The small intestine contains the duodenum, jejunum, and ileum.

The duodenum is not only involved in the secretion of intestinal juice with a high content of bicarbonate ions, but is also the dominant zone of digestion regulation..

It is the duodenum that sets a certain rhythm to the distal parts of the digestive tract through nervous, humoral and intracavitary mechanisms..

Together with the antrum of the stomach, the duodenum, jejunum and ileum constitute an important single endocrine organ.. The duodenum is part of the contractile (motor) complex, generally consisting of the antrum, pyloric canal, duodenum, and sphincter of Oddi.

It takes in the acidic contents of the stomach, secretes its secrets, changes the pH of the chyme to the alkaline side.. The contents of the stomach affect the endocrine cells and nerve endings of the mucous membrane of the duodenum, which ensures the coordinating role of the antrum of the stomach and duodenum, as well as the relationship of the stomach, pancreas, liver, small intestine.

Outside of digestion, on an empty stomach, the contents of the duodenum have a slightly alkaline reaction (pH 7.2-8.0). When portions of acidic contents from the stomach pass into it, the reaction of the duodenal contents also becomes acidic, but then it quickly changes, since the hydrochloric acid of the gastric juice is neutralized here by bile, pancreatic juice, as well as duodenal (Brunner) glands and intestinal crypts (Lieberkun glands.

This stops the action of gastric pepsin.. The higher the acidity of the duodenal contents, the more pancreatic juice and bile are secreted, and the more the evacuation of the contents of the stomach into the duodenum slows down.. In the hydrolysis of nutrients in the duodenum, the role of enzymes of pancreatic juice, bile is especially great..

Digestion in the small intestine is the most important step in the overall digestive process.. It ensures the depolymerization of nutrients to the stage of monomers, which are absorbed from the intestines into the blood and lymph..

Digestion in the small intestine occurs first in its cavity (abdominal digestion), and then in the zone of the brush border of the intestinal epithelium with the help of enzymes embedded in the membrane of microvilli of intestinal cells, as well as fixed in the glycocalyx (membrane digestion). Cavitary and membrane digestion is carried out by enzymes supplied with pancreatic juice, as well as intestinal enzymes proper (membrane or transmembrane). Bile plays an important role in the breakdown of lipids..

For humans, the combination of cavitary and membrane digestion is most characteristic.. The initial stages of hydrolysis are carried out by cavitary digestion. Most of the supramolecular complexes and large molecules (proteins and products of their incomplete hydrolysis, carbohydrates, fats) are cleaved in the cavity of the small intestine in neutral and slightly alkaline environments, mainly under the action of endohydrolases secreted by pancreatic cells..

Some of these enzymes may be adsorbed on mucus structures or mucosal overlays.. Peptides formed in the proximal intestine and consisting of 2-6 amino acid residues provide 60-70% of ?-amino nitrogen, and in the distal part of the intestine - up to 50%.

Carbohydrates (polysaccharides, starch, glycogen) are broken down by pancreatic ?-amylase to dextrins, tri- and disaccharides without significant accumulation of glucose. Fats are hydrolyzed in the cavity of the small intestine by pancreatic lipase, which gradually cleaves off fatty acids, which leads to the formation of di- and monoglycerides, free fatty acids and glycerol.. Bile plays an important role in the hydrolysis of fats..

The products of partial hydrolysis formed in the cavity of the small intestine, due to intestinal motility, come from the cavity of the small intestine to the zone of the brush border, which is facilitated by their transfer in the flows of the solvent (water) resulting from the absorption of sodium and water ions.. It is on the structures of the brush border that membrane digestion occurs..

At the same time, the intermediate stages of hydrolysis of biopolymers are realized by pancreatic enzymes adsorbed on the structures of the apical surface of enterocytes (glycocalix), and the final stages are carried out by intestinal membrane enzymes proper (maltase, sucrase, a-amylase, isomaltase, trehalase, aminopeptidase, tri- and dipeptidases, alkaline phosphatase,. ), embedded in the enterocyte membrane covering the microvilli of the brush border. Some enzymes (a-amylase and aminopeptidase) also hydrolyze highly polymerized products.

Peptides entering the area of \u200b\u200bthe brush border of intestinal cells are cleaved to oligopeptides, dipeptides and amino acids capable of absorption.. Peptides consisting of more than three amino acid residues are hydrolyzed mainly by brush border enzymes, while tri- and dipeptides are hydrolyzed both by brush border enzymes and intracellularly by cytoplasmic enzymes..

Glycylglycine and some dipeptides containing proline and hydroxyproline residues and not having a significant nutritional value are absorbed partially or completely in an unsplit form.

Disaccharides from food (for example, sucrose), as well as those formed during the breakdown of starch and glycogen, are hydrolyzed by intestinal glycosidases proper to monosaccharides, which are transported through the intestinal barrier into the internal environment of the body. Triglycerides are broken down not only by pancreatic lipase, but also by intestinal monoglyceride lipase..

Secretion.

In the mucous membrane of the small intestine there are glandular cells located on the villi that produce digestive secrets that are secreted into the intestine. Brunner's glands of the duodenum, Lieberkun's crypts of the jejunum, goblet cells. Endocrine cells produce hormones that enter the intercellular space, and from where they are transported to the lymph and blood.

Cells with acidophilic granules in the cytoplasm (Paneth cells) that secrete a protein secret are also localized here.. The volume of intestinal juice (normally up to 2.5 liters) may increase with local exposure to certain food or toxic substances on the intestinal mucosa. Progressive dystrophy and atrophy of the mucous membrane of the small intestine are accompanied by a decrease in the secretion of intestinal juice..

Glandular cells form and accumulate a secret and, at a certain stage of their activity, are rejected into the intestinal lumen, where, disintegrating, they give this secret to the surrounding fluid.. Juice can be divided into liquid and solid parts, the ratio between which varies depending on the strength and nature of the irritation of the intestinal cells..

The liquid part of the juice contains about 20 g/l of dry matter, which consists partly of the contents of desquamated cells coming from organic blood (mucus, proteins, urea, etc.).. ) and inorganic substances - approximately 10 g / l (such as bicarbonates, chlorides, phosphates). The dense part of the intestinal juice looks like mucous lumps and consists of undestroyed desquamated epithelial cells, their fragments and mucus (goblet cell secretion).

In healthy people, periodic secretion is characterized by relative qualitative and quantitative stability, which contributes to maintaining the homeostasis of the enteric environment, which is primarily chyme..

According to some calculations, in an adult with digestive juices, up to 140 g of protein per day enters food, another 25 g of protein substrates is formed as a result of desquamation of the intestinal epithelium.

It is not difficult to imagine the significance of protein losses that can occur with prolonged and severe diarrhea, with any form of indigestion, pathological conditions associated with enteral insufficiency - increased intestinal secretion and impaired reabsorption (reabsorption).

The mucus produced by the goblet cells of the small intestine is an important component of secretory activity.. The number of goblet cells in the villi is greater than in the crypts (up to approximately 70%), and increases in the distal small intestine. This seems to reflect the importance of the non-digestive functions of mucus..

It has been established that the cellular epithelium of the small intestine is covered with a continuous heterogeneous layer up to 50 times the height of the enterocyte.. This epithelial layer of mucous overlays contains a significant amount of adsorbed pancreatic and a small amount of intestinal enzymes that implement the digestive function of mucus..

The mucous secretion is rich in acidic and neutral mucopolysaccharides, but poor in proteins.. This ensures the cytoprotective consistency of the mucous gel, mechanical, chemical protection of the mucous membrane, prevention of penetration into the deep tissue structures of large molecular compounds and antigenic aggressors..

Suction.

Absorption is understood as a set of processes, as a result of which food components contained in the digestive cavities are transferred through the cell layers and intercellular pathways into the internal circulatory environments of the body - blood and lymph..

The main organ of absorption is the small intestine, although some food components can be absorbed in the large intestine, stomach, and even the oral cavity.. Nutrients coming from the small intestine are carried throughout the body with the blood and lymph flow and then participate in the intermediate (intermediate) metabolism..

Up to 8-9 liters of fluid are absorbed per day in the gastrointestinal tract. Of these, approximately 2.5 liters comes from food and drink, the rest is the liquid secretions of the digestive apparatus..

The absorption of most nutrients occurs after their enzymatic processing and depolymerization, which occur both in the cavity of the small intestine and on its surface due to membrane digestion..

Within 3-7 hours after eating, all its main components disappear from the cavity of the small intestine.. The intensity of absorption of nutrients in different parts of the small intestine is not the same and depends on the topography of the corresponding enzymatic and transport activities along the intestinal tube (Fig.. four).

There are two types of transport through the intestinal barrier into the internal environment of the body. These are transmembrane (transcellular, through the cell) and paracellular (shunt, going through the intercellular spaces).

The main type of transport is transmembrane. Conventionally, two types of transmembrane transport of substances through biological membranes can be distinguished - these are macromolecular and micromolecular.. Macromolecular transport refers to the transfer of large molecules and molecular aggregates through cell layers..

This transport is discontinuous and occurs primarily through pinocytosis and phagocytosis, collectively referred to as endocytosis.. Due to this mechanism, proteins, including antibodies, allergens and some other compounds that are important for the body, can enter the body..

Micromolecular transport is the main type, as a result of which the products of hydrolysis of nutrients, mainly monomers, various ions, drugs and other compounds with a small molecular weight, are transferred from the intestinal environment to the internal environment of the body.. The transport of carbohydrates across the plasma membrane of intestinal cells occurs in the form of monosaccharides (glucose, galactose, fructose, etc.).. ), proteins - mainly in the form of amino acids, fats - in the form of glycerol and fatty acids.

During the transmembrane movement, the substance crosses the microvilli membrane of the brush border of intestinal cells, enters the cytoplasm, then through the basolateral membrane - into the lymphatic and blood vessels of the intestinal villi and then into the general circulation system. The cytoplasm of the intestinal cells serves as a compartment forming a gradient between the brush border and the basolateral membrane..

In micromolecular transport, in turn, it is customary to distinguish between passive and active transport.. Passive transport can occur due to the diffusion of substances through a membrane or water pores along a concentration gradient, osmotic or hydrostatic pressure..

It is accelerated due to water flows moving through the pores, changes in the pH gradient, as well as transporters in the membrane (in the case of facilitated diffusion, their work is carried out without energy consumption). Exchange diffusion provides microcirculation of ions between the cell periphery and the surrounding microenvironment.

Facilitated diffusion is realized with the help of special transporters - special protein molecules (specific transport proteins), which contribute to the penetration of substances through the cell membrane without energy expenditure due to the concentration gradient.

The actively transported substance moves through the apical membrane of the intestinal cell against its electromechanical gradient with the participation of special transport systems that function as mobile or conformational transporters (carriers) with energy consumption. This is how active transport differs sharply from facilitated diffusion..

The transport of most organic monomers across the brush border membrane of intestinal cells depends on sodium ions.. This is true for glucose, galactose, lactate, most amino acids, some conjugated bile acids, and a number of other compounds.. The driving force of this transport is the concentration gradient of Na+. However, in the cells of the small intestine, there is not only a Na+-dependent transport system, but also a Na+-independent one, which is characteristic of some amino acids..

Water is absorbed from the intestines into the blood and flows back according to the laws of osmosis, but most of it is from isotonic solutions of intestinal chyme, since hyper- and hypotonic solutions are quickly diluted or concentrated in the intestines..

The absorption of sodium ions in the intestine occurs both through the basolateral membrane into the intercellular space and further into the blood, and transcellularly.. During the day, 5-8 g of sodium enters the human digestive tract with food, 20-30 g of this ion is secreted with digestive juices (t. only 25-35 g). Part of the sodium ions are absorbed together with chloride ions, as well as during the oppositely directed transport of potassium ions due to Na+, K+-ATPase.

Absorption of divalent ions (Ca2+, Mg2+, Zn2+, Fe2+) occurs along the entire length of the gastrointestinal tract, and Cu2+ occurs mainly in the stomach. Divalent ions are absorbed very slowly. Ca2+ absorption most actively occurs in the duodenum and jejunum with the participation of simple and facilitated diffusion mechanisms, it is activated by vitamin D, pancreatic juice, bile and a number of other compounds..

Carbohydrates are absorbed in the small intestine in the form of monosaccharides (glucose, fructose, galactose). Glucose absorption occurs actively with the expenditure of energy. At present, the molecular structure of the Na+-dependent glucose transporter is already known.. It is a high molecular weight protein oligomer with extracellular loops that has glucose and sodium binding sites..

Proteins are absorbed through the apical membrane of intestinal cells mainly in the form of amino acids and to a much lesser extent in the form of dipeptides and tripeptides.. As with monosaccharides, the energy for amino acid transport is provided by the sodium cotransporter..

In the brush border of enterocytes, there are at least six Na+-dependent transport systems for various amino acids and three independent of sodium.. The peptide (or amino acid) transporter, like the glucose transporter, is an oligomeric glycosylated protein with an extracellular loop.

As for the absorption of peptides, or the so-called peptide transport, the absorption of intact proteins takes place in the small intestine in the early stages of postnatal development.. It is now accepted that, in general, the absorption of intact proteins is a physiological process necessary for the selection of antigens by subepithelial structures..

However, against the background of the general intake of food proteins mainly in the form of amino acids, this process has a very small nutritional value.. A number of dipeptides can enter the cytoplasm by a transmembrane route, like some tripeptides, and be cleaved intracellularly.

Lipids are transported differently. Long-chain fatty acids and glycerol formed during the hydrolysis of food fats are practically passively transferred through the apical membrane to the enterocyte, where they are resynthesized into triglycerides and enclosed in a lipoprotein shell, the protein component of which is synthesized in the enterocyte..

Thus, a chylomicron is formed, which is transported to the central lymphatic vessel of the intestinal villi and then enters the blood through the thoracic lymphatic duct system.. Medium-chain and short-chain fatty acids enter the bloodstream immediately, without resynthesis of triglycerides.

The rate of absorption in the small intestine depends on the level of its blood supply (affects the processes of active transport), the level of intra-intestinal pressure (affects the processes of filtration from the intestinal lumen) and the topography of absorption.

medbe. en.