Nutrients enter the blood and lymphatic capillaries through the epithelial lining of the digestive tract. This mainly occurs in the small intestine, which is adapted to ensure that absorption is as efficient as possible.

From the inside, the intestines are lined with a mucous membrane with a huge number of outgrowths: more than 2,500 villi are placed on each square centimeter of the inner surface of this organ. Each villus cell forms up to 3000 microvilli. Thanks to the villi and microvilli, the inner surface of the small intestine is larger than a football field. So, for parietal digestion in the body there is a huge surface - substances are absorbed through it.

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The cavities of the villi contain blood and lymph capillaries, elements of smooth muscle tissue, and nerve fibers. Villi and microvilli is the main "device" that ensures the absorption of nutrients.

How is the absorption of substances?

There are two ways of transporting substances through the intestinal epithelium: through the gaps between the cells and through the epithelial cells themselves. In the first case, it is carried out by diffusion. Thus, water and some mineral salts and organic compounds enter the internal environment. However, only a small part of the nutrients get to the internal environment of the villus by diffusion. Many molecules have to penetrate into the villi through the epithelial cells themselves. First of all, these molecules must overcome their plasma membranes. In this they are assisted by special carrier molecules. Once in the cell, nutrient molecules move in the cytoplasm to another cell and exit through the membrane into the intercellular fluid. Overcoming these barriers by molecules of substances that are absorbed usually requires a large expenditure of energy.

What happens to the substances that got into the intercellular fluid of the villi? their molecules are sent to the blood or lymphatic capillaries of the villi. Glucose, amino acids, mineral salts dissolved in water pass directly into the blood. The products of fat breakdown (glycerol and fatty acids) first enter the lymph, and with it enter the circulatory system.

The human large intestine is 1.2-1.5 m long, its diameter reaches 9 cm. Digestion and absorption are mainly completed in the small intestine. The only exceptions are some substances, such as cellulose. It is partially digested in the large intestine by numerous lactic acid bacteria. These bacteria- mutualists synthesize substances useful for humans: some amino acids, vitamin K, B vitamins, which enter the bloodstream and are transported to every cell of the human body.

Digestive juice, which is produced by the glands of the walls of the colon, contains almost no enzymes. Its main component is mucus, which acts on undigested residues, and they become like butter.

Digestion in the large intestine - the main stages

Why do food particles thicken in the large intestine? It is in it that intensive absorption of water into the blood vessels occurs. Therefore chyme, advancing, gradually turns into dense fecal masses. Feces can remain in the large intestine for up to 36 hours and then move to the rectum. From the rectum, they are brought out through the anus, surrounded by sphincter. This sphincter, unlike those located in the esophagus and stomach, contracts voluntarily. This means that the person controls the excretion of feces. Therefore, absorption occurs in all parts of the digestive tract. However, on each of them, various substances enter the internal environment. Nutrients are almost not absorbed in the oral cavity and esophagus. Small amounts of water, glucose, amino acids, etc. are absorbed in the stomach. Intensive absorption of nutrients occurs in the small intestine. The large intestine absorbs mostly water.

Digestion begins in the mouth and stomach, but most of the absorption of food occurs in the small intestine. This part of the digestive tract is divided into three sections: the duodenum, jejunum and ileum.

The total length of the small intestine is 6.5 m. The duodenum is 25 cm long. It is in it that the contents of the stomach (chyme) are mixed with digestive juices. The jejunum, which is 2.5 m long, connects to the ileum, which forms the remainder of the small intestine. There is no clear boundary between the departments, although the jejunum has thicker walls and a larger diameter (about 3.8 cm) than the rest.

Food moves through the intestines with the help of peristalsis (wave-like muscle contractions). The process of digestion continues throughout the small intestine. The main function of the jejunum and ileum is to absorb the products of digestion into the body.

digestive juices

The digestive juices of the duodenum contain sodium bicarbonate. It neutralizes the acid produced by the stomach and creates an alkaline environment favorable for intestinal enzymes.

Juices come from two sources. Glands located in the walls of the duodenum produce the enzymes maltase, sucrase, enteropeptidase, and a mixture of intestinal enzymes called erepsin. The second source of juices is the pancreas, which, in addition to its endocrine function, produces three digestive enzymes: lipase, amylase and trypsinogen, which is converted into trypsin in the intestine. Together, these enzymes continue to break down proteins, sugars, and fats into simpler substances.

Digestion of proteins, fats and carbohydrates

Some proteins break down in the stomach into peptides (small chains of amino acids that make up proteins). Enteropeptidase activates pancreatic trypsin in the small intestine. With the help of this enzyme, both proteins and peptides are broken down into amino acids. Erepsin is also involved in the cleavage of peptides.

Digestion of fats occurs with the help of salts found in bile. It is produced by the liver and stored in the gallbladder. Bile enters the duodenum through the bile duct. Bile salts emulsify fats. This increases the surface area that lipase can act on, which breaks down fats into fatty acids and glycerol.

Starch that has not reacted with salivary enzymes is converted to maltose by the enzyme amylase. Further, maltose under the influence of maltase is converted into glucose. Sucrose is broken down into glucose and fructose by sucrase.

How are nutrients absorbed?

The mucous membrane of the jejunum and ileum is the main surface for the absorption of digestive products. The total volume of fluid that is absorbed every day by the intestines can reach 9 liters. About 7.5 liters of them are absorbed by the small intestine.

The inner surface of the jejunum and ileum is covered with small finger-like protrusions - villi, which protrude about 1 mm into the intestine. The purpose of these villi is to increase the surface area on which nutrients can be absorbed.

The walls of each villus are formed by long epithelial cells. Inside the villi is a network of small capillaries and a single lacteal vessel - a tube that connects to the body's lymphatic system.

Epithelial cells absorb digestion products and liters of water, transferring amino acids and sugars into the blood. Fatty acids and glycerol are converted by epithelial cells into fats, which are sent in the form of a whitish emulsion to the lacteal vessels.

Suction occurs relatively slowly, and therefore it is feasible to a sufficient extent only if there is a large surface of the mucous membrane, on which there is contact with the split food substances.

absorption in the stomach occurs only to a small extent. Mineral salts, monosaccharides, alcohol and water are very slowly absorbed here.

The amount of absorbed substances is also relatively small in the cavity of the duodenum, where, as shown by the experiments of E. S. London, about 53-63% of carbohydrates and proteins and a small amount of fat are digested. If we take into account digestion in the stomach, then more than ⅔ of the proteins and carbohydrates of food are split in the duodenum. Absorption in the duodenum fluctuates within 5-8% of incoming food, which is of little physiological significance, in particular with respect to proteins, since more of them are excreted with digestive juices and enter the intestinal cavity than are absorbed at the same time.

Most intense absorption in the small intestine where the suction surface is very large. Due to the presence of a large number of folds and protrusions of the mucous membrane - villi - its area is many times greater than the outer surface of the body.

The membrane through which absorption occurs is formed by the so-called border epithelium. Border cells have the form of elongated cylinders, the diameter of which is about 8 microns, and the height is about 25 microns. On the surface of these cells facing the lumen of the intestine, under a conventional light microscope, a narrow border 1-3 microns thick is visible, because of which the cells got their name.

The electron microscope made it possible to see the structure of this border. It turned out that it is formed by the thinnest filamentous processes - microvilli ( rice. 91). On the surface of one cell there are 31500/3000 microvilli, inside which microtubules pass. The height of each microvillus is 1-3 microns, and the diameter is about 0.08 microns. Their presence increases the absorption surface of the intestinal mucosa so much that it reaches a very large value - up to 500 m2. Processes take place on the same surface . Rice. 91. Microvilli of the border epithelium of the monkey small intestine. Magnification with an electron microscope 66,000 times (according to N. M. Shestopalova). 1 - microvilli; 2 - microtubules.

In experiments with the removal of the entire small intestine below the duodenum, the animals die quite soon, since there is no entry of substances from the intestine into the blood.

If, in an experiment on an animal, the mucous membrane of the intestinal loop is damaged or poisoned (sodium fluoride is used for this) and this causes, to one degree or another, a violation of the viability of the intestinal epithelium, then absorption in this loop is sharply disturbed. Similar experiments have shown that absorption is associated with the normal physiological function of the mucosal epithelium.

Absorption is a physiological process consisting in the fact that aqueous solutions of nutrients formed as a result of the digestion of food penetrate through the mucous membrane of the gastrointestinal canal into the lymphatic and blood vessels. Through this process, the body receives the nutrients necessary for life.

In the upper parts of the digestive tube (mouth, esophagus, stomach), absorption is very small. In the stomach, for example, only water, alcohol, some salts and products of the breakdown of carbohydrates are absorbed, and in small quantities. Minor absorption also occurs in the duodenum.

The bulk of the nutrients are absorbed in the small intestine, and absorption occurs in different parts of the intestine at a different rate. Maximum absorption occurs in the upper parts of the small intestines (Table 22).

Table 22. Absorption of substances in various parts of the dog's small intestine

	Absorption of substances in the intestine, %
Substances	25 cm below		2-3 cm up
	gatekeeper	above the caecum	from the caecum
Alcohol
grape sugar
starch paste
Palmitic acid
Butyric acid

In the walls of the small intestine there are special organs of absorption - villi (Fig. 48).

The total surface of the intestinal mucosa in humans is approximately 0.65 m 2, and due to the presence of villi (18-40 per 1 mm 2), it reaches 5 m 2. This is approximately 3 times the outer surface of the body. According to Verzar, a dog has about 1,000,000 villi in the small intestine.

Rice. 48. Cross section of the human small intestine:

/ - villus with nerve plexus; d - central lacteal vessel of the villi with smooth muscle cells; 3 - Lieberkuhn crypts; 4 - muscularis mucosa; 5 - plexus submucosus; g _ submucosa; 7 - plexus of lymphatic vessels; c - layer of circular muscle fibers; 9 - plexus of lymphatic vessels; 10 - ganglion cells of the plexus myente; 11 - a layer of longitudinal muscle fibers; 12 - serous membrane

The height of the villi is 0.2-1 mm, the width is 0.1-0.2 mm, each contains 1-3 small arteries and up to 15-20 capillaries located under the epithelial cells. During absorption, the capillaries expand, which significantly increases the surface of the epithelium and its contact with the blood flowing in the capillaries. The villi contain a lymphatic vessel with valves that open in one direction only. Due to the presence of smooth muscles in the villus, it can perform rhythmic movements, as a result of which soluble nutrients are absorbed from the intestinal cavity and lymph is squeezed out of the villus. For 1 minute, all the villi can absorb 15-20 ml of liquid from the intestine (Verzar). Lymph from the lymphatic vessel of the villus enters one of the lymph nodes and then into the thoracic lymphatic duct.

After eating, the villi move for several hours. The frequency of these movements is about 6 times per minute.

Contractions of the villi occur under the influence of mechanical and chemical irritations of substances in the intestinal cavity, such as peptones, albumose, leucine, alanine, extractives, glucose, bile acids. The movement of the villi is also excited by the humoral way. It has been proven that in the mucous membrane of the duodenum a specific hormone villikinin is formed, which is brought to the villi by the blood flow and excites their movements. The action of the hormone and nutrients on the musculature of the villi occurs, apparently, with the participation of the nerve elements embedded in the villus itself. According to some reports, the Meissnerog plexus, located in the submucosal layer, takes part in this process. When the intestine is isolated from the body, the movement of the villi stops after 10-15 minutes.

In the large intestine, the absorption of nutrients under normal physiological conditions is possible, but in small quantities, as well as substances that are easily decomposed and well absorbed. The use of nutritional enemas is based on this in medical practice.

Water is absorbed quite well in the large intestine, and therefore the feces acquire a dense texture. If the absorption process is disturbed in the large intestine, loose stools appear.

E. S. London developed the technique of angiostomy, with the help of which it was possible to study some important aspects of the absorption process. This technique consists in the fact that the end of a special cannula is sewn to the stacks of large vessels, the other end is brought out through the skin wound. Animals with such angiostomy tubes live with special care for a long time, and the experimenter, having pierced the wall of the vessel with a long needle, can obtain blood from the animal for biochemical analysis at any moment of digestion. Using this technique, E. S. London found that the products of protein breakdown are absorbed mainly in the initial sections of the small intestine; their absorption in the large intestine is small. Usually animal protein is digested and absorbed from 95 to 99%,

and vegetable - from 75 to 80%. The following protein breakdown products are absorbed in the intestine: amino acids, di- and polypeptides, peptones and albumoses. Can be absorbed in small quantities and non-split proteins: serum proteins, egg and milk proteins - casein. The amount of absorbed unsplit proteins is significant in young children (R. O. Feitelberg). The process of absorption of amino acids in the small intestine is under the regulatory influence of the nervous system. Thus, transection of the splanchnic nerves causes an increase in absorption in dogs. Transection of the vagus nerves under the diaphragm is accompanied by inhibition of the absorption of a number of substances in an isolated loop of the small intestine (Ya-P. Sklyarov). Increased absorption is observed after extirpation of the solar plexus nodes in dogs (Nguyen Tai Luong).

The absorption rate of amino acids is influenced by some endocrine glands. The introduction of thyroxin, cortisone, pituitrin, ACTH to animals led to a change in the rate of absorption, however, the nature of the change depended on the doses of these hormonal drugs and the duration of their use (N. N. Kalashnikova). Change the rate of absorption of secretin and pancreozymin. It has been shown that the transport of amino acids is carried out not only through the apical membrane of the enterocyte, but also through the entire cell. This process involves subcellular organelles (in particular, mitochondria). The rate of absorption of undigested proteins is influenced by many factors, in particular, intestinal pathology, the amount of proteins administered, intra-intestinal pressure, and excessive intake of whole proteins into the blood. All this can lead to sensitization of the body, the development of allergic diseases.

Carbohydrates, being absorbed in the form of monosaccharides (glucose, levulose, galactose) and partly disaccharides, directly enter the blood, with which they are delivered to the liver, where they are synthesized into glycogen. Absorption occurs very slowly, and the rate of absorption of various carbohydrates is not the same. If monosaccharides (glucose) combine with phosphoric acid in the wall of the small intestine (phosphorylation process), absorption is accelerated. This is proved by the fact that when an animal is poisoned with monoioacetic acid, which inhibits the phosphorylation of carbohydrates, their absorption is significantly

slows down. Absorption in different parts of the intestine is not the same. According to the rate of absorption of isotonic glucose solution, the sections of the small intestine in humans can be arranged in the following order: duodenum> jejunum> ileum. Lactose is most absorbed in the duodenum; maltose - in lean; sucrose - in the distal part of the jejunum and ileum. In dogs, the involvement of the different parts of the intestine is basically the same as in humans.

The cerebral cortex is involved in the regulation of carbohydrate absorption in the small intestine. So, A. V. Rikkl worked out conditioned reflexes both to increase absorption and to delay. The intensity of absorption changes with food arousal, with the act of eating. Under experimental conditions, it was possible to influence the absorption of carbohydrates in the small intestine by changing the functional state of the central nervous system, using pharmacological agents, and stimulating current of different cortical areas in dogs with electrodes implanted in the frontal, parietal, temporal, occipital, and posterior limbic areas of the cerebral cortex (P O. Feitelberg). The effect depended on the nature of the shift in the functional state of the cerebral cortex, in experiments with the use of pharmacological preparations, on the areas of the cortex that were irritated by the current, and also on the strength of the stimulus. In particular, a greater importance in the regulation of the absorption function of the small intestine of the limbic cortex was revealed.

What is the mechanism by which the cerebral cortex is involved in the regulation of absorption? At present, there is reason to believe that information about the ongoing process of absorption in the intestine is carried to the central nervous system by impulses that occur both in the receptors of the digestive tract and blood vessels, the latter being irritated by chemicals that have entered the bloodstream from the intestine.

An important role is played by subcortical structures in the regulation of absorption in the small intestine. During stimulation of the lateral and posteroventral nuclei of the thalamus, changes in sugar absorption were not the same: upon stimulation of the former, a weakening was observed, and upon stimulation of the latter, an increase. Changes in the intensity of absorption were observed with different

irritations of the globus pallidus, amygdala and with

irritation with current of the hypothalamic region (P. G. Bogach).

Thus, the participation of subcortical formations in re-

The absorption activity of the small intestine is influenced by the reticular formation of the brain stem. This is evidenced by the results of experiments with the use of chlorpromazine, blocking adrenoreactive structures of the reticular formation. The cerebellum is involved in the regulation of absorption, which contributes to the optimal course of the absorption process, depending on the body's needs for nutrients.

According to the latest data, impulses arising in the cerebral cortex and underlying parts of the central nervous system reach the absorptive apparatus of the small intestine through the vegetative part of the nervous system. This is evidenced by the fact that switching off or irritation of the vagus or splanchnic nerves significantly, but not unidirectionally, changes the intensity of absorption (in particular, glucose).

The glands of internal secretion are also involved in the regulation of absorption. Violation of the activity of the adrenal glands is reflected in the absorption of carbohydrates in the small intestine. The introduction of cortin, prednisolone into the body of animals changes the intensity of absorption. Removal of the pituitary gland is accompanied by a weakening of glucose absorption. Administration of ACTH to an animal stimulates absorption; removal of the thyroid gland reduces the rate of glucose absorption. A decrease in glucose absorption is also noted with the introduction of antithyroid substances (6-MTU). There are some grounds for recognizing that pancreatic hormones can influence the function of the absorptive apparatus of the small intestine (Fig. 49).

Neutral fats are absorbed in the intestine after splitting into glycerol and higher fatty acids. Absorption of fatty acids usually occurs when they are combined with bile acids. The latter, entering the liver through the portal vein, are excreted by the liver cells with bile and thus can again take part in the process of fat absorption. Absorbed fat breakdown products in the epithelium of the intestinal mucosa are again synthesized into fat.

R. O. Feitelberg believes that the absorption process consists of four stages:

Rice. 49. Neuroendocrine regulation of absorption processes in the intestine (according to R. O. Feitelberg and Nguyen Tai Luong): Black arrows - afferent information, white - efferent transmission of impulses, shaded - hormonal regulation

foot and parietal lipolysis through the apical membrane; transport of fatty particles along the membranes of the tubules of the cytoplasmic reticulum and the vacuole of the lamellar complex; transport of chylomicrons through the lateral and. basement membranes; transport of chylomicrons across the endothelial membrane of lymphatic and blood vessels. The rate of absorption of fats probably depends on the synchronization of all stages of the conveyor (Fig. 50).

It has been established that some fats can affect the absorption of others, and the absorption of a mixture of two fats is better than either separately.

Neutral fats absorbed in the intestine enter the blood through the lymphatic vessels into the large thoracic duct. Fats such as butter and lard are absorbed up to 98%, and stearin and spermaceti - up to 9-15%. If the animal's abdominal cavity is opened 3-4 hours after ingestion of fatty foods (milk), then it is easy to see with the naked eye the lymphatic vessels of the mesentery of the intestine filled with a large amount of lymph. Lymph has a milky appearance and is called milky juice or chyle. However, not all fat after absorption enters the lymphatic vessels, some of it can be sent to the blood. This can be verified by ligating the thoracic lymphatic duct in an animal. Then the content of fat in the blood increases sharply.

Water enters the gastrointestinal tract in large quantities. In an adult, the daily water intake reaches 2 liters. During the day, a person secretes up to 5-6 liters of digestive juices into the stomach and intestines (saliva - 1 l, gastric juice - 1.5-2 l, bile - 0.75-1 l, pancreatic juice - 0.7-0 .8 l, intestinal juice - 2 l). Only about 150 ml is excreted from the intestine to the outside. Water absorption occurs partially in the stomach, more intensively in the small and especially the large intestine.

Salt solutions, mainly table salt, are absorbed quite quickly if they are hypotonic. At a salt concentration of up to 1%, absorption is intense, and up to 1.5%, salt absorption stops.

Solutions of calcium salts are absorbed slowly and in small quantities. At a high salt concentration, water is released from the blood into the intestines.

Rice. 50. The mechanism of digestion and absorption of fats. Four-stage

transport of long chain lipids through enterocytes

(according to R. O. Feitelberg and Nguyen Tai Luong)

Nick. On this principle, the use of certain concentrated salts as laxatives is built in the clinic.

The role of the liver in the process of absorption. It is known that blood from the vessels of the walls of the stomach and intestines enters through the portal vein to the liver, and then through the hepatic veins into the inferior vena cava and then into the general circulation. Poisonous substances formed in the intestine during food decay (indole, skatole, tyramine, etc.) and absorbed into the blood are neutralized in the liver by adding sulfuric and glucuronic acids to them and forming slightly toxic ethereal sulfuric acids. This is the barrier function of the liver. It was found out by IP Pavlov and VN Ekk, who performed the following original operation on animals, which was called the Pavlov-Ekk operation. The portal vein by anastomosis connects with the inferior vena cava, and thus the blood flowing from the intestine enters the general circulation, bypassing the liver. Animals after such an operation die after a few days due to poisoning by toxic substances absorbed in the intestines. Feeding meat especially quickly leads animals to death.

The liver is an organ in which a number of synthetic processes take place: the synthesis of urea and lactic acid, the synthesis of glycogen from mono- and disaccharides, etc. The synthetic function of the liver underlies its antitoxic function. With the introduction of sodium benzoate into the gastrointestinal tract in the liver, it is neutralized by the formation of hippuric acid, which is then excreted from the body by the kidneys. This is the basis of one of the functional tests used in the clinic in determining the synthetic function of the liver in humans.

absorption mechanisms. The absorption process is e that nutrients penetrate through the intestinal epithelial cells into the blood and lymph. At the same time, one part of the nutrients passes through the epithelium without changing, the other part undergoes synthesis. The movement of substances goes in one direction: from the intestinal cavity to the lymphatic and blood vessels. This is due to the structural features of the mucous membrane of the intestinal wall and the composition of the substances contained in the cells. Define-

Of great importance is the pressure in the intestinal cavity, which partly determines the process of filtering water and solutes into the epithelial cells. With an increase in pressure in the intestinal cavity by 2-3 times, absorption, for example, of sodium chloride solution, increases

At one time, it was believed that the filtration process completely determines the absorption of substances from the intestinal cavity into the epithelial cells. However, this point of view is mechanistic, since it considers the process of absorption, which is the most complex physiological process, firstly, from purely physical principles, secondly, without taking into account the biological specialization of the organs of absorption, and, finally, thirdly, in isolation from the whole organism in in general and the regulatory role of the central nervous system and its higher department - the cerebral cortex. The failure of the filtration theory is already evident from the fact that the pressure in the intestine is approximately equal to 5 mm Hg. Art., and the value of blood pressure inside the capillaries of the villi reaches 30-40 mm Hg. Art., i.e. 6-8 times more than in the intestine. This is also evidenced by the fact that the penetration of nutrients under normal physiological conditions goes only in one direction: from the intestinal cavity to the vessels of the lymph and blood; finally, experiments on animals have proven the dependence of the absorption process on cortical regulation. It has been established that impulses arising from conditioned reflex stimulation can either accelerate or slow down the rate of absorption of substances in the intestine.

Theories that explain the absorption process only by the laws of diffusion and osmosis are also untenable and metaphysical. In physiology, a sufficient number of facts have accumulated that contradict this. So, for example, if you introduce a solution of grape sugar into the intestine of a dog in a concentration lower than the blood sugar content, then at first it is not sugar that is absorbed, but water. Sugar absorption in this case begins only when its concentration in the blood and the intestinal cavity is the same. When a glucose solution is introduced into the intestine in a concentration exceeding the concentration of glucose in the blood, glucose is first absorbed, and then water. In the same way, if highly concentrated solutions are introduced into the intestine

salts, then at first water enters the intestinal cavity from the blood, and then, when the concentration of salts in the intestinal cavity and in the blood (isotony) is equalized, the salt solution is already absorbed. Finally, if blood serum, the osmotic pressure of which corresponds to the osmotic pressure of the blood, is introduced into the ligated section of the intestine, then soon the serum is completely absorbed into the blood.

All these examples indicate the presence of unilateral conduction and specificity for nutrient permeability in the intestinal wall mucosa. Therefore, the phenomenon of absorption cannot be explained solely by the processes of diffusion and osmosis. However, these processes undoubtedly play a role in the absorption of nutrients in the intestine. The processes of diffusion and osmosis occurring in a living organism are radically different from these processes observed under artificially created conditions. The intestinal mucosa cannot be considered, as some researchers did, only as a semi-permeable membrane, a membrane.

The intestinal mucosa, its villous apparatus is such an anatomical formation that is specialized in the process of absorption and its functions are strictly subordinated to the general laws of the living tissue of the whole organism, where any process is regulated by the nervous and endocrine systems.

Table of contents of the topic "Digestion in the Small Intestine. Digestion in the Large Intestine.":
1. Digestion in the small intestine. Secretory function of the small intestine. Brunner's glands. Lieberkuhn's glands. cavity and membrane digestion.
2. Regulation of the secretory function (secretion) of the small intestine. local reflexes.
3. Motor function of the small intestine. rhythmic segmentation. pendulum contractions. peristaltic contractions. tonic contractions.
4. Regulation of motility of the small intestine. myogenic mechanism. motor reflexes. Brake reflexes. Humoral (hormonal) regulation of motility.

6. Digestion in the large intestine. Movement of chyme (food) from the jejunum to the cecum. Bisphincter reflex.
7. Juice secretion in the large intestine. Regulation of sap secretion of the mucous membrane of the large intestine. Enzymes of the large intestine.
8. Motor activity of the large intestine. Peristalsis of the large intestine. peristaltic waves. Antiperistaltic contractions.
9. Microflora of the large intestine. The role of the microflora of the large intestine in the process of digestion and the formation of the immunological reactivity of the body.
10. The act of defecation. Bowel emptying. Defecation reflex. Chair.
11. The immune system of the digestive tract.
12. Nausea. Causes of nausea. Nausea mechanism. Vomit. The act of vomiting. Causes of vomiting. Vomiting mechanism.

general characteristics absorption processes in the digestive tract were outlined in the first topics of the section.

Small intestine is the main part of the digestive tract where suction hydrolysis products of nutrients, vitamins, minerals and water. High speed suction and a large volume of transport of substances through the intestinal mucosa are explained by the large area of ​​​​its contact with chyme due to the presence of macro- and microvilli and their contractile activity, a dense network of capillaries located under the basement membrane of enterocytes and having a large number of wide pores (fenestres) through which they can penetrate large molecules.

Through the pores of the cell membranes of the enterocytes of the mucous membrane of the duodenum and jejunum, water easily penetrates from the chyme into the blood and from the blood into the chyme, since the width of these pores is 0.8 nm, which significantly exceeds the width of the pores in other parts of the intestine. Therefore, the contents of the intestine is isotonic with blood plasma. For the same reason, the main amount of water is absorbed in the upper sections of the small intestine. In this case, water follows osmotically active molecules and ions. These include ions of mineral salts, monosaccharide molecules, amino acids and oligopeptides.

With the fastest speed are absorbed Na+ ions (about 500 m/mol per day). There are two ways of transporting Na + ions - through the membrane of enterocytes and through intercellular channels. They enter the cytoplasm of enterocytes in accordance with the electrochemical gradient. Na+ is transported from the enterocyte to the interstitium and blood by Na+/K+-Hacoca localized in the basolateral part of the enterocyte membrane. In addition to Na +, K + and Cl ions are absorbed through intercellular channels by the diffusion mechanism. High speed suction Cl is due to the fact that they follow the Na + ions.

Rice. 11.14. Diagram of protein digestion and absorption. Dipeptidases and aminopeptidases of the enterocyte microvillus membrane cleave oligopeptides to amino acids and small fragments of the protein molecule, which are transported to the cell cytoplasm, where cytoplasmic peptidases complete the hydrolysis process. Amino acids pass through the basement membrane of the enterocyte into the intercellular space, and then into the blood.

Transport HCO3 is coupled with Na+ transport. In the process of its absorption, in exchange for Na +, the enterocyte secretes H + into the intestinal cavity, which, interacting with HCO3, forms H2CO3. H2CO3 under the influence of the enzyme carbonic anhydrase turns into a molecule of water and CO2. Carbon dioxide is absorbed into the blood and removed from the body with exhaled air.

Ion suction Ca2+ is carried out by a special transport system, which includes the Ca2+-binding protein of the enterocyte brush border and the calcium pump of the basolateral part of the membrane. This explains the relatively high absorption rate of Ca2+ (compared to other divalent ions). At a significant concentration of Ca2+ in the chyme, the volume of its absorption increases due to the diffusion mechanism. Ca2+ absorption is enhanced by parathyroid hormone, vitamin D, and bile acids.

Suction Fe2+ ​​is carried out with the participation of a carrier. In the enterocyte, Fe2+ combines with apoferritin to form ferritin. As part of ferritin, iron is used in the body. Ion suction Zn2+ and Mg+ occurs according to the laws of diffusion.

At a high concentration of monosaccharides (glucose, fructose, galactose, pentose) in the chyme that fills the small intestine, they are absorbed by the mechanism of simple and clothed diffusion. suction mechanism glucose and galactose is active sodium-dependent. Therefore, in the absence of Na +, the rate of absorption of these monosaccharides slows down by 100 times.

The products of protein hydrolysis (amino acids and tripeptides) are absorbed into the blood mainly in the upper part of the small intestine - the duodenum and jejunum (about 80-90%). Main mechanism of amino acid absorption- active sodium-dependent transport. A minority of amino acids are absorbed by diffusion mechanism. Hydrolysis processes and suction cleavage products of the protein molecule are closely related. A small amount of protein is absorbed without splitting to monomers - by pinocytosis. So from the intestinal cavity enter the body of immunoglobulins, enzymes, and in the newborn - proteins contained in breast milk.

Rice. 11.15. Scheme of the transfer of fat hydrolysis products from the intestinal lumen to the enterocyte cytoplasm and to the intercellular space.
Triglycerides are resynthesized from the products of fat hydrolysis (monoglycerides, fatty acids and glycerol) in the smooth endoplasmic reticulum, and chylomicrons are formed in the granular endoplasmic reticulum and the Golgi apparatus. Chylomicrons through the lateral sections of the enterocyte membrane enter the intercellular space, and then into the lymphatic vessel.

Suction process products of hydrolysis of fats (monoglycerides, glycerol and fatty acids) is carried out mainly in the duodenum and jejunum and has significant features.

Monoglycerides, glycerol and fatty acids interact with phospholipids, cholesterol and bile salts to form micelles. On the surface of enterocyte microvilli, the lipid components of the micelle are easily dissolved in the membrane and penetrate into its cytoplasm, while bile salts remain in the intestinal cavity. In the smooth endoplasmic reticulum of the enterocyte, triglycerides are resynthesised, from which the smallest droplets of fat (chylomicrons) are formed in the granular endoplasmic reticulum and the Golgi apparatus with the participation of phospholipids, cholesterol and glycoproteins, the diameter of which is 60-75 nm. Chylomicrons accumulate in secretory vesicles. Their membrane "embeds" into the lateral membrane of the enterocyte, and through the hole formed, chylomicrons enter the intercellular spaces, and then into the lymphatic vessel (Fig. 11.15).