14.8. SUCTION
14.8.1. GENERAL SUCTION CHARACTERISTICS
Suction- the physiological process of the transfer of substances from the lumen of the digestive tract into the blood and lymph. It should be noted that the transport of substances through the mucous membrane of the digestive tract constantly occurs from the blood capillaries into the cavity of the digestive tract. If the transport of substances from the blood capillaries into the lumen of the digestive tract predominates, the resulting effect of two differently directed flows is secretion, and if the flow from the cavity of the digestive tract dominates, absorption.
Absorption occurs throughout the digestive tract, but with varying intensity in its various sections. In the oral cavity, absorption is insignificantly expressed due to the short stay of food in it. However, the suction capacity of the oral mucosa is clearly manifested in relation to certain substances, including drugs, which is widely used in clinical practice. The mucous membrane in the region of the bottom of the mouth and the lower surface of the tongue is thinned, has a rich blood supply, and the absorbed substances immediately enter the systemic circulation. The stomach absorbs water and
mineral salts soluble in it, alcohol, glucose and a small amount of amino acids. The main section of the digestive tract, where the absorption of water, minerals, vitamins, hydrolysis products of nutrients, is the small intestine. This part of the digestive tract has an exceptionally high rate of nutrient transfer. Within 1-2 minutes after the entry of food substrates into the intestine, nutrients appear in the blood flowing from the mucous membrane, and after 5-10 minutes their concentration in the blood reaches its maximum values. Part of the liquid (about 1.5 l), together with chyme, enters the large intestine, where it is almost completely absorbed.
The structure of the small intestine is adapted to perform absorptive function. In humans, the surface of the mucous membrane of the small intestine increases 600 times due to circular folds, villi and microvilli and reaches 200 m 2. Absorption of nutrients occurs mainly in the upper part of the intestinal villi. Of essential importance for the transport of nutrients are the features of the organization of the microcirculation of the villi. The blood supply to the intestinal villi is based on a dense network of capillaries located directly under the basement membrane. Characteristic features of the microvasculature of the villi are a high degree of fenestration of the capillary endothelium and a large pore size, which allows rather large molecules to penetrate through them. Fenestra are located in the endothelial zone facing the basement membrane, which facilitates the exchange between the vessels and intercellular spaces of the epithelium. After eating, the blood flow increases by 30-130%, and the increased blood flow is always directed to that part of the intestine where the bulk of the chyme is currently located.
Absorption in the small intestine is also facilitated by the contraction of its villi. Due to the rhythmic contractions of the intestinal villi, the contact of their surface with the chyme improves, and the lymph is squeezed out of the blind ends of the lymphatic capillaries, which creates a suction effect of the central lymphatic vessel.
In an adult, each intestinal cell provides nutrients to approximately 100,000 other cells in the body. This suggests a high activity of enterocytes in the hydrolysis and absorption of nutrients.
body substances. The absorption of substances into the blood and lymph is carried out using all types of primary and secondary transport mechanisms.
14.8.2. ABSORPTION OF WATER, MINERAL SALT AND CARBOHYDRATES
A. The absorption of water is carried out according to the law of osmosis. Water enters the digestive tract as part of food and liquids (2-2.5 l), secretions of the digestive glands (6-8 l), and only 100-150 ml of water is excreted with feces. The rest of the water is absorbed from the digestive tract into the blood, a small amount - into the lymph. Water absorption begins in the stomach, but it occurs most intensively in the small and large intestines (about 9 liters per day). About 60% of water is absorbed in the duodenum and about 20% in the ileum. The mucous membrane of the upper parts of the small intestine is well permeable to dissolved substances. The effective pore size in these sections is about 0.8 nm, while in the ileum and colon it is 0.4 and 0.2 nm, respectively. Therefore, if the osmolarity of the chyme in the duodenum differs from the osmolarity of the blood, then this parameter levels off within a few minutes.
Water easily passes through cell membranes from the intestinal cavity into the blood and back into the chyme. Due to such movements of water, the contents of the intestine are isotonic with respect to blood plasma. When hypotonic chyme enters the duodenum due to the intake of water or liquid food, water enters the bloodstream until the contents of the intestine become isoosmotic to the blood plasma. On the contrary, when hypertonic chyme enters the duodenum from the stomach, water passes from the blood into the intestinal lumen, due to which the contents also become isotonic to the blood plasma. In the process of further moving through the intestine, the chyme remains isoosmotic to the blood plasma. Water moves into the blood following osmotically active substances (ions, amino acids, glucose).
B. Absorption of mineral salts. The absorption of sodium ions in the intestine is very efficient: from 200-300 mmol Na + daily entering the intestine with food, and 200 mmol contained in the composition of digestive juices, excreted with feces
only 3-7 mmol. The main part of sodium ions is absorbed in the small intestine. The concentration of sodium ions in the contents of the duodenum and jejunum is close to their concentration in blood plasma. Despite this, there is a constant absorption of Na + in the small intestine.
The transfer of Na + from the intestinal cavity into the blood can be carried out both through intestinal epitheliocytes and through intercellular channels. Na + comes from the intestinal lumen to the cytoplasm through the apical membrane of enterocytes according to the electrochemical gradient (the electric charge of the cytoplasm of enterocytes is 40 mV relative to the outer side of the apical membrane). The transfer of sodium ions from enterocytes to the interstitium and blood is carried out through the basolateral membranes of enterocytes using the Na/K pump localized there. Na + , K + and SG ions also move along intercellular channels according to the laws of diffusion.
In the upper small intestine, SG is absorbed very rapidly, mainly along an electrochemical gradient. In this regard, negatively charged chloride ions move from the negative to the positive pole and enter the interstitial fluid after the sodium ions.
HCO3 contained in the composition of pancreatic juice and bile are absorbed indirectly. When Na + is absorbed into the intestinal lumen, H + is secreted in exchange for Na +. Hydrogen ions with HCO^ form H 2 CO 3, which under the action of carbonic anhydrase turns into H 2 O and CO 2. Water remains in the intestines as part of the chyme, while carbon dioxide is absorbed into the blood and excreted through the lungs.
The absorption of calcium ions and other divalent cations in the small intestine is slow. Ca 2+ is absorbed 50 times slower than Na + , but faster than other divalent ions: magnesium, zinc, copper and iron. Calcium salts supplied with food dissociate and dissolve in the acidic contents of the stomach. Only half of the calcium ions are absorbed, mainly in the upper part of the small intestine. At low concentrations, Ca 2+ is absorbed by primary transport. The specific Ca2+-binding protein of the brush border is involved in the transfer of Ca 2+ through the apical membrane of the enterocyte, and the transport through the basolateral membranes is carried out with the help of a calcium pump localized there. At a high concentration
walkie talkie Ca 2+ in chyme, it is transported by diffusion. Parathyroid hormone and vitamin D play an important role in the regulation of absorption of calcium ions in the intestine. Bile acids stimulate the absorption of Ca 2+.
The absorption of magnesium, zinc and iron ions occurs in the same sections of the intestine as Ca 2+, and Cu 2+ - mainly in the stomach. The transport of Mg 2+ , Zn 2+ and Cu 2+ occurs by diffusion. The absorption of Fe 2+ is carried out primarily and secondarily actively with the participation of carriers. When Fe 2+ enters the enterocyte, they combine with apoferritin, resulting in the formation of ferritin, in the form of which iron is deposited in the body.
B. Absorption of carbohydrates. Polysaccharides and disaccharides are practically not absorbed in the gastrointestinal tract. Absorption of monosaccharides occurs mainly in the small intestine. Glucose is absorbed at the highest rate, and during the period of feeding with mother's milk - galactose.
The entry of monosaccharides from the cavity of the small intestine into the blood can be carried out in various ways, however, the sodium-dependent mechanism plays the main role in the absorption of glucose and galactose. In the absence of Na +, glucose is transferred through the apical membrane 100 times slower, and in the absence of a concentration gradient, its transport naturally stops completely. Glucose, galactose, fructose, pentose can be absorbed by simple and facilitated diffusion in the case of their high concentration in the intestinal lumen, which usually occurs when eating carbohydrate-rich foods. Glucose is absorbed faster than other monosaccharides.
14.8.3. ABSORPTION OF PROTEIN AND FAT HYDROLYSIS PRODUCTS
Products of hydrolytic cleavage of proteins- free amino acids, di- and tri-peptides are absorbed mainly in the small intestine. The bulk of the amino acids are absorbed in the duodenum and jejunum (up to 80-90%). Only 10% of the amino acids reach the colon, where they are broken down by bacteria.
The main mechanism of absorption of amino acids in the small intestine is secondary active - sodium-dependent transport. At the same time, the diffusion of amino acids according to the electrochemical gradient is also possible. The presence of two transport mechanisms
amino acids explains the fact that D-amino acids are absorbed in the small intestine faster than L-isomers that enter the cell by diffusion. There are complex relationships between the absorption of various amino acids, as a result of which the transport of some amino acids is accelerated, while others are slowed down.
Intact protein molecules in very small amounts can be absorbed in the small intestine by pinocytosis (endocytosis). Endocytosis, apparently, is not essential for the absorption of proteins, but may play an important role in the transfer of immunoglobulins, vitamins, enzymes from the intestinal cavity into the blood. In newborns, breast milk proteins are absorbed by pinocytosis. In this way, antibodies enter the body of the newborn with mother's milk, providing immunity to infections.
Absorption of fat breakdown products. The digestibility of fats is very high. More than 95% of triglycerides and 20-50% of cholesterol are absorbed into the blood. A person with a normal diet with feces excretes up to 5-7 g of fat per day. The bulk of the products of fat hydrolysis is absorbed in the duodenum and jejunum.
The mixed micelles formed as a result of the interaction of monoglycerides, fatty acids with the participation of bile salts, phospholipids and cholesterol enter the enterocyte membranes. Micelles do not penetrate into cells, but their lipid components dissolve in the plasma membrane and, according to the concentration gradient, enter the cytoplasm of enterocytes. The bile acids of the micelles remaining in the intestinal cavity are transported to the ileum, where they are absorbed by the primary transport mechanism.
In intestinal epitheliocytes, resynthesis of triglycerides from monoglycerides and fatty acids occurs on microsomes of the endoplasmic reticulum. From the newly formed triglycerides, cholesterol, phospholipids and glycoproteins, chylomicrons are formed - the smallest fatty particles enclosed in the thinnest protein shell. The diameter of chylomicrons is 60-75 nm. Chylomicrons accumulate in secretory vesicles, which merge with the lateral membrane of the enterocyte, and through the opening formed in this case they enter the intercellular space, from where they enter the blood through the central lymphatic and thoracic ducts. The main amount of fat
absorbed into the lymph. Therefore, 3-4 hours after a meal, the lymphatic vessels are filled with a large amount of lymph, reminiscent of milk (milky juice).
Fatty acids with short and medium chains are quite soluble in water and can diffuse to the surface of enterocytes without forming micelles. They penetrate through the cells of the intestinal epithelium directly into the portal blood, bypassing the lymphatic vessels.
The absorption of fat-soluble vitamins (A, D, E, K) is closely related to the transport of fats in the intestine. In violation of the absorption of fats, the absorption and assimilation of these vitamins are inhibited.
Water begins to be absorbed in the stomach, but since it quickly passes into the intestines, its main absorption occurs in the latter. In this case, the absorbed water passes into the blood.
Water and mineral salts are vital for the body, but getting clean water is becoming more and more difficult every year. One of the simple options is bottled water with delivery. This will make it possible to constantly drink clean water without wasting time getting it.
Huge amounts of water can be absorbed through the intestines (a person has 15-20 liters per day). The main mechanism of water absorption is osmosis, since the osmotic pressure of the blood is higher than the osmotic pressure of the chyme. When giving a significant amount of poorly absorbed salts, for example, Na2SO4, MgSO4, the osmotic pressure in the intestine increases sharply and water passes into it from the blood. The laxative effect of these salts is partly based on this. However, we must not forget that the water content in the intestine can increase not only due to diffusion from the blood through the intestinal wall, but also due to increased secretion of intestinal juice.
Most of the substances that are absorbed from the intestine pass into the blood and lymph in the form of aqueous solutions. If the solute is absorbed quickly, then the solution becomes hypotonic and the water leaves the intestine too quickly. If the absorption of dissolved substances is slow, then water is retained in the intestine by salts, maintaining the osmotic balance between the blood and the contents of the intestine. For example, from an isotonic solution of xylose (4.5%), water is not absorbed after an hour, although about half of the sugar disappears during this time. Large amounts of water quickly enter the intestinal lumen and the volume of the intestinal contents increases. This shows that even with isotonic solutions, water cannot be absorbed if the substances dissolved in it (in this case, xylose) pass into the blood more slowly than salts from the blood into the intestine. Therefore, water is most rapidly absorbed from hypotonic solutions of those substances that quickly diffuse through the intestinal wall.
The absorption of alkali metal salts into the blood occurs through the cells of the intestinal epithelium, and not through the intercellular spaces. The higher the diffusion rate, the faster the ion is absorbed. Salts of hydrohalic acids are absorbed better than sulfate or carbonic ones.
Salts, especially sodium chloride, under certain conditions can flow from the blood into the intestine, sometimes in very large quantities, thereby equalizing the osmotic pressure between the contents of the intestine and the blood. The intensity of absorption of sodium chloride solution increases with increasing concentration up to 1%. Absorption stops if the concentration of sodium chloride solution increases to 1.5%. At this and higher concentrations, sodium chloride solution acts as a causative agent for the secretion of intestinal juice.
Calcium salts are absorbed only in relatively small amounts, so that there is no sharp increase in the calcium content in the blood. In recent years, it has been shown that calcium salts are best absorbed when significant amounts of fat are taken with food; this forms a soluble salt of calcium and a fatty acid. The facts obtained in experiments with the use of isotopes have shown that iron is absorbed in significant quantities only if the body needs it.
1. Tell us about the structure of the stomach.
The stomach serves as a reservoir for the accumulation and digestion of food. Outwardly, it resembles a large pear, its capacity is up to 2-3 liters. The shape and size of the stomach depends on the amount of food eaten. The stomach has a body, a bottom and a pyloric section (a section bordering the duodenum), an inlet (cardia) and an outlet (pylorus) openings. The wall of the stomach consists of three layers: mucous membrane (the mucous membrane is collected in folds, into which the excretory ducts of the glands that produce gastric juice open; also in the mucous membrane there are endocrine cells that produce hormones, in particular gastrin), muscle (three layers of muscle cells: longitudinal, circular, oblique), serous.
2. What processes take place in the stomach?
Digestion of proteins begins under the action of enzymes in the stomach. This process proceeds gradually, as the digestive juice soaks the food lump, penetrating into its depth. This is facilitated by the constant mixing of food in the stomach, due to the alternate contraction of various muscle fibers. Food is retained in the stomach for up to 4-6 hours and, as it turns into a semi-liquid or liquid slurry and is digested in portions, passes into the intestines.
3. How is the regulation of the separation of gastric juice?
The regulation of juice secretion by the glands of the stomach occurs in reflex and humoral ways. It begins with conditional and unconditional secretion of juice at the sight or smell of food and when food enters the mouth immediately after the salivary glands of the oral cavity begin to work. Under the influence of the sympathetic nervous system, the secretion of digestive juices increases, while the parasympathetic nervous system decreases.
4. What is the composition of gastric juice?
Gastric juice is a clear liquid, 0.25% of its volume is hydrochloric acid (pH ≈ 2), mucins (protect the walls of the stomach) and inorganic salts and directly digestive enzymes. Digestive enzymes are activated by hydrochloric acid. These are pepsin (breaks down proteins), gelatinase (breaks down gelatin), lipase (breaks down milk fats to glycerol and fatty acids), chymosin (curdles milk casein).
5. It is known that proteins are digested in the stomach. Why are the walls of the stomach itself not damaged?
From self-digestion, the mucous membrane is protected by mucus (mucin), which abundantly covers the walls of the stomach.
6. What substances are digested in the duodenum?
In the duodenum, food is exposed to the action of pancreatic juice, bile and intestinal juice. Their enzymes break down proteins into amino acids, fats into glycerol and fatty acids, and carbohydrates into glucose.
7. Using additional sources of information, as well as the figure "The movement of blood in the liver", explain how the liver performs its barrier function.
The gates of the liver include the hepatic artery and the portal vein, which collects blood from all unpaired organs of the abdominal cavity. Blood passes through liver cells - hepatocytes, collected in hepatic acini, in which it is cleared of toxic substances, hemoglobin breakdown products, and some microorganisms. Further, the purified blood is collected in the hepatic vein, and the rest is mixed with the secretion of hepatocytes (together they make up bile) and moves through the bile ducts, which are collected at the gates of the liver into the common bile duct. Further, bile either directly enters the duodenum, or is collected in the gallbladder and enters the intestine from the bladder as needed.
8. What role does bile play in the process of digestion?
Bile increases the activity of intestinal juice and pancreatic enzymes, and also under its action, large drops of fat break up into small drops, which facilitates their digestion. Bile also activates absorption processes in the small intestine; has a detrimental effect on some microorganisms; creates an alkaline environment in the intestines; enhances motor activity (motility) of the intestine.
9. What stages can be identified in the process of digestion in the small intestine?
The process of digestion in the small intestine consists of three stages: cavity digestion, parietal digestion and absorption.
10. What is parietal digestion? What is its meaning?
Parietal digestion, the second stage of the digestion process, which takes place on the very surface of the intestinal mucosa. Digestion with the help of appropriate enzymes is carried out by food particles that penetrate into the spaces between the villi. Larger particles cannot get here. They remain in the intestinal cavity, where they are exposed to digestive juices and split into smaller sizes. The process of parietal digestion provides the final stage of hydrolysis and the transition to the final stage of digestion - absorption.
11. What is the significance of pendulum movements of the small intestine?
The small intestine is also capable of pendulum movements due to the alternating lengthening and shortening of the intestine in a certain area. The contents of the intestine are mixed and move in both directions.
12. What is the significance of the folding of the inner wall of the small intestine?
Due to folding, the surface area of the intestinal mucosa increases dramatically, so almost complete processing of food occurs here.
13. Where does the pancreatic duct flow into? What is the role of the enzymes secreted by it?
The pancreatic duct, like the common bile duct, opens at the major duodenal papilla on the lateral wall of the duodenum. The following digestive enzymes are produced in the pancreas: trypsin, chymotrypsin, elastase (break down proteins into peptides and amino acids); amylase (translates carbohydrates into glucose); lipase (breaks down fats to glycerol and fatty acids); nucleases (break down nucleic acids into nucleotides).
14. What is the essence of suction? Where does most of the absorption of nutrients take place? water?
Absorption is the process by which nutrients move from the intestines into the blood vessels; a complex physiological process based on the phenomena of filtration, diffusion, and some others. Absorption occurs in the wall of the small and large intestines. The walls of the villi of the small intestine are covered with a single-layer epithelium, under which there are networks of blood and lymphatic capillaries and nerve fibers with nerve endings. Between the dissolved nutrient in the intestinal cavity and the blood there is only the thinnest barrier of two layers of cells - the walls of the intestine and capillaries. Cells of the intestinal epithelium are active. They pass some substances (only in one direction), others do not.
15. Name the end products of the breakdown of proteins, fats and carbohydrates. Which of them is absorbed into the blood, and which - into the lymph?
Proteins in our body are broken down into amino acids, carbohydrates into glucose, fats into glycerol and fatty acids. The breakdown products of glucose, amino acids, solutions of mineral salts are directly absorbed into the blood. In the cells of the body, these substances are converted into proteins and carbohydrates that are characteristic of humans. Fatty acids and glycerol are absorbed into the lymphatic capillaries.
From the small intestine several hundred grams of carbohydrates, 100 g or more of fat, 50-100 g of amino acids, 50-100 g of ions and 7-8 liters of water are absorbed daily. The absorption capacity of the small intestine is normally much greater, up to several kilograms per day: 500 g of fat, 500-700 g of protein and 20 liters or more of water. The large intestine can absorb additional water and ions, even some nutrients.
Isotonic suction. Water passes through the intestinal membrane completely by diffusion, which obeys the normal laws of osmosis. Consequently, when the chyme is sufficiently diluted, water is absorbed by the villi of the intestinal mucosa into the blood almost exclusively by osmosis.
Conversely, water can be transported in the opposite direction from plasma to chyme. In particular, this occurs when a hypertonic solution enters the duodenum from the stomach. In order to make the chyme isotonic to the plasma, the required amount of water will be moved into the intestinal lumen by osmosis within a few minutes.
Physiology of ion absorption in the intestine
Active sodium transport. In the composition of the intestinal secretion, 20-30 g of sodium is secreted daily. In addition, the average person eats 5-8 g of sodium daily. Thus, in order to prevent direct loss of sodium in the faeces, 25-35 g of sodium should be absorbed per day in the intestines, which is approximately 1/7 of the total sodium in the body.
In situations where significant amount of intestinal secretion excreted, such as with extreme diarrhea, sodium stores in the body can be depleted, reaching deadly levels within a few hours. Usually, less than 0.5% of intestinal sodium is lost daily with faeces, because. it is rapidly absorbed by the intestinal mucosa. Sodium also plays an important role in the absorption of sugars and amino acids, as we will see in further discussions.
Main mechanism absorption of sodium from the intestine shown in the figure. The principles of this mechanism are basically similar to the absorption of sodium from the gallbladder and renal tubules.
driving strength to absorb sodium is provided by the active excretion of sodium from the inside of the epithelial cells through the basal and lateral walls of these cells into the intercellular space. In the figure, this is indicated by wide red arrows. This active transport obeys the usual laws of active transport: it needs energy, and energy processes are catalyzed in the cell membrane by adenosine triphosphatase-dependent enzymes. Part of the sodium is absorbed along with chloride ions; in addition, negatively charged chloride ions are passively attracted to positively charged sodium ions.
Active sodium transport through the basolateral membrane of cells reduces the sodium concentration inside the cell to low values (about 50 meq/l), which is also shown in the figure. Because the sodium concentration in the chyme is normally about 142 mEq/L (i.e. approximately equal to that in plasma), sodium moves inward along this steep electrochemical gradient from the chyme through the brush border into the cytoplasm of the epithelial cells, which provides the main transport of sodium ions by epithelial cells into the intercellular space.
Water osmosis. The next step in transport processes is the osmosis of water into the intercellular space. It occurs because a high osmotic gradient is created due to the increased concentration of ions in the intercellular space. Most of the osmosis occurs through the tight junctions of the apical rim of the epithelial cells, as well as through the cells themselves. The osmotic movement of water creates a fluid flow through the intercellular space. As a result, water ends up in the circulating blood of the villi.
Water enters the gastrointestinal tract as part of food and drinking liquids (2-2.5 l), secrets of the digestive glands (6-7 l), but 100-150 ml of water is excreted with feces per day. The rest of the water is absorbed from the digestive tract into the blood, a small amount - into the lymph. Water absorption begins in the stomach, but it occurs most intensively in the small and especially the large intestine - about 8 liters per day. The movement of water through the mucosa is always associated with the transfer of substances dissolved in it - bearing and not carrying a charge.
The absorption of a certain amount of water occurs along the osmotic gradient, but it is also possible in the absence of a difference in osmotic pressure. The main amount of water is absorbed from isotonic solutions of intestinal chyme, since hyper- and hypotonic solutions are quickly concentrated or diluted in the intestine. The absorption of water from isotonic and hypertonic solutions requires energy. Water follows osmotically active molecules and ions. These include ions of mineral salts, monosaccharide molecules, amino acids and oligopeptides. The most intensive absorption of sodium and water in the intestine occurs at pH 6.8 (at pH 3.0, water absorption stops).
Water absorption is regulated by the hormones of the endocrine glands. Adrenocorticotropic hormone enhances the absorption of water and chlorides without affecting the absorption of glucose; thyroxine increases the absorption of water, glucose and lipids. Some gastrointestinal hormones: gastrin, secretin, cholecystokinin, vasointestinal polypeptide, bombesin, serotonin - reduce water absorption.
Absorption of sodium ions. More than 1 mole of sodium chloride is absorbed in the gastrointestinal tract per day. In the human stomach, sodium is almost not absorbed, but this process is intensively carried out in the large intestine and ileum. In the jejunum, its intensity is much less. Sodium ions are transferred from the cavity of the small intestine into the blood through intestinal epithelial cells and through intercellular channels. The flow of Na + into the epitheliocyte occurs along the electrochemical gradient in a passive way. There is also a Na + transport system associated with the transport of sugars and amino acids, possibly with Cl - and HCO3 - . Sodium ions from epitheliocytes are actively transported through their basolateral membranes into the intercellular fluid, blood and lymph. The intensity of sodium absorption depends on the pH of the intestinal contents, the hydration of the body and the content of this element in it. Na + transport through intercellular channels occurs passively along the concentration gradient.
In different parts of the intestine, the transport of Na + has features. In the large intestine, its absorption does not depend on the presence of sugars and amino acids, and in the small intestine it depends on these substances. In the small intestine, the transfer of Na + and Cl - is conjugated; in the large intestine, the absorbed Na + is exchanged for K +. With a decrease in the content of sodium in the body, its absorption by the intestine increases sharply. Increased absorption of sodium occurs under the influence of the hormones of the pituitary and adrenal glands, inhibition - under the influence of gastrin, secretin and cholecystokinin.
absorption of potassium ions. Potassium ions are absorbed mainly in the small intestine mainly due to passive transport along the concentration gradient, since the concentration of K + ions in the cell is 14 mM, and in plasma - 4 mM. In the process of absorption of K +, the role of active transport is small, and it seems to be associated with Na + transport in the basolateral membranes of epitheliocytes.
Absorption of chloride ions occurs in the stomach, most actively - in the ileum by the type of active and passive transport. Passive Cl - transport is coupled with Na + transport. Active transport of Cl - is carried out through the apical membranes, it is associated with the transport of Na + or the exchange of Cl - for HCO3 -.
Absorption of Ca 2+ ions carries out a special transport system, which includes the Ca 2+ -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 Ca 2+ (in comparison with other divalent ions). At a significant concentration of Ca 2+ in the chyme, the volume of its absorption increases due to the diffusion mechanism. The absorption of Ca 2+ is enhanced under the influence of parathyroid hormone, vitamin D and bile acids.
Absorption of Fe 2+ carried out with the participation of the carrier. In the enterocyte, Fe 2+ combines with apoferritin to form ferritin. As part of ferritin, iron is used in the body.
Manganese is mainly absorbed in the duodenum and jejunum by facilitated diffusion. Magnesium is also most intensively absorbed in the upper small intestine by active transport at low cation concentrations in the chyme, and by simple diffusion at high concentrations. In the upper part of the small intestine, zinc is also absorbed along the concentration gradient. Copper is absorbed mainly in the stomach and upper small intestine, mainly by the mechanism of passive transport and a small part - by the active way, together with amino acids in the form of complexes.