by the cells of the small intestine and transported to the surface of the cell membrane. Relatively large nutrient molecules, falling between the fibers to the base of glycocalyx, are finally hydrolyzed. Here, on the membrane of enterocytes, there are transport systems that ensure the absorption of hydrolysis products.

Thus, abdominal and membrane digestion are interrelated. Abdominal digestion prepares the original substrates for membrane digestion, which in turn completes their hydrolysis and provides a transition to absorption.

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