What is absorbed in the human small intestine. Physiological mechanisms of absorption in the intestine
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. Thanks to this process, the body receives the nutrients necessary for life.
In the upper parts of the digestive tube (mouth, esophagus, stomach) absorption is very insignificant. In the stomach, for example, only water, alcohol, some salts and carbohydrate breakdown products are absorbed, and in small quantities. Minor absorption also occurs in the duodenum.
The bulk of nutrients are absorbed in the small intestine, and absorption occurs in different parts of the intestine at different rates. 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
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Absorption of substances in the intestinal region, % |
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Substances |
25 cm lower |
2-3 cm up |
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gatekeeper |
superior to the cecum |
from the cecum |
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Alcohol | |||
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Grape sugar | |||
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Starch paste | |||
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Palmitic acid | |||
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Butyric acid | |||
In the walls of the small intestine there are special absorption organs - villi (Fig. 48).
The total surface of the intestinal mucosa in humans is approximately 0.65 m2, and due to the presence of villi (18-40 per 1 mm2) it reaches 5 m2. This is approximately 3 times the outer surface of the body. According to Verzar, a dog has about 1,000,000 villi in its small intestine.
Rice. 48. Cross section of the human small intestine:
/ - villus with nerve plexus; d - central milk vessel of the villi with smooth muscle cells; 3 - Lieberkühn 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 - layer of longitudinal muscle fibers; 12 - serous membrane
The height of the villi is 0.2-1 mm, width 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, due to which the surface of the epithelium and its contact with the blood flowing in the capillaries significantly increases. The villi contain a lymphatic vessel with valves that open only in one direction. 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. In 1 minute, all villi can absorb 15-20 ml of liquid from the intestine (Verzar). Lymph from the lymphatic vessel of the villi 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 1 minute.
Contractions of the villi occur under the influence of mechanical and chemical irritations of substances located in the intestinal cavity, for example peptones, albumin, leucine, alanine, extractives, glucose, bile acids. The movement of the villi is also stimulated by the humoral route. It has been proven that a specific hormone, villikinin, is formed in the mucous membrane of the duodenum, which is carried by the bloodstream to the villi and stimulates their movements. The effect of the hormone and nutrients on the muscles of the villi occurs, apparently, with the participation of nerve elements embedded in the villi itself. According to some data, the Meissnerian plexus, located in the submucosal layer, takes part in this process. When the intestine is isolated from the body, the movements of the villi stop after 10-15 minutes.
In the large intestine, absorption of nutrients under normal physiological conditions is possible, but in small amounts, as well as substances that are easily broken down and well absorbed. This is the basis for the use of nutritional enemas in medical practice.
Water is absorbed quite well in the large intestine, which is why stool acquires a dense consistency. When the absorption process in the large intestine is disrupted, loose stool appears.
E. S. London developed an angiostomy technique, with the help of which it was possible to study some important aspects of the absorption process. This technique consists of sewing the end of a special cannula to the stitches of large vessels, and 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, by piercing the vessel wall with a long needle, can obtain blood from the animal for biochemical analysis at any time during digestion. Using this technique, E. S. London found that the products of protein breakdown are absorbed primarily in the initial parts of the small intestine; their absorption in the large intestines is small. Typically, animal protein is digested and absorbed from 95 to 99%,
and vegetable - from 75 to 80%. The following products of protein breakdown are absorbed in the intestine: amino acids, di- and polypeptides, peptones and albumoses. Undigested proteins can also be absorbed in small quantities: serum proteins, egg and milk proteins - casein. The amount of absorbed unsplit proteins can be significant in young children (R. O. Faitelberg). 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 increased 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 Thai Luong).
The rate of absorption of amino acids is influenced by some endocrine glands. Administration of thyroxine, cortisone, pituitrin, and ACTH to animals led to a change in the rate of absorption, but the nature of the change depended on the doses of these hormonal drugs and the duration of their use (N. N. Kalashnikova). The rate of absorption of secretin and pancreozymin changes. It has been shown that amino acid transport occurs not only through the apical membrane of the enterocyte, but also through the entire cell. Subcellular organelles (in particular, mitochondria) participate in this process. The rate of absorption of undigested proteins is influenced by many factors, including intestinal pathology, the amount of proteins administered, intraintestinal pressure, and excess intake of whole proteins into the blood. All this can lead to sensitization of the body and the development of allergic diseases.
Carbohydrates, absorbed in the form of monosaccharides (glucose, levulose, galactose) and partly disaccharides, directly enter the blood, from which they are delivered to the liver, where they are synthesized into glycogen. Absorption occurs very slowly, and the rate of absorption of different carbohydrates is not the same. If monosaccharides (glucose) combine with phosphoric acid in the wall of the small intestine (phosphorylation), absorption is accelerated. This is proven by the fact that when an animal is poisoned with monoiodoacetic acid, which inhibits the phosphorylation of carbohydrates, their absorption is significantly
slows down. Absorption varies in different parts of the intestine. Based on 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 absorbed to the greatest extent in the duodenum; maltose - in skinny; sucrose - in the distal part of the jejunum and ileum. In dogs, the involvement of different parts of the intestine is basically the same as in humans.
The cerebral cortex takes part in regulating the process of carbohydrate absorption in the small intestine. Thus, A.V. Rikkl developed conditioned reflexes both to enhance suction and to delay. The intensity of absorption changes during food stimulation, during 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, pharmacological agents, irritation with current of different cortical areas in dogs with implanted electrodes in the frontal region, parietal, temporal, occipital and posterior limbic areas of the cerebral cortex (R . O. Faitelberg). The effect depended on the nature of the shift in the functional state of the cerebral cortex, in experiments with the use of pharmacological drugs, on the areas of the cortex exposed to current irritation, as well as on the strength of irritation. In particular, the limbic cortex has been found to be of greater importance in the regulation of the absorption function of the small intestine.
What is the mechanism of involvement of the cerebral cortex in the regulation of absorption? Currently, there is reason to believe that information to the central nervous system about the ongoing process of absorption in the intestine is carried by impulses that arise both in the receptors of the digestive tract and in the blood vessels, the latter being irritated by chemicals entering the bloodstream from the intestine.
Subcortical structures play an important role in the regulation of absorption in the small intestine. With stimulation of the lateral and posteroventral nuclei of the thalamus, changes in sugar absorption were unequal: with stimulation of the former, a weakening was observed, and with stimulation of the latter, an increase. Changes in absorption intensity were observed with different
irritation by electric current in the subtubercular region (P. G. Bogach).
Thus, the participation of subcortical formations in the 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 using chlorpromazine, which blocks the adrenoreactive structures of the reticular formation. The cerebellum is involved in the regulation of absorption, contributing 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 autonomic part of the nervous system. This is evidenced by the fact that turning off or irritating the vagus or splanchnic nerves significantly, but not unidirectionally, changes the intensity of absorption (in particular, glucose).
Endocrine glands also participate in the regulation of absorption. Disruption of the adrenal glands affects the absorption of carbohydrates in the small intestine. The introduction of cortin and 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 observed with the administration of antithyroid substances (6-MTU). There is some reason to admit 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 being broken down 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 secreted by liver cells with bile and thus can again take part in the process of fat absorption. The absorbed products of fat breakdown in the epithelium of the intestinal mucosa are again synthesized into fat.
R. O. Faitelberg believes that the absorption process consists of four stages: transport of cavity products -
Rice. 49. Neuroendocrine regulation of absorption processes in the intestine (according to R. O. Faitelberg and Nguyen Thai Luong): Black arrows - afferent information, white - efferent transmission of impulses, shaded - hormonal regulation
lipolysis 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 fat absorption probably depends on the synchronicity of the operation 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 occurs better than each one separately.
Neutral fats absorbed in the intestines enter the blood through the lymphatic vessels into the large thoracic duct. Fats such as butter and pork fat are absorbed up to 98%, and stearin and spermaceti - up to 9-15%. If you open the abdominal cavity of an animal 3-4 hours after eating fatty food (milk), you can easily see with the naked eye the lymphatic vessels of the intestinal mesentery 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 into the blood. This can be verified by ligating the animal's thoracic lymphatic duct. Then the fat content in the blood increases sharply.
Water enters the gastrointestinal tract in large quantities. An adult's daily water consumption reaches 2 liters. During the day, a person secretes up to 5-6 liters of digestive juices into the stomach and intestines (saliva - 1 liter, gastric juice - 1.5-2 liters, bile - 0.75-1 liters, pancreatic juice - 0.7-0 .8 l, intestinal juice - 2 l). Only about 150 ml is excreted from the intestines. Absorption of water occurs partially in the stomach, more intensely in the small and especially large intestines.
Solutions of salts, mainly table salt, are absorbed quite quickly if they are hypotonic. When the concentration of table salt is 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. With a high concentration of salts, 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 across enterocytes
(according to R. O. Feitelberg and Nguyen Thai Luong)
Nick. The clinical use of certain concentrated salts as laxatives is based on this principle.
The role of the liver in the absorption process. It is known that blood from the vessels of the walls of the stomach and intestines enters through the portal vein into the liver, and then through the hepatic veins into the inferior vena cava and further into the general circulation. Toxic substances formed in the intestines during the rotting of food (indole, skatole, thyramine, etc.) and absorbed into the blood are neutralized in the liver by adding sulfuric and glucuronic acids to them and forming slightly toxic ester-sulfuric acids. This is the barrier function of the liver. It was clarified by I.P. Pavlov and V.N. Eck, who performed the following original operation on animals, called the Pavlov-Eck operation. The portal vein is connected by anastomosis to 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 within a few days due to poisoning by toxic substances absorbed in the intestines. Feeding animals with meat leads to death especially quickly.
The liver is an organ in which a number of synthetic processes occur: 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. When sodium benzoate is introduced into the gastrointestinal canal, it is neutralized in the liver 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 clinically to determine the synthetic function of the liver in humans.
Suction mechanisms. The absorption process consists e that nutrients penetrate through the intestinal epithelial cells into the blood and lymph. In this case, one part of the nutrients passes through the epithelium without changing, the other 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 particular importance is the pressure in the intestinal cavity, which partly determines the process of filtering water and dissolved substances into the epithelial cells. When the pressure in the intestinal cavity increases by 2-3 times, absorption, for example, of table salt 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 a complex physiological process, firstly, from purely physical principles, secondly, without taking into account the biological specialization of the absorption organs 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 inconsistency of the filtration theory is already evident from the facts that the pressure in the intestine is approximately 5 mm Hg. Art., and the 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 occurs in only one direction: from the intestinal cavity to the lymph and blood vessels; finally, animal experiments 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. Physiology has accumulated a sufficient number of facts that contradict this. So, for example, if you introduce a solution of grape sugar into a dog’s intestine in a concentration lower than the sugar content in the blood, then first absorption occurs not of sugar, but of water. In this case, sugar absorption begins only when its concentration in the blood and 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 first water enters the intestinal cavity from the blood, and then, when the concentration of salts in the intestinal cavity and in the blood is equalized (isotonia), the salt solution is absorbed. Finally, if blood serum is injected into the bandaged area of the intestine, the osmotic pressure of which corresponds to the osmotic pressure of the blood, then the serum is soon completely absorbed into the blood.
All these examples indicate the presence in the mucous membrane of the intestinal wall of one-way conduction and specificity for the permeability of nutrients. Therefore, it is impossible to explain the phenomenon of absorption 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 fundamentally different from these processes observed in artificially created conditions. The intestinal mucosa cannot be considered, as some researchers did, only as a semi-permeable membrane, a membrane.
The intestinal mucosa and its villous apparatus are an anatomical formation that is specialized for the process of absorption and its functions are strictly subordinate to the general laws of living tissue of the whole organism, where any process is regulated by the nervous and endocrine systems.
S.T. Metelsky Doctor of Biological Sciences, Chief Researcher of the State Research Institute of General Pathology and Pathophysiology of the Russian Academy of Medical Sciences; contact information for correspondence - This email address is being protected from spambots. You must have JavaScript enabled to view it.; Moscow, 125315, Baltiyskaya 8.
Purpose of the lecture. Consider the physiological mechanisms of absorption in gastrointestinal tract(Gastrointestinal tract).
Basic provisions. In the literature, these issues are covered from three sides: 1) topography of absorption of substances in various parts of the gastrointestinal tract - stomach, duodenum, jejunum, ileum and colon; 2) the main functions of enterocytes; 3) the main mechanisms of absorption in the intestine. 7 main mechanisms of absorption of substances in the intestine are considered.
Conclusion. Of the entire gastrointestinal tract, the jejunum and ileum are characterized by the widest spectrum of absorption of various compounds. Understanding the physiological mechanisms of absorption in the small intestine is of great importance in practical gastroenterology.
Key words:
Absorption, ions, sodium, nutrients, gastrointestinal tract, simple diffusion, facilitated diffusion, osmosis, filtration, pericellular transport, active transport, coupled transport, secondary energized transport, endocytosis, transcytosis, P-glycoprotein.
Basic mechanisms of absorption
The wall of the small intestine, where the most intensive absorption of essential nutrients, or nutrients, occurs, consists of the mucosa (villi and intestinal glands), submucosa (where the blood and lymphatic vessels are located), the muscular layer (where the nerve fibers are located) and the serosa. The mucous membrane is formed by villi, covered with single-layer epithelium interspersed with goblet cells; Lymphatic vessels, a capillary network, and nerve fibers pass inside the villi.A characteristic feature of the transport of substances in the epithelium of the small intestine is that it occurs through a monolayer of cells. The absorption surface of such a monolayer is significantly increased due to microvilli. Enterocytes of the small intestine, where the absorption of nutrients (nutrients) mainly occurs, are asymmetrical, or polarized: the apical and basement membranes differ from each other in permeability, a set of enzymes, the magnitude of the electrical potential difference and perform unequal transport functions.
Ions enter cells using ion channels or special molecular machines - pumps. Energy for the entry of ions into the cell is usually provided through the plasma membrane by an electrochemical sodium gradient generated and maintained by the functioning of the Na + , K + -ATPase pump. This pump is localized on the basolateral membrane facing the blood (Fig. 1).
The energy that can be obtained from the Na + electrochemical potential (ion concentration difference + electrical potential difference across the membrane) and which is released when incoming sodium crosses the plasma membrane can be used by other transport systems. Consequently, the Na + , K + -ATPase pump performs two important functions - it pumps Na + out of cells and generates an electrochemical gradient that provides energy to the solute entry mechanisms.
The term “absorption” refers to a set of processes that ensure the transfer of substances from the intestinal lumen through the epithelial layer into the blood and lymph; secretion is a movement in the opposite direction.
Absorption in various parts of the gastrointestinal tract
20% of alcohol consumed is absorbed in the stomach, as well as short-chain fatty acids. IN duodenum– vitamins A and B1, iron, calcium, glycerin, fatty acids, monoglycerides, amino acids, mono- and disaccharides. IN jejunum– glucose, galactose, amino acids and dipeptides, glycerol and fatty acids, mono- and diglycerides, copper, zinc, potassium, calcium, magnesium, phosphorus, iodine, iron, fat-soluble vitamins D, E and K, a significant part of the B vitamin complex, vitamin C and alcohol residues. IN ileum– disaccharides, sodium, potassium, chloride, calcium, magnesium, phosphorus, iodine, vitamins C, D, E, K, B 1, B 2, B 6, B 12 and most of the water. In the large intestine - sodium, potassium, water, gases, some fatty acids formed during the metabolism of plant fibers and undigested starch, vitamins synthesized by bacteria - biotin (vitamin H) and vitamin K.
Main functions of enterocytes
The main functions of enterocytes include the following.Ion absorption, including sodium, calcium, magnesium and iron, according to the mechanism of their active transport.
Water absorption(transcellular or pericellular) - occurs due to the osmotic gradient formed and maintained by ion pumps, in particular Na +, K + -ATPase.
Absorption of sugars. Enzymes (polysaccharidases and disaccharidases) located in the glycocalyx break down large sugar molecules into smaller ones, which are then absorbed. Glucose is transported across the apical membrane of the enterocyte using the Na+-dependent glucose transporter. Glucose moves through the cytosol (cytoplasm) and leaves the enterocyte through the basolateral membrane (into the capillary system) using the GLUT-2 transporter. Galactose is transported using the same transport system. Fructose crosses the apical membrane of the enterocyte using the GLUT-5 transporter.
Absorption of peptides and amino acids. In the glycocalyx, peptidase enzymes break down proteins into amino acids and small peptides. Enteropeptidases activate the conversion of pancreatic trypsinogen to trypsin, which, in turn, activates other pancreatic zymogens.
Lipid absorption. Lipids - triglycerides and phospholipids - are broken down and passively diffuse into enterocytes, and free and esterified sterols are absorbed in mixed micelles (see below). Small lipid molecules are transported into the intestinal capillaries through tight junctions. Sterols that enter the enterocyte, including cholesterol, are esterified by the enzyme acyl-CoA: cholesterol acyltransferase (ACAT), together with resynthesized triglycerides, phospholipids and apolipoproteins, is included in chylomicrons, which are secreted into the lymph and then into the bloodstream.
Resorption of unconjugated bile salts. Bile that enters the intestinal lumen and is not used in the process of lipid emulsification is reabsorbed in the ileum. The process is known as enterohepatic circulation.
Vitamin absorption. For the absorption of vitamins, as a rule, the absorption mechanisms of other substances are used. A special mechanism exists for the absorption of vitamin B12 (see below).
Secretion of immunoglobulins. IgA from mucosal plasma cells is absorbed through the basolateral surface through the mechanism of receptor-mediated endocytosis and released into the intestinal lumen as a receptor-IgA complex. The presence of a receptor gives the molecule additional stability.
Basic mechanisms of absorption of compounds in the intestine
In Fig. 2 presents the main mechanisms of absorption of substances. Let us consider these mechanisms in more detail.Presystemic metabolism, or metabolism (effect) of the first passage of the intestinal wall. A phenomenon in which the concentration of a substance decreases sharply before entering the bloodstream. However, if the administered substance is a substrate of P-glycoprotein (see below), its molecules can be repeatedly transferred into and out of enterocytes, as a result of which the likelihood of metabolism of this compound in enterocytes increases.
P-glycoprotein is highly expressed in normal cells lining the intestine, proximal tubules of the kidneys, capillaries of the blood-brain barrier, and liver cells. P-glycoprotein transporters are members of the largest and most ancient transporter superfamily, present in organisms from prokaryotes to humans. These are transmembrane proteins whose function is to transport a wide range of
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Passive transport of substances through the epithelial layer. Passive transport of substances through a monolayer of enterocytes occurs without the expenditure of free energy and can be carried out either transcellular or pericellular. This type of transport includes simple diffusion (Fig. 3), osmosis (Fig. 4) and filtration (Fig. 5). The driving force for the diffusion of solute molecules is its concentration gradient.
The dependence of the rate of diffusion of a substance on its concentration is linear. Diffusion is the least specific and, apparently, the slowest transport process. With osmosis, which is a type of diffusion transfer, movement occurs in accordance with the concentration gradient of free (not associated with the substance) solvent (water) molecules.
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Pericellular transport- This is the transport of connections between cells through the area of tight junctions (Fig. 7), it does not require energy expenditure. The structure and permeability of tight junctions of the small intestine are currently being actively studied and debated. For example, it is known that claudin-2 is responsible for the selectivity of tight junctions for sodium.
Another possibility is that intercellular transfer occurs due to some defect in the epithelial layer. Such movement can occur along intercellular areas in those places where desquamation of individual cells occurs. This path may be a gateway for the penetration of foreign macromolecules directly into the blood or tissue fluids.
Endocytosis, exocytosis, receptor-mediated transport(Fig. 8) and transcytosis. Endocytosis is the vesicular uptake of fluid, macromolecules or small particles into the cell. There are three mechanisms of endocytosis: pinocytosis (from the Greek words "to drink" and "cell"), phagocytosis (from the Greek words "to eat" and "cell"), and receptor-mediated endocytosis or clathrin-dependent endocytosis. Violations of this mechanism lead to the development of certain diseases. Many intestinal toxins, in particular cholera, enter enterocytes precisely through this mechanism.
During pinocytosis, the flexible plasma membrane forms an invagination (invagination) in the form of a pit. Such a hole is filled with liquid from the external environment. Then it detaches from the membrane and moves into the cytoplasm in the form of a vesicle, where its membrane walls are digested and the contents are released. Thanks to this process, cells can absorb both large molecules and various ions that are not able to penetrate the membrane on their own. Pinocytosis is often observed in cells whose function is related to absorption. This is an extremely intensive process: in some cells, 100% of the plasma membrane is absorbed and restored in just an hour.
During phagocytosis (a phenomenon discovered by the Russian scientist I.I. Mechnikov in 1882), the outgrowths of the cytoplasm capture droplets of liquid containing any dense (living or nonliving) particles (up to 0.5 microns), and draw them into the thickness of the cytoplasm, where hydrolyzing enzymes digest the absorbed material, breaking it down into fragments that can be absorbed by the cell. Phagocytosis occurs via a clathrin-independent actin-dependent mechanism; This is the main mechanism of defense of the host body against microorganisms. Phagocytosis of damaged or aged cells is necessary for tissue renewal and wound healing.
In receptor-mediated endocytosis (see Fig. 8), specific surface receptors are used to transfer molecules. This mechanism has the following properties: specificity, the ability to concentrate the ligand on the cell surface, and refractoriness. If a specific receptor, after binding the ligand and its uptake, does not return to the membrane, the cell becomes refractory to this ligand.
With the help of the endocytotic vesicular mechanism, both high-molecular-weight compounds such as vitamin B12, ferritin and hemoglobin, and low-molecular-weight compounds such as calcium, iron, etc. are absorbed. The role of endocytosis is especially great in the early postnatal period. In an adult, the pinocytotic type of absorption apparently does not have a significant role in providing the body with nutrients.
Transcytosis is a mechanism by which molecules that enter a cell from outside can be delivered to various compartments within the cell or even move from one layer of cells to another. One well-studied example of transcytosis is the penetration of some maternal immunoglobulins through the intestinal epithelial cells of the newborn. Maternal antibodies enter the child's body with milk. Antibodies bound to the appropriate receptors are sorted into early endosomes of digestive tract cells, then, with the help of other vesicles, pass through the epithelial cell and fuse with the plasma membrane at the basolateral surface. Here the ligands are released from the receptors. The immunoglobulins then collect in the lymphatic vessels and enter the newborn's bloodstream.
A review of absorption mechanisms from the point of view of individual groups of substances and compounds will be presented in one of the next issues of the journal.
The work was supported by the Russian Foundation for Basic Research grant 09-04-01698
References:
1. Metelsky S.T. Transport processes and membrane digestion in the mucous membrane of the small intestine. Electrophysiological model. – M.: Anacharsis, 2007. – 272 p.
2. General course of human and animal physiology. - Book 2. Physiology of visceral systems / Ed. HELL. Nozdracheva. – M.: Higher School, 1991. – P. 356–404.
3. Membrane digestion. New facts and concepts / Ed. A.M. Ugolev. – M.: MIR Publishers, 1989. – 288 p.
4. Tansey T., Christie D.A., Tansey E.M. Intestinal absorption. – London: Wellcome Trust, 2000. – 81 p
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What is absorbed in the small intestine
Absorption function of the gastrointestinal tract
Absorption is the physiological process of transferring substances from the lumen of the gastrointestinal tract into the internal environment of the body (blood, lymph, tissue fluid).
The total amount of fluid reabsorbed daily in the gastrointestinal tract is 8-9 liters (about 1.5 liters of fluid is consumed with food, the rest is fluid from the secretions of the digestive glands).
Absorption occurs in all parts of the digestive tract, but the intensity of this process in different parts is not the same.
In the oral cavity, absorption is insignificant due to the short-term presence of food here.
The stomach absorbs water, alcohol, small amounts of some salts and monosaccharides.
The small intestine is the main section of the digestive tract, where water, mineral salts, vitamins and hydrolysis products of substances are absorbed. In this section of the digestive tube, the rate of transfer of substances is extremely high. Already 1-2 minutes after food substrates enter the intestine, they appear in the blood flowing from the mucous membrane, and after 5-10 minutes the concentration of nutrients in the blood reaches its maximum values. Part of the liquid (about 1.5 l) along with chyme enters the large intestine, where almost all of it is absorbed.
The mucous membrane of the small intestine is adapted in its structure to ensure the absorption of substances: folds are formed throughout its entire length, increasing the absorption surface by approximately 3 times; the small intestine has a huge number of villi, which also increases its surface many times; Each epithelial cell of the small intestine contains microvilli (each length is 1 μm, diameter 0.1 μm), due to which the absorption surface of the intestine increases 600 times.
The peculiarities of the organization of microcirculation of intestinal villi are essential for the transport of nutrients. The blood supply to the villi is based on a dense network of capillaries, which are located directly under the basement membrane. A characteristic feature of the vascular system of intestinal villi is the high degree of fenestration of the capillary endothelium and the large size of the fenestrae (45-67 nm). This allows not only large molecules, but also supramolecular structures to penetrate through them. Fenestrae are located in the endothelial zone facing the basement membrane, which facilitates exchange between the vessels and the intercellular space of the epithelium.
Two processes are constantly taking place in the mucous membrane of the small intestine:
1. Secretion - the transfer of substances from the blood capillaries into the intestinal lumen,
2. Absorption - transport of substances from the intestinal cavity into the internal environment of the body.
The intensity of each of them depends on the physicochemical parameters of chyme and blood.
Absorption occurs through passive transfer of substances and active energy-dependent transport.
Passive transport is carried out in accordance with the presence of transmembrane concentration gradients of substances, osmotic or hydrostatic pressure. Passive transport includes diffusion, osmosis, and filtration (see Chapter 1).
Active transport occurs against a concentration gradient, is unidirectional in nature, and requires energy expenditure due to high-energy phosphorus compounds and the participation of special carriers. It can pass along a concentration gradient with the participation of carriers (facilitated diffusion), is characterized by high speed and the presence of a saturation threshold.
Absorption (water absorption) occurs according to the laws of osmosis. Water easily passes through cell membranes from the intestine into the blood and back into the chyme (Fig. 9.7).
Fig.9.7. Scheme of active and passive transfer of water and electrolytes through the membrane. When hyperosmic chyme enters the intestine from the stomach, a significant amount of water is transferred from the blood plasma into the intestinal lumen, which ensures isosmicity of the intestinal environment. When substances dissolved in water enter the blood, the osmotic pressure of chyme decreases. This causes water to quickly penetrate through cell membranes into the blood. Consequently, the absorption of substances (salts, glucose, amino acids, etc.) from the intestinal lumen into the blood leads to a decrease in the osmotic pressure of chyme and creates conditions for the absorption of water.
Every day, 20-30 g of sodium is secreted into the digestive tract with digestive juices. In addition, a person normally consumes 5-8 g of sodium in food daily and the small intestine must absorb 25-35 g of sodium, respectively. Sodium absorption occurs through the basal and lateral walls of epithelial cells into the intercellular space - this is active transport catalyzed by the corresponding ATPase. Some of the sodium is absorbed simultaneously with chlorine ions, which passively penetrate along with positively charged sodium ions. Absorption of sodium ions is also possible during the oppositely directed transport of potassium and hydrogen ions in exchange for sodium ions. The movement of sodium ions causes water to penetrate into the intercellular space (due to the osmotic gradient) and into the bloodstream of the villi.
In the upper small intestine, chlorides are absorbed very quickly, mainly by passive diffusion. The absorption of sodium ions through the epithelium creates greater electronegativity of the chyme and a slight increase in electropositivity on the basal side of the epithelial cells. In this regard, chlorine ions move along the electrical gradient following the sodium ions.
Bicarbonate ions, contained in significant quantities in pancreatic juice and bile, are absorbed indirectly. When sodium ions are absorbed into the intestinal lumen, a certain amount of hydrogen ions is secreted in exchange for a certain amount of sodium. Hydrogen ions with bicarbonate ions form carbonic acid, which then dissociates to form water and carbon dioxide. Water remains in the intestine as part of the chyme, and carbon dioxide is quickly absorbed into the blood and excreted through the lungs.
Calcium ions are actively absorbed along the entire length of the gastrointestinal tract. However, the greatest activity of its absorption remains in the duodenum and proximal small intestine. The process of calcium absorption involves the mechanisms of simple and facilitated diffusion. There is evidence of the existence of a calcium transporter in the basement membrane of enterocytes, which transports calcium against an electrochemical gradient from the cell to the blood. Bile acids stimulate the absorption of Ca++.
Absorption of Mg++, Zn++, Cu++, Fe++ ions occurs in the same parts of the intestine as calcium, and Cu++ - mainly in the stomach. The transport of Mg++, Zn++, Cu++ is ensured by diffusion mechanisms, and the absorption of Fe++ both with the participation of carriers and by the mechanism of simple diffusion. Important factors regulating calcium absorption are parathyroid hormone and vitamin D.
Monovalent ions are absorbed easily and in large quantities, divalent ions to a much lesser extent.
Fig.9.8. Transport of carbohydrates in the small intestine. Carbohydrates are absorbed in the small intestine in the form of monosaccharides, glucose, fructose, and during feeding with mother's milk - galactose (Fig. 9.8). Their transport across the intestinal cell membrane can occur against large concentration gradients. Different monosaccharides are absorbed at different rates. Glucose and galactose are most actively absorbed, but their transport stops or is significantly reduced if active sodium transport is blocked. This is because the transporter cannot transport the glucose molecule in the absence of sodium. The epithelial cell membrane contains a transporter protein that has receptors sensitive to both glucose and sodium ions. Transport of both substances into the epithelial cell occurs if both receptors are excited simultaneously. The energy that causes the movement of sodium ions and glucose molecules from the outer surface of the membrane inward is the difference in sodium concentrations between the inner and outer surfaces of the cell. The described mechanism is called sodium cotransport or secondary active glucose transport mechanism. It ensures the movement of glucose only into the cell. An increase in intracellular glucose concentrations creates conditions for its facilitated diffusion through the basement membrane of the epithelial cell into the intercellular fluid.
Most proteins are absorbed through the membranes of epithelial cells in the form of dipeptides, tripeptides and free amino acids (Fig. 9.9).
Fig.9.9. Scheme of the breakdown and absorption of proteins in the intestine. The energy for transport of most of these substances is provided by a sodium cotransport mechanism similar to glucose transport. Most peptides or amino acid molecules bind to transport proteins, which also need to interact with sodium. The sodium ion, moving along the electrochemical gradient into the cell, “conducts” the amino acid or peptide with it. Some amino acids are not required; sodium cotransport mechanism, and are transported by special membrane transport proteins.
Fats are broken down to form monoglycerides and fatty acids. Absorption of monoglycerides and fatty acids occurs in the small intestine with the participation of bile acids (Fig. 9.10).
Fig.9.10. Diagram of the breakdown and absorption of fats in the intestine. Their interaction leads to the formation of micelles, which are captured by the membranes of enterocytes. Once captured by the micelle membrane, bile acids diffuse back into the chyme, are released, and promote the absorption of new amounts of monoglycerides and fatty acids. Fatty acids and monoglycerides entering the epithelial cell reach the endoplasmic reticulum, where they participate in the resynthesis of triglycerides. Triglycerides formed in the endoplasmic reticulum, together with absorbed cholesterol and phospholipids, are combined into large formations - globules, the surface of which is covered with beta-lipoproteins synthesized in the endoplasmic reticulum. The formed globule moves to the basement membrane of the epithelial cell and, through exocytosis, is excreted into the intercellular space, from where it enters the lymph in the form of chylomicrons. Beta lipoproteins promote the penetration of globules through the cell membrane.
About 80-90% of all fats are absorbed in the gastrointestinal tract and transported into the blood through the thoracic duct in the form of chylomicrons. Small amounts (10-20%) of short-chain fatty acids are absorbed directly into the portal blood before they are converted to triglycerides.
The absorption of fat-soluble vitamins (A, D, E, K) is closely related to the absorption of fats. If fat absorption is impaired, the absorption of these vitamins is also inhibited. Proof of this is that vitamin A is involved in the resynthesis of triglycerides and enters the lymph as part of chylomicrons. The absorption mechanisms of water-soluble vitamins are different. Vitamin C and riboflavin are transported by diffusion. Folic acid is absorbed in the jejunum in conjugated form. Vitamin B12 combines with the intrinsic factor of Castle and in this form is actively absorbed in the ileum.
The bulk of water and electrolytes (5-7 liters per day) are absorbed in the large intestine, and only less than 100 ml of liquid is excreted in humans as part of feces. Basically, the absorption process in the colon occurs in its proximal section. This part of the large intestine is called the absorptive colon. The distal part of the colon performs a storage function and is therefore called the storage colon.
The mucous membrane of the colon has a high ability to actively transport sodium ions into the blood; it absorbs them against a higher concentration gradient than the mucosa of the small intestine, since as a result of its absorption and secretory function, the chyme entering the large intestine is isotonic.
The entry of sodium ions into the intercellular space of the intestinal mucosa, as a result of the created electrochemical potential, promotes the absorption of chlorine. The absorption of sodium and chlorine ions creates an osmotic gradient, which in turn promotes the absorption of water through the mucous membrane of the colon into the blood. Bicarbonates, which enter the lumen of the colon in exchange for an equal amount of chlorine, help neutralize the acidic end products of bacteria in the colon.
When a large amount of fluid enters the colon through the ileocecal valve or when the colon secretes juice in large quantities, excess fluid is created in the feces and diarrhea occurs.
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Absorption in the small intestine
The mucous membrane of the small intestine contains circular folds, villi and crypts (Fig. 22–8). Due to the folds, the absorption area increases 3 times, due to the villi and crypts - 10 times, and due to the microvilli of the border cells - 20 times. In total, folds, villi, crypts and microvilli provide a 600-fold increase in the absorption area, and the total absorption surface of the small intestine reaches 200 m2. Single-layer cylindrical bordered epithelium (Fig. 22–8) contains border, goblet, enteroendocrine, Paneth and cambial cells. Absorption occurs through the border cells.
· Border cells (enterocytes) have more than 1000 microvilli on their apical surface. This is where the glycocalyx is present. These cells absorb broken down proteins, fats and carbohydrates (see caption to Fig. 22–8).
à Microvilli form an absorptive or brush border on the apical surface of enterocytes. Through the absorption surface, active and selective transport occurs from the lumen of the small intestine through the border cells, through the basement membrane of the epithelium, through the intercellular substance of the own layer of the mucous membrane, through the wall of the blood capillaries into the blood, and through the wall of the lymphatic capillaries (tissue slits) into the lymph.
à Intercellular contacts (see Fig. 4–5, 4–6, 4–7). Since the absorption of amino acids, sugars, glycerides, etc. occurs through cells, and the internal environment of the body is far from indifferent to the contents of the intestine (recall that the intestinal lumen is the external environment), the question arises of how the penetration of intestinal contents into the internal environment through the spaces between epithelial cells is prevented. “Closing” of actually existing intercellular spaces is carried out due to specialized intercellular contacts that bridge the gaps between epithelial cells. Each cell in the epithelial layer along the entire circumference in the apical region has a continuous belt of tight junctions that prevent the entry of intestinal contents into the intercellular gaps.
Rice. 22–9. ABSORPTION IN THE SMALL INTESTINE. I - Emulsification, breakdown and entry of fats into the enterocyte. II - Entry and exit of fats from the enterocyte. 1 - lipase, 2 - microvilli. 3 - emulsion, 4 - micelles, 5 - bile acid salts, 6 - monoglycerides, 7 - free fatty acids, 8 - triglycerides, 9 - protein, 10 - phospholipids, 11 - chylomicron. III - The mechanism of secretion of HCO3– by epithelial cells of the mucous membrane of the stomach and duodenum: A - the release of HCO3– in exchange for Cl– is stimulated by some hormones (for example, glucagon), and the Cl– transport blocker furosemide suppresses. B - active transport of HCO3–, independent of Cl– transport. C and D - transport of HCO3– through the membrane of the basal part of the cell into the cell and through the intercellular spaces (depends on the hydrostatic pressure in the subepithelial connective tissue of the mucous membrane). .
· Water. Hypertonicity of the chyme causes the movement of water from the plasma into the chyme, while the transmembrane movement of water itself occurs through diffusion, obeying the laws of osmosis. Crypt border cells release Cl– into the intestinal lumen, which initiates the flow of Na+, other ions and water in the same direction. At the same time, the cells of the villi “pump” Na+ into the intercellular space and thus compensate for the movement of Na+ and water from the internal environment into the intestinal lumen. Microorganisms that lead to the development of diarrhea cause water loss by inhibiting the absorption of Na+ by villous cells and increasing the hypersecretion of Cl– by crypt cells. The daily turnover of water in the digestive tract is shown in table. 22–5.
Table 22–5. Daily water turnover (ml) in the digestive tract
· Sodium. Daily intake of 5 to 8 g of sodium. From 20 to 30 g of sodium is secreted with digestive juices. To prevent loss of sodium excreted in feces, the intestines need to absorb 25 to 35 g of sodium, which is approximately 1/7 of the total sodium content in the body. Most Na+ is absorbed through active transport. Active transport of Na+ is associated with the absorption of glucose, some amino acids and a number of other substances. The presence of glucose in the intestine facilitates the reabsorption of Na+. This is the physiological basis for restoring water and Na+ losses during diarrhea by drinking salted water with glucose. Dehydration increases aldosterone secretion. Aldosterone activates all mechanisms for enhancing Na+ absorption within 2–3 hours. An increase in Na+ absorption entails an increase in the absorption of water, Cl– and other ions.
· Chlorine Cl– ions are secreted into the small intestinal lumen through cAMP-activated ion channels. Enterocytes absorb Cl– along with Na+ and K+, and sodium serves as a carrier (Fig. 22–7,III). The movement of Na+ through the epithelium creates electronegativity in the chyme and electropositivity in the intercellular spaces. Cl– ions move along this electrical gradient, “following” the Na+ ions.
· Bicarbonate. The absorption of bicarbonate ions is associated with the absorption of Na+ ions. In exchange for Na+ absorption, H+ ions are secreted into the intestinal lumen, combine with bicarbonate ions and form h3CO3, which dissociates into h3O and CO2. Water remains in the chyme, and carbon dioxide is absorbed into the blood and released by the lungs.
· Potassium. A certain amount of K+ ions are secreted along with mucus into the intestinal cavity; Most of the K+ ions are absorbed through the mucous membrane by diffusion and active transport.
· Calcium. From 30 to 80% of absorbed calcium is absorbed in the small intestine by active transport and diffusion. Active Ca2+ transport is enhanced by 1,25-dihydroxycalciferol. Proteins activate the absorption of Ca2+, phosphates and oxalates inhibit it.
· Other ions. Iron, magnesium, and phosphate ions are actively absorbed from the small intestine. With food, iron comes in the form of Fe3+; in the stomach, iron passes into the soluble form of Fe2+ and is absorbed in the cranial parts of the intestine.
· Vitamins. Water-soluble vitamins are absorbed very quickly; the absorption of fat-soluble vitamins A, D, E and K depends on the absorption of fats. If pancreatic enzymes are absent or bile does not enter the intestines, the absorption of these vitamins is impaired. Most vitamins are absorbed in the cranial portions of the small intestine, with the exception of vitamin B12. This vitamin combines with intrinsic factor (a protein secreted in the stomach), and the resulting complex is absorbed in the ileum.
· Monosaccharides. The absorption of glucose and fructose in the brush border of small intestinal enterocytes is ensured by the GLUT5 transporter protein. GLUT2 of the basolateral part of enterocytes realizes the release of sugars from cells. 80% of carbohydrates are absorbed predominantly in the form of glucose - 80%; 20% comes from fructose and galactose. Transport of glucose and galactose depends on the amount of Na+ in the intestinal cavity. A high concentration of Na+ on the surface of the intestinal mucosa facilitates, and a low concentration inhibits the movement of monosaccharides into epithelial cells. This is explained by the fact that glucose and Na+ have a common transporter. Na+ moves into the intestinal cells along a concentration gradient (glucose moves along with it) and is released into the cell. Next, Na+ actively moves into the intercellular spaces, and glucose, due to secondary active transport (the energy of this transport is provided indirectly due to the active transport of Na+), enters the blood.
· Amino acids. The absorption of amino acids in the intestine is realized using transporters encoded by SLC genes. Neutral amino acids - phenylalanine and methionine - are absorbed through secondary active transport due to the energy of sodium active transport. Na+-independent transporters carry out the transfer of some neutral and alkaline amino acids. Special carriers transport dipeptides and tripeptides into enterocytes, where they are broken down into amino acids and then enter the intercellular fluid through simple and facilitated diffusion. Approximately 50% of digested proteins come from food, 25% from digestive juices and 25% from shed mucosal cells.
· Fats. Fat absorption (see caption to Fig. 22–8 and Fig. 22–9,II). Monoglycerides, cholesterol and fatty acids delivered by micelles to enterocytes are absorbed depending on their size. Fatty acids containing less than 10–12 carbon atoms pass through the enterocytes directly into the portal vein and from there enter the liver as free fatty acids. Fatty acids containing more than 10–12 carbon atoms are converted into triglycerides in enterocytes. A certain amount of absorbed cholesterol is converted into cholesterol esters. Triglycerides and cholesterol esters are covered with a layer of proteins, cholesterol and phospholipid, forming chylomicrons, which leave the enterocyte and enter the lymphatic vessels.
Absorption in the colon. Every day, about 1500 ml of chyme passes through the ileocecal valve, but every day the colon absorbs from 5 to 8 liters of fluid and electrolytes (see Table 22-5). Most of the water and electrolytes are absorbed in the colon, leaving no more than 100 ml of fluid and some Na+ and Cl– in the stool. Absorption occurs predominantly in the proximal part of the colon, the distal part serves for the accumulation of waste and the formation of feces. The mucous membrane of the colon actively absorbs Na+ and along with it Cl–. The absorption of Na+ and Cl– creates an osmotic gradient, which causes water to move across the intestinal mucosa. The colonic mucosa secretes bicarbonate in exchange for an equivalent amount of absorbed Cl–. Bicarbonates neutralize the acidic end products of colon bacteria.
Formation of feces. The composition of feces is 3/4 water and 1/4 solid matter. The dense substance contains 30% bacteria, 10 to 20% fat, 10–20% inorganic substances, 2–3% protein and 30% undigested food debris, digestive enzymes, and desquamated epithelium. Colon bacteria are involved in the digestion of small amounts of cellulose, producing vitamins K, B12, thiamine, riboflavin and various gases (carbon dioxide, hydrogen and methane). The brown color of stool is determined by bilirubin derivatives - stercobilin and urobilin. The smell is created by the activity of bacteria and depends on the bacterial flora of each individual and the composition of the food consumed. Substances that give feces a characteristic odor are indole, skatole, mercaptans and hydrogen sulfide.
In the small intestine, transport (absorption) of the bulk of digested nutrients into the blood and lymph occurs. Contractions of the villi, as well as motility of the walls of the small intestine, play an important role in the transfer of substances into the blood and lymph.
Absorption is an active process that requires energy; often it occurs against the concentration gradient, i.e. when the level of nutrients in the blood is higher than in the intestinal juice.
The main products of protein hydrolysis are amino acids. Their absorption in the intestine, as well as transport through other cell membranes, is carried out using special transport systems for amino acids. The most universal is the Na + , K + - ATphase system (sodium pump). When transporting amino acids through the membrane of the intestinal epithelium, Na + ions enter the cell with them. Sodium is again “pumped out” of the cell by Na +, K + - ATphase, and amino acids remain inside the cell. In the intestine, small amounts of dipeptides and non-hydrolyzed proteins can be absorbed.
Some amino acids and nucleotide hydrolysis products are absorbed by diffusion.
Carbohydrates are transported into the blood mainly in the form of glucose (in the slightly alkaline environment of intestinal juice, fructose is partially converted into glucose). Galactose is absorbed most quickly, followed by glucose.
Glucose absorption occurs both by active transport (sodium pump) and by diffusion.
The products of lipid digestion are absorbed in different ways. Glycerol, phosphoric acid, choline and other soluble components are absorbed by diffusion. Short-chain (up to 10-12 carbon atoms) fatty acids can also be absorbed in the same way.
Long-chain (more than 14 carbon atoms) fatty acids, cholesterol and fat-soluble vitamins are absorbed through the wall of the small intestine with the participation of bile acids, with which they form complexes. These complexes are called choleic acids. Inside the intestinal wall, the cholein complex disintegrates, and bile acids go into the blood of the portal vein and into the liver. From the liver they return again with bile to the intestines.
Most of the released fatty acids, before entering the lymph, are synthesized in the intestinal wall into lipids specific to the human body (fats, phospholipids, cholesterol). They form fat droplets - chylomicrons, which are absorbed mainly into the lymph, from where they enter the blood, which becomes cloudy. In the blood, chylomicrons are broken down by lipoproteinase and the blood plasma becomes clear.
Water-soluble vitamins are absorbed from the small intestine into the blood by diffusion, where they form complexes with the corresponding proteins and in this form are transported to various tissues.
In the absorption of water and minerals, their active transport through the membranes of the intestinal wall plays a significant role. Here, on average, 8-9 liters of water pass per day. Its main sources are the digestive juices of the upper parts of the digestive system; only about 1.5 liters of water comes from the outside. This is an important way to maintain water balance in the body. Bile acids are necessary for the absorption of calcium and magnesium. Iron is absorbed as a supervalent ion.
The function of the small intestine is regulated by the central nervous system. Stimulators of the function of the small intestines are the juices of the stomach and duodenum.
The motor and secretory activity of the small intestines is enhanced by dense pieces of food, mainly ballast substances (fiber, etc.), and relatively coarse particles are more effective than finely chopped ones (intestinal muscle training).
In the small intestine, in addition to digestive functions, regulatory and homeostatic functions are carried out; in conditions of insufficient supply of plastic material from the outside, the small intestine participates in providing the internal environment with necessary substances. The source of essential amino acids is the proteins of digestive juices and exfoliated cells. In this section of the digestive tract, the synthesis of phospholipids, the formation of retinol (vitamin A from carotenes) and some other biologically active substances important for the body, as well as the neutralization of some toxic substances occur.
After completion of the digestion process of substances in the small intestine, their selective transport into the blood and lymph, all undigested and unabsorbed mass enters the large intestine.
Absorption is the process of transporting substances from the intestinal cavity into the internal environment of the body - blood and lymph. Absorption of hydrolysis products of proteins, fats, carbohydrates, as well as vitamins, salts and water begins in the duodenum and ends in the upper 1/3-1/2 parts of the small intestine. The remaining part of the small intestine is a reserve for absorption. Of course, hydrolysates are absorbed: 50-100 g of protein, about 100 g of fat, several hundred grams of carbohydrates, 50-100 g of salts, 8-9 liters of water (of which 1.5 liters entered the body with drink, food, and 8 liters isolated as part of various secretions). Only 0.5-1 liters of water passes through the ileocecal sphincter into the large intestine.
Features of absorption of various substances
Suction carbohydrates into the blood occurs in the form of monosaccharides. Glucose And galactose transported across the apical membrane of the enterocyte via secondary active transport - together with ions Nα+ located in the intestinal lumen. Glucose and Na + ions on the membrane bind to the GLUT transporter, which transports them into the cell. In a cage
RICE. 13.29. Electronic photograph of microvilli and apical membrane of columnar epithelial cells of the small intestine: A - low magnification, B - high magnification
the complex is split. Na + - ions are transported by active transport thanks to sodium-potassium pumps into the lateral intercellular spaces, and glucose and galactose are transported to the basolateral membrane with the help of GLUT and pass into the interstitial space, and from it into the blood. Fructose transported by facilitated diffusion(GLUT) due to the concentration gradient and is independent of Na + ions (Fig. 13.30).
Protein absorption occurs in the form of amino acids, dipeptides, tripeptides mainly by secondary active transport through apical membrane. Absorption and transport of amino acids is achieved using transport systems. Five of them operate similar to the glucose transport system and require cotransport of Na + ions. These include carrier proteins of basic, acidic, neutral, beta and gamma amino acids and proline. Two transport systems depend on the presence of Cl- ions.
Dipeptides and tripeptides, thanks to hydrogen ions (H +), are absorbed into enterocytes, in which they are hydrolyzed to amino acids, transported by active carriers into the blood through the basolateral membranes of the cell (Fig. 13.31).
Lipid absorption after their emulsification with bile salts and hydrolysis of pancreatic lipase occurs in the form fatty acids, monoglycerides, cholesterol. Bile acids together with fatty acids, monoglycerides, phospholipids and cholesterol form micelles - hydrophilic compounds, in which they are transported to the apical surface of enterocytes, through which fatty acids diffuse into a cage. Bile acids remain in the intestinal lumen and are absorbed into the blood in the ileum, which is carried to the liver. Glycerol is hydrophilic and does not enter into micelles, but enters the cell by diffusion. Occurs in enterocytes re-registration products of lipid hydrolysis, diffuse through the membrane, into triglycerides , which, together with cholesterol and apoproteins, form chylomicrons . Chylomicrons are transported from enterocytes to lymphatic capillaries by exocytosis (Fig. 13.32). Short chain fatty acids transported into the blood.
Hormones stimulate fat absorption processes: secretin, CCK-PZ, thyroid and adrenal hormones.
Ion absorption Να + occurs by an electrochemical gradient across the apical membrane of enterocytes due to the following mechanisms:
■ diffusion through the apical membrane by ion channels;
■ combined transport (cotransport) together with glucose or amino acids;
■ cotransport together with SG ions;
■ in exchange for H+ ions.
Through the basolateral membranes of enterocytes, Na + ions are transported into the blood by active transport - Na + - TO + -pump(Fig. 13.33).
RICE. 13.30.
RICE. 13.31.
RICE. 13.32.
RICE. 13.33.
Sodium absorption is regulated by the adrenal hormone aldosterone.
Ion absorption Ca 2+ is carried out using the following mechanisms
■ passive diffusion from the intestinal cavity through intercellular connections;
■ cotransport with Na + ions;
■ transport in exchange for HCO3-.
K ion absorption + is carried out passively through intercellular connections.
Ca ions 2+ are absorbed through transporters in the apical membrane of enterocytes, which are activated by calcitriol (the active form of vitamin D). Transport of Ca 2+ ions from the enterocyte to the blood occurs by two mechanisms: a) due to calcium pumps; b) in exchange for Na + ions.
The hormone calcitonin inhibits the absorption of Ca 2+ ions.
Water suction occurs by an osmotic gradient following the transport of osmotically active substances (mineral salts, carbohydrates). Absorption of iron and other substances:
Iron absorbed in the form of heme or free Fe2+. Vitamin C promotes the absorption of iron, converting it from Fe3 + to Fe2 +.
The mechanisms of its transport are as follows:
1 Iron is transported through the apical membrane by carrier proteins.
2 In the cell, Fe2+ is destroyed and released, heme and non-heme iron binds to apoferritin, forming ferritin.
3 Iron is broken down from ferritin and bound to intracellular transport protein, where the basolateral membrane is released from the enterocyte into the interstitial space.
3 April from the interstitial space to the plasma, iron is transported by the protein transferrin.
The amount of iron absorbed depends on the concentration of intracellular and extracellular transport proteins, in particular transferrin, compared to the amount of ferritin. If the number of transport proteins predominates, iron is absorbed. If there is little transferrin, then ferritin remains in the enterocytes, which are desquamated into the intestinal cavity. After bleeding, transferrin synthesis increases. Vitamin absorption:
■ fat-soluble vitamins A, D, E and K are part of micelles and are reabsorbed along with lipids;
■ water soluble vitamins absorbed by secondary active transport along with Na + ions;
■ vitamin 12 is also absorbed in the ileum by secondary active transport, but its absorption requires Castle's intrinsic factor(secreted by gastric parietal cells), which binds to receptors on the apical membrane of enterocytes, after which secondary active transport is possible.
Secretion of water and electrolytes in the small intestine
If the function of absorption of electrolytes and water is localized in enterocytes, which are located on the tips of the villi, then secretory mechanism - in crypts.
Ions Cl- secreted by enterocytes into the intestinal cavity, their movement through ion channels is regulated by cAMP. Na + ions follow Cl- ions passively, water follows an osmotic gradient, due to which the solution is maintained isosmotic.
Toxins from Vibrio cholerae and other bacteria activate adenylate cyclase on the basolateral membranes of enterocytes located in the crypts, which increases the formation of cAMP. cAMP activates the secretion of Cl- ions, which leads to passive transport of Na + ions and water into the intestinal cavity, resulting in stimulation of motility and diarrhea.
