Digestion in the small and large intestines. The role of bile in digestion. Features of digestion in the small intestine
The contents of the stomach enter the intestines, namely the duodenum. She is a department small intestine(small intestine), which also includes the jejunum (2-2.5 m long) and the ileum (2.5-3.2 m).
The duodenum is the thickest with a length of 25-30 cm. On its inner surface there are many villi, and in the submucosal layer there are small glands, the secret of which breaks down proteins and carbohydrates.
in the cavity duodenum the main pancreatic duct and the common bile duct are located, here pancreatic juice, bile, and intestinal juice affect food. This is where carbohydrates, fats and proteins are digested so that they can be absorbed by the body.
pancreatic juice
Pancreatic juice is also called pancreatic juice from the Latin "pancreas" - pancreas. It is the second largest gland in a person with a length of 15 - 22 cm, weight - 60 - 100 g. It consists of two glands - exocrine, synthesizing 500 - 700 ml of pancreatic juice, and endocrine - producing hormones.
Pancreatic juice is a clear, colorless liquid with an alkaline reaction with a pH of 7.8 - 8.4. It begins to be produced 2-3 minutes after eating, and this process continues for 6-14 hours. The longest juice secretion causes the intake of fatty foods.
pancreatic juice enzymes
The protein-splitting enzyme trypsin is synthesized by the cells of the gland in an inactive form (trypsinogen), the enterokinase enzyme of intestinal juice makes it active, as a result of which trypsin breaks down proteins into amino acids.
The enzyme lipase converts fats into glycerol and fatty acid, its activity enhances bile.
The pancreatic juice also contains the enzyme amylase, which breaks down starch into disaccharides, and maltase, which converts disaccharides into monosaccharides.
Enzyme composition under gastric juice due to the nature of the diet. A diet rich in fat has been found to increase lipase activity in pancreatic juice. The systematic use of carbohydrate foods increases the activity of amylase, protein foods - the protease enzyme.
In this way, pancreatic juice neutralizes acidic contents in the duodenum and breaks down fats, carbohydrates, proteins, nucleic acids through abdominal digestion.
Bile in digestion
A large role in digestion is given to the liver, the largest gland in the body. It synthesizes and secretes bile, which is stored in the gallbladder. Its volume is approximately 40 ml, but the bile is concentrated here - dark with a greenish tint due to the large amount of bile acids and pigments. In concentration, it exceeds hepatic bile by 3-5 times, since mineral salts, water and a number of other substances are constantly absorbed from it.
Bile begins to flow into the duodenum 5-10 minutes after a meal and ends when the last portion leaves the stomach. Bile stops the action of gastric juice and its enzymes.
Functions of bile:
- leads to the active state of the enzyme lipase, which breaks down fats;
- mixes with fats, forming an emulsion and thus improving their splitting, since the contact surface of fatty particles with enzymes increases many times;
- takes part in the absorption of fatty acids;
- increases the production of pancreatic juice;
- activates peristalsis (motility) of the intestine.
Violations in the synthesis of bile or in its entry into the intestine cause problems in the digestion and absorption of fats.
Bile contains fatty acids, fats, bile pigment bilirubin, cholesterol, lecithin, mucin (mucus), soaps and inorganic salts.
The reaction of bile is slightly alkaline. Per day, the volume of bile secreted in an adult is 500 - 1000 ml, a rather impressive amount.
intestinal juice
The inner lining of the small intestine contains special glands that produce and secrete intestinal juice. It complements the process with its action.
intestinal juice is a colorless liquid, cloudy from impurities of mucus and epithelial cells. It has an alkaline reaction and contains a complex digestive enzymes- over 20 (aminopeptidases, dipeptidases, etc.).
Types of digestion in the small intestine
In the intestine, 2 types of digestion are distinguished: cavitary and parietal. Cavitary digestion is carried out by enzymes in the cavity of the organ, parietal - by enzymes that are localized on the mucous membrane of the inner surface of the small intestine, and here the concentration of enzymes is much higher. This kind digestion in small intestine also called contact or membrane.
Contact digestion (enzymes lactase, maltase, sucrase) breaks down disaccharides into monosaccharides and small peptides into amino acids. Nutrients, crushed in the intestine as a result of the action of bile and pancreatic juice, penetrate into a dense border formed by the villi of intestinal cells, where large molecules, and even more so bacteria, are not able to enter.
Enzymes are secreted into the same zone by intestinal cells, and nutrients are separated into elementary components - amino acids, fatty acids, monosaccharides, which are then absorbed. Both processes - splitting and absorption into the blood - are carried out within a limited space and quite often represent one interconnected process.
Absorption in the small intestine
The intestines are able to absorb 2-3 liters of liquid in 1 hour, which contains nutrients dissolved in it. This is possible due to the large total absorbing surface of the intestine, a significant number of folds and protrusions of the mucous membrane - villi, including due to the special structure of the epithelial cells lining the intestine.
The surface of these cells is covered with the thinnest filamentous processes (microvilli). One cell contains from 1600 to 3000 microvilli, inside of which there are microtubules. Villi and especially microvilli 
NKs expand the suction surface of the intestinal mucosa to a huge size - 500 m2.
As a result of the process absorption in the small intestine the resulting nutrients penetrate into the blood, but not into the general circulation, otherwise the person would die after the first meal. All the blood that is sent from the stomach and from the intestines accumulates in the portal vein and moves to the liver, since not only useful compounds, but also by-products- toxins that are released by the intestinal microflora, drugs and poisons contained in products at the level of modern ecology. In addition, the entry of nutritional components into the general bloodstream at once would exceed all permissible limits.
It is not for nothing that the liver is otherwise called the biochemical laboratory of the body, since harmful compounds are disinfected here, in addition, fat, protein and carbohydrate metabolism is regulated.
The degree of intensity of the liver is determined by the energy expended: with a weight of 1.5 kg, it consumes 1/7 of the body's energy. Within one minute, actually 1.5 liters of blood passes through the liver, and the vessels of the organ contain up to 20% of the total blood volume.
At the end of the digestion process in the small intestine, undigested food remains from the ileum through the valve (sphincter) enter the large intestine, where this process continues.
After the food bolus has passed the stomach, digestion continues in the small intestine. The initial section of the small intestine is the duodenum. How the digestive process proceeds and the absorption of food in the duodenum and the rest of the small intestine largely determines the state of our health and longevity prospects.
Food enters the duodenum through the pyloric valve. The configuration of the duodenum looks like a horseshoe, its length is 25-30 cm. Inside the duodenum is the pancreas.
Digestion in the duodenum is provided by the bile produced by the liver, pancreatic juice produced by the pancreas and the juice of the glands located in the mucous membrane of this intestine entering its cavity. All of them have a pronounced alkaline reaction.
The pancreas includes a gland that produces pancreatic juice (up to 500 - 700 ml per day), and an endocrine gland that produces hormones that enter the blood or lymph.
Pancreatic juice contains a complex of enzymes that break down proteins, fats, and carbohydrates.
The flow of juice into the duodenum begins 2-3 minutes after a meal and lasts from 6 to 14 hours. The longest release of pancreatic juice occurs when fatty foods are taken.
Before meals, the secretion of the pancreas is completely absent, during and after meals, the secretion of pancreatic juice becomes continuous. At the same time, the amount of secreted juice, its digestive capacity and the duration of secretion depend on the composition and amount of food taken. Pancreatic juice enzymes (trypsin, chymotrypsin, lipase, amylase, maltose, lactase, nuclease, etc.) have the ability to break down proteins to free amino acids, fats - to glycerol and fatty acids. The enzymatic composition of pancreatic juice varies depending on the nature of the diet. It was found that with a diet rich in fats, the activity of lipase in the pancreatic juice increases (several times). At systematic use food rich in carbohydrates, amylase activity increases; with protein rich meat diet increases the activity of the protease enzyme.
The reflex mechanism of pancreatic juice secretion is the same as that of the gastric one: there is a brain phase and an intestinal (neurohumoral) phase.
Bile enters the duodenum during digestion - the flow begins as early as 5-10 minutes after the start of the meal and ends when the last portion of food leaves the stomach. But the secretion of bile is carried out continuously. When digestion does not occur, bile enters gallbladder. At healthy person 0.5-1.2 liters of bile are secreted per day. Bile consists of water, fatty acids, cholesterol and inorganic substances.
Bile neutralizes the action of gastric juice, due to which gastric digestion is replaced by intestinal. It also emulsifies fats, multiplying the contact surface of fatty particles with the enzymes that act on them. It also improves the absorption of fat breakdown products and other nutrients- amino acids, vitamins, promotes the promotion of food masses and prevents their decay.
Bile also activates the lipase of pancreatic and intestinal juices, promotes the breakdown and absorption of fats, amino acids, vitamins, enhances intestinal motility, and prevents rotting of food masses.
Pancreatic hormones (insulin, glucagon, etc.) are also involved in the regulation of carbohydrate and fat metabolism. For example, insulin stops the breakdown of glycogen (animal starch) in the liver and switches the body's cells to feed primarily on glucose.
When portions of acidic gastric contents pass into the duodenum, the reaction in it is initially acidic, and then gradually normalizes. Thus, in the duodenum, the acidic contents are neutralized and carbohydrates, fats, proteins, nucleic acids are broken down due to abdominal digestion.
Chyme is found in the duodenum a short time, so there is no real chemical processing of food. Chyme is only wetted by the juices of the pancreas and the intestine itself, with bile it enters further into the small intestine. Food products pass into a liquid or semi-liquid state in it and then pass into the small intestine.
However, the role of the duodenum does not end there. She is the most important endocrine organ, releasing up to 20 digestive hormones into the blood, affecting the activity of all parts of the gastrointestinal tract. Also, the duodenum is a reflexogenic zone, from which reflexes begin that regulate the excretion of bile, the evacuation of food from the stomach, the functioning of the intestines, salivary glands, and the entire gastrointestinal tract as a whole.
One of the most important is the endocrine function of the small intestine. Cells of the small intestine synthesize peptide hormones (secretin, pancreozymin, intestinal glucagon, gastroinhibiting polypeptide, vasoactive intestinal peptide, motilin, neurotensin, etc.), which ensure the regulation of activity digestive system and other body systems. The largest number such cells are concentrated in the duodenum.
The small intestine takes an active part in immune processes. Along with the bone marrow, spleen, lymph nodes, bronchial mucosa, it is a source of immunoglobulins; in the intestine, also, various subpopulations of T-lymphocytes were found, with the help of which cellular immunity is realized.
Further, the 12 duodenum passes into the jejunum (length 2-2.5 m), and that connects to the ileum (2.5-3.5 m). Due to the presence of transverse folds, the number of which reaches 600-650, many villi, its suction capacity increases many times over.
The motor function of this part of the small intestine is carried out due to three types of movements: rhythmic, pendulum and peristaltic. Rhythmic contractions occur due to pacemakers, which work automatically and generate slow electrical waves in the muscles of the intestine. Pendulum movements - due to alternating contractions of longitudinal and circular muscle fibers (rhythm - up to 20 contractions per minute). And peristaltic movements include several types of biomechanical contractions.
Humoral stimuli, also stimulating intestinal movements are acetylcholine, choline, histamine, duodenal hormones. From food components - meat extracts, cabbage broth, bile, salt, roughage containing a large number of fiber, vegetables.
The hormones of the adrenal glands - adrenaline and norepinephrine - inhibit the motor activity of the intestine. Therefore, in such emotional states of the body as fear, fright, anger, anger, rage, etc., a large amount of adrenaline enters the bloodstream, which causes inhibition of motor function gastrointestinal tract.
The rhythmic contractions accompanying the digestion process divide the food gruel into separate segments, which contributes to its better rubbing and mixing with digestive juices.
In the regulation of the motor activity of the small intestine, nervous mechanisms are also involved, combined into a single regulatory system. Depending on their impact, the motor function of the small intestine may increase or decrease.
In the small intestine, the stage of mechanical and chemical processing food.
The inner lining of the small intestine is covered with a layer of mucus and along the entire mucosa of the small intestine are glands that secrete intestinal juice. The mucous membrane of the small intestine of an adult produces about 2.5 liters of juice per day.
Intestinal juice, which has a complete set of enzymes for further breakdown nutrients, complements the action of gastric and pancreatic juices, as well as the liver. Intestinal juice enzymes are able to break down any food substances (poly - and oligomers) to monomers, which, as they form, are absorbed by the mucous membrane of the small intestine into the blood and lymph.
In the small intestine, there is cavity and parietal (membrane) digestion. Cavitary digestion provides the initial hydrolysis of nutrients to intermediate products.
As a result of cavity digestion, complex polymers (proteins, fats, carbohydrates, nucleic acids) are hydrolyzed to oligomers - polypeptides and disaccharides. And membrane digestion provides hydrolysis of oligomers at the intermediate and final stages of digestion. Here there is a further splitting of the formed compounds to monosaccharides, amino acids, fatty acids and monoglycerides, as well as the transition to absorption. It should be noted that parietal (membrane) digestion was discovered relatively recently and it turned out that it is characterized by high efficiency, since it occurs over a very large area. Food components, largely already crushed under the action of pancreatic juice and bile, fall between the microvilli of intestinal cells. Numerous enzymes are secreted into this zone by intestinal cells, under the action of which the remaining components of nutrients are broken down into elementary components - amino acids, fatty acids, monosaccharides.
It should be noted that only 20-30% of the enzymes of the small intestine enter the intestinal cavity, while most of the enzymes remain on the surface of the microvilli membranes, where the final hydrolysis of oligomers occurs. Moreover, the villi are located so densely that for large molecules, and even more so for bacteria, the surface of the intestine is inaccessible. In the small intestine It should be noted that digestion and absorption are often combined into one complex interrelated process.
Thus, the hydrolysis of food components is sequentially carried out using cavity digestion (in the cavity of the small intestine), parietal digestion (in the supraepithelial layer of mucous formations), membrane digestion (on the membranes of the mucous villi) and intracellular digestion (after the penetration of incompletely split substrates into the cells of the mucous membrane). ).
The small intestine is the main section of the digestive tract, where the absorption of hydrolysis products of nutrients, vitamins, minerals and water. As already noted, the large volume and high rate of absorption of nutrients are explained by the large area of contact between the surface of the small intestine and chyme due to the presence of macro- and microvilli, their contractile activity, and a dense network of capillaries located in the mucous surface of the small intestine.
It should be noted that the main amount of water is absorbed in the upper sections of the small intestine, while water follows osmotically active molecules and ions, which include ions of mineral salts, monosaccharide molecules, amino acids and oligopeptides.
The process of absorption occurs throughout the small intestine, but the most intensive absorption occurs in the jejunum and ileum (it is believed that the volume of absorption here can reach 2-3 liters per hour). This is due to the fact that, as noted above, the suction surface of this section of the gastrointestinal tract is very large due to the presence of folds and villi.
Proteins are absorbed in the form of amino acids, and when fed with proteins of animal origin, 95-99% of the introduced protein is digested and absorbed, when fed with proteins vegetable origin- 75 - 80%. Carbohydrates are absorbed in the form of glucose and galactose. Unlike other substances, monosaccharides are most actively absorbed at the beginning of the small intestine. Insulin enhances the absorption of glucose in the intestine. Most difficult process- Absorption of fats. Fatty acids and glycerol, formed as a result of the breakdown of fats, enter the villus cells, where they are again converted into lipids. The proteins present there cover the lipid molecules with a thin layer, forming lipoprotein globules.
Further, the formed globules enter the lymph and near the heart enter the blood vessels, where they enter the liquid part of the blood - the plasma. Here, plasma enzymes again hydrolyze lipids to fatty acids and glycerol, which are absorbed by cells and in this form can be used for respiration or storage as fat in the liver, muscles, subcutaneous layer. It should be noted that according to research data, only 35–70% of those admitted to digestive tract fats. Unsplit fats after emulsification can also be absorbed in the digestive tract.
Monosaccharides, dipeptides and amino acids, when absorbed, enter the blood capillaries emerging from the villi, which, when combined, form the portal vein of the liver, through which the absorbed digestion products enter the liver. In the liver, harmful compounds are decontaminated and protein, fat and carbohydrate metabolism is regulated. All these substances can be synthesized and broken down in the liver - as needed, ensuring the constancy of our internal environment.
In the small intestine, the absorption of inorganic salts, vitamins and water also occurs.
Articles related to the subject.
Digestion in the small intestine
From the stomach, chyme enters the small intestine. In ensuring intestinal digestion essential have processes occurring in the duodenum. Here, food masses are exposed to intestinal juice, bile and pancreatic juice. The length of the duodenum is small, so food does not linger here, and the main processes of digestion occur in the underlying sections of the intestine. Intestinal juice, formed by the glands of the mucous membrane of the duodenum, contains a large amount of mucus and the enzyme peptidase, which breaks down proteins. This juice has a weaker effect on fats and starch. It also contains the enzyme enterokinase, which activates pancreatic trypsinogen. The cells of the duodenum produce two hormones - secretin and cholecystokinin - pancreozymin, which enhances the secretion of the pancreas.
The acidic contents of the stomach, when passing into the duodenum, acquire an alkaline reaction under the influence of bile, intestinal and pancreatic juice. In humans, the pH of duodenal contents ranges from 4.0 to 8.0. In the hydrolysis of nutrients, carried out in the duodenum, pancreatic juice plays a particularly important role. The bulk of the pancreatic tissue produces digestive juice (pancreatic), which is excreted through the duct into the duodenal cavity. In a normal adult, 1.5-2.0 liters of pancreatic juice is secreted per day, which is a clear liquid with an alkaline reaction (pH = 7.8-8.5). Pancreatic juice is rich in enzymes that break down proteins, fats and carbohydrates. Amylase, lactase, nuclease and lipase are secreted by the pancreas in the active state and break down starch, respectively, milk sugar, nucleic acids and fats. Nucleases (trypsin and chymotrypsin) are produced by the cells of the gland in an inactive state in the form of trypsinogen and chymotrypsinogen. Trypsinogen in the duodenum is converted into trypsin by the action of its enterokinase enzyme. In turn, trypsin converts chymotrypsinogen into active chymotrypsin. Under the influence of trypsin and chymotrypsin, proteins and high molecular weight polypeptides are cleaved to low molecular weight peptides and free amino acids.
The role of the liver in digestion is also significant. Liver cells continuously secrete bile, which is one of the most important digestive juices. A person produces about 500-1200 ml of bile per day. The process of bile formation is continuous, and its entry into the duodenum - periodically, mainly in connection with food intake. On an empty stomach, bile does not enter the intestines, it goes to the gallbladder and accumulates there. The composition of cystic bile differs from hepatic bile.
Bile contains bile acids, bile pigments and other substances. Bile acids are involved in the process of fat digestion. The bile pigment bilirubin is formed both by liver cells and from hemoglobin in the process of destruction of red blood cells there. Dark color bile is due to the presence of this pigment in it.
Bile increases the activity of pancreatic and intestinal juice enzymes, especially lipase. It emulsifies fats and dissolves the products of their hydrolysis, which contributes to their absorption. By creating an alkaline reaction in the duodenum, bile prevents the destruction of trypsin by pepsin. It also performs a regulatory role, stimulating bile formation, bile secretion, motor and secretory activity of the small intestine. Bile also has bacteriostatic properties. It delays putrefactive processes in the intestines. The role of bile in the absorption of fat-soluble vitamins, cholesterol, amino acids and calcium salts from the intestines is great.
The liver, forming bile, performs not only a secretory, but also an excretory (excretory) function. The main organic excretions of the liver are bile salts, bilirubin, cholesterol, fatty acids and lecithin, as well as calcium, sodium, chlorine, and bicarbonates. Once in the bile in the intestines, all these substances are excreted from the body.
Food masses (chyme) from the duodenum move to the small intestine, where they continue to be digested by digestive juices released into the duodenum. At the same time, its own intestinal juice, produced by the Lieberkühn and Brunner glands of the mucous membrane of the small intestine, begins to act here. Intestinal juice contains enterokinase, as well as a complete set of enzymes that break down proteins, fats and carbohydrates. These enzymes are involved only in parietal digestion, since they are not released into the intestinal cavity. Cavitary digestion in the small intestine is carried out by enzymes supplied with food chyme. Cavitary digestion is most effective for the hydrolysis of large molecular substances.
Parietal (membrane) digestion, discovered by Academician A.M. Coal in the 1950s and 60s, occurs on the surface of the microvilli of the small intestine. It completes the intermediate and final stages of digestion by hydrolyzing intermediate cleavage products. Microvilli are cylindrical outgrowths of the intestinal epithelium with a height of 1-2 microns. Their number is huge - from 50 to 200 million per 1 mm 2 of the surface of the intestine, which increases the inner surface of the small intestine by 300-500 times. The large total surface area of the microvilli also improves absorption processes. The products of intermediate hydrolysis enter the zone of the so-called brush border formed by microvilli, where the final stage of hydrolysis and the transition to absorption take place. The main enzymes involved in parietal digestion are amylase, lipase, and proteases. Thanks to this digestion, 80-90% of peptide and glycolytic bonds and 55-60% of triglycerides are cleaved.
Parietal digestion closely interacts with abdominal digestion. Cavitary digestion prepares the initial food substrates for parietal digestion, which reduces the volume of processed chyme in cavity digestion due to the transition of partial hydrolysis products to the brush border of the intestinal mucosa. These processes contribute to the most complete digestion of all food components and prepare them for absorption.
The motor activity of the small intestine ensures the mixing of chyme with digestive secrets and its movement through the intestine due to the contraction of the circular and longitudinal muscles. The duration of periods of contraction and relaxation of sections of the intestine during pendulum movements is 4-6 s. This periodicity of contractions is due to the automaticity of the smooth muscles of the intestine - the ability of the muscles to periodically contract and relax without external influences. Contractions of the circular muscles of the intestine cause peristaltic movements that help move food forward. Several peristaltic waves simultaneously move along the length of the intestine.
The contraction of the longitudinal and circular muscles is regulated by the vagus and sympathetic nerves. The vagus nerve stimulates intestinal motility. The sympathetic nerve transmits inhibitory signals that reduce muscle tone and inhibit mechanical bowel movements. Humoral factors also influence the motor function of the intestine: serotonin, choline and enterokinin stimulate bowel movement.
AT small intestine mixing of acidic chyme with alkaline secretions of the pancreas, intestinal glands and liver, depolymerization of nutrients to final products(monometers) that can enter the bloodstream, the promotion of chyme in the aboral direction, the excretion of metabolites, etc.
Digestion in this section begins in the intestinal cavity. (according to
flat digestion) and then continues in the glycocalyx zone and
striated border of enterocytes of the intestinal mucosa (parietal
digestion). Both cavitary and parietal digestion of the wasp
ferments pancreatic secretions and
intestinal juice under the influence of bile.
Secretory activity of the pancreas. The pancreas is a large gland of mixed secretion. The endocrine pancreas, represented by cells of the islets of Langerhans, produces a number of hormones (insulin, glucagon, etc.) directly into the blood. The exocrine section is represented by ocinus pancreatocytes that secrete digestive enzymes, as well as racinous epithelial cells and cells of the intercalary sections of the pancreas of small excretory ducts that secrete water, carbonates, and electrolytes. The emerging pancreatic juice enters through the excretory ducts into the duodenum. The composition and properties of pancreatic juice depend on the quantity and quality of food.
Composition and properties of pancreatic juice. A person produces 1.5-2.5 liters of pancreatic juice per day, isotonic to blood plasma, alkaline reaction (pH 7.5-8.8). This reaction is due to the content of bicarbonate ions, which neutralize the acidic gastric contents and create an alkaline environment in the duodenum that is optimal for the action of pancreatic enzymes. In addition, the composition of pancreatic juice includes cations Na +, K +, Ca 2+ M 2+ and anions
Cl - , HCO 3 2- , HPO 4 2- , as well as mucous substances. The concentration of bicarbonates in the juice is directly proportional to the rate of its secretion. Between the concentration of bicarbonates and chlorides, the relationship is inversely proportional.
Pancreatic juice contains enzymes for the hydrolysis of all kinds of nutrients: proteins, fats and carbohydrates.
Proteins cleave proteolytic enzymes, which are divided into two groups according to the mechanism of hydrolysis: endopeptidases (trypsin, chymotrypsin, elastase) cleave the internal peptide bonds of proteins, forming peptides and amino acids; exopeptidases (carboxyeptidase A and B and aminopeptidase) cleave end bonds in proteins and peptides, releasing amino acids one by one. Pancreatocytes secrete proteolytic enzymes in the form of inactive proenzymes of trypsinogens, chymotrypsinogens, procarboxypeptidases A and B, which are activated in the duodenum.
Trypsinogen is converted to active trypsin when the hexapeptide is cleaved off by the enzyme enterokinase (a product of the enterocytes of the Brunner glands). After the formation of trypsin, the process of activation of its precursor at pH 6.8-8.0 becomes autocatalytic. The resulting active trypsin, in turn, becomes an activator for chymotrypsinogen, procarboxypeptidases A and B, and proelastase.
The pancreatic juice contains lipolytic enzymes that are secreted in an inactive (prophospholipase A) and active state (pancreatic lipase, lecithinase).
Pancreatic lipase hydrolyzes neutral fats to fatty acids and monoglycerides, phospholipase A breaks down phospholipids to fatty acids. Hydrolysis of fats by lipase is enhanced in the presence of bile acids and calcium ions.
The amylolytic enzyme of the juice (pancreatic alpha-amylase) breaks down starch and glycogen into di- and monosaccharides. Disaccharides are further converted into monosaccharides under the influence of maltase and lactase.
Nucleotic enzymes belong to the phosphodiesterases. In pancreatic juice, they are represented by ribonuclease (glycolysis of ribonucleic acid) and deoxynuclease (hydrolysis of deoxy nucleic acid).
To prevent self-digestion, the same cells that secrete proteolytic enzymes simultaneously secrete a substance called a trypsin inhibitor. This substance accumulates in the cytoplasm of glandular cells, surrounding the enzyme granules, which prevents the activation of trypsin both inside the secretory cells and in the acini and pancreatic ducts. Since trypsin activates other pancreatic proteolytic enzymes, its inhibitor also prevents their subsequent activation.
Trypsin-activated kallikrein is biologically active substance, stimulates the formation in the blood of kallidin, a hypotensive peptide identical to bradykinin.
Secretion of bicarbonate and water ions. Unlike enzymes, bicarbonate and water ions are secreted in large quantities by the epithelial cells of the pancreatic ducts. Abundant secretion of pancreatic juice is accompanied by a significant increase in the concentration of bicarbonate ions - up to 145 Meq / l, which is five times higher than their content in plasma. Such a multiple increase in secreted bicarbonate is provided by closely interrelated processes, starting with the fact that carbon dioxide diffuses into cells from the blood and, under the influence of carbonic anhydrase, interacts with water, forming carbonic acid, which dissociates into bicarbonate and hydrogen ions. Bicarbonate passes through the cell membrane into the lumen of the ducts. Hydrogen ions formed during the dissociation of carbonic acid inside the cell are actively exchanged for sodium ions, which are transported to the pancreatic duct. The movement of sodium and bicarbonate ions from the blood into the lumen of the duct creates an osmotic gradient, causing the movement of water into the pancreatic duct and the formation of a bicarbonate solution.
regulation of pancreatic secretion. The secretory cells of the pancreas outside the period of digestion are at rest and separate the juice only in connection with the periodic activity of the gastrointestinal tract. The secretion of the pancreas occurs under the influence of nerve influences and humoral stimuli that occur when food enters the digestive tract, a also at the sight, smell of food and in the case of the action of the usual environment for its reception. As in the case of gastric secretion, the process of separation of pancreatic juice is divided into three phases: complex reflex (cerebral or cephalic), gastric and intestinal.
Complex reflex phase secretion begins not only with the direct action of the elements of food on the receptors of the oral cavity, but also with the sight, smell of food and under the action of the environment in which the act of eating takes place. The entry of food into the oral cavity and pharynx causes reflex excitation, superimposed on the already begun pancreatic secretion. This phase is much less pronounced than in the previous sections of the digestive tract.
Gastric phase secretion is the result of food entering the stomach. Excitation of the pancreas occurs with mechanical, chemical and humoral stimulation of the stomach receptors. Afferent impulses resulting from irritation of the chemoreceptors of the gastric mucosa, through sensitive nerve fibers, enter the central nervous system, from where efferent influences are sent along the vagus nerve to the pancreas. Chemical irritants are
substances produced in the gastrointestinal tract itself, and contained in food. Natural irritants that cause excitation of the pancreas are HC1, vegetable juices, fats and their hydrolysis products. The humoral regulator of the pancreas in this phase is the hormone of the antrum gastrin. The latter, being absorbed into the blood, stimulates the secretion of the pancreas.
Intestinal phase secretion begins after the entry of chyme into the duodenum. At this time, most of the pancreatic juice is produced. The amount and composition of the pancreatic secretion depends on the quality and quantity of food, is controlled by the receptive cells of the intestine and, first of all, the duodenum. This phase of secretion develops under the influence of reflex influences and intestinal hormones. The existence of powerful duodenopancreatic reflexes has been proven. The commonality of the innervation of the pancreas, duodenum and liver with the bile ducts determines their functional relationship.
The entry of hydrochloric acid and digestion products into the upper small intestine stimulates the secretion of the pancreas. Stimulation continues with the entry of bile into the duodenum and new particles of digested food into the small intestine. However, the pancreas in this phase of secretion is predominantly stimulated by the intestinal hormones secretin and cholecystokinin. Secretin is released by the action of HC1 on the S-cells of the duodenum. Under its influence, a large amount of pancreatic juice is produced, rich in bicarbonates and poor in enzymes, since it mainly acts on the functions of the epithelial cells of the ducts and has almost no effect on pancreatocytes. Cholecystokinin acts predominantly on acinar cells, causing the secretion of pancreatic juice rich in enzymes. The release of cholecystokinin from I-cells of the mucous membrane of the duodenum and jejunum to the greatest extent stimulates the products of initial hydrolysis dietary protein and fat, as well as amino acids, this process is stimulated to a lesser extent by hydrochloric acid and carbohydrates. Enzyme-rich pancreatic juice is secreted only with the joint action of secretin and cholecystokinin, potentiated by acetylcholine, on the gland.
Pancreatic secretion is also stimulated by vasoactive intestinal polypeptide (VIP), serotonin, which is formed in the enterochromaffin cells of the mucosa of the gastrointestinal tract and pancreatic tissue, insulin, bombesin, substance P, bile salts. Glucagon, calcitonin, PP, somatostatin, GIP have an inhibitory effect on secretion. The effects of intestinal peptides are mediated by their influence on the secretory activity of the gastric glands, which is accompanied by an increase in the acidity of the gastric chyme, which, upon entering the duodenum, stimulates the release of its hormones.
Parasympathetic nerve influences have a starting, and sympathetic - inhibitory effect on the gland. Nervous influences are less pronounced here than humoral ones. A polypeptide that stimulates the secretion of chymotrypsinogen was isolated from the duodenal mucosa.
A decrease in pancreatic secretion occurs with painful stimuli, during sleep, during intense physical and mental work.
In response to the consumption of protein and carbohydrate foods (meat, bread), there is a sharp increase in secretion in the first two hours, with a maximum of juice separation in the second hour after eating. In this case, the duration of secretion can be from 4-5 hours (meat) to 9-10 hours (bread). When fatty food (milk) is taken, the maximum rise in secretion occurs at the third hour, the duration of secretion for this stimulus is 5 hours (Fig. 9.4).
The role of the liver in digestion. The liver is a gland in which numerous and most complex biochemical processes take place, providing homeostasis of vital systems in the body that are closely related to metabolism. It affects the metabolism of proteins, peptides, carbohydrates, pigment metabolism, performs detoxification (neutralizing) and bile-forming functions.
Bile-forming and biliary functions of the liver. Bile is a secret and, at the same time, an excretion, constantly produced by liver cells-hepatocytes. Bile formation occurs in the liver through active and passive transport of water, glucose, creatinine, electrolytes, vitamins and hormones through cells and intercellular spaces, as well as active transport of bile acids by cells and reabsorption of water, mineral and organic substances from bile capillaries, ducts and gallbladder in which it is filled with the product of mucin-secreting cells.
Having entered the lumen of the duodenum, bile is included in the digestion process and is involved in the change of gastric digestion to intestinal, inactivating pepsin and neutralizing the acid of the contents of the stomach, creating favorable conditions for the activity of pancreatic enzymes, especially lipases. Bile acids of bile emulsify fats, reducing the surface tension of fat droplets, which creates conditions for the formation of fine particles that can be absorbed without prior hydrolysis, and increase its contact with lipolytic enzymes. Bile provides absorption in the small intestine of water-insoluble higher fatty acids, cholesterol, fat-soluble vitamins (D, E, K) and calcium salts, enhances the hydrolysis of proteins and carbohydrates, as well as the absorption of their hydrolysis products, promotes the resynthesis of triglycerides in enterocytes. Due to the alkaline reaction, bile is involved in the regulation of the pyloric sphincter. It has a stimulating effect on motor activity
small intestine, including the activity of intestinal villi, resulting in an increase in the rate of absorption of substances in the intestine; participates in parietal digestion, creating favorable conditions for the fixation of enzymes on the intestinal surface. Bile is one of the stimulators of pancreatic secretion, gastric mucus, motor and secretory activity of the small intestine, proliferation and desquamation of epithelial cells, and most importantly, the bile-forming function of the liver. The presence of digestive enzymes allows bile to participate in the processes of intestinal digestion, it also prevents the development of putrefactive processes, providing a bacteriostatic effect on the intestinal flora.
The secret of hepatocytes is a golden liquid, almost isotonic to blood plasma, its pH is 7.8-8.6. The daily secretion of bile in humans is 0.5-1.0 liters. Bile contains 97.5% water and 2.5% solids. Its constituent parts are bile acids, bile pigments, cholesterol, inorganic salts (sodium, potassium, calcium, magnesium, phosphates, iron and traces of copper). Bile contains fatty acids and neutral fats, lecithin, soaps, urea, uric acid, vitamins A, B, C, some enzymes (amylase, phosphatase, protease, catalase, oxidase), amino acids, glycoproteins. The qualitative originality of bile is determined by its main components: bile acids, bile pigments and cholesterol. Bile acids are specific metabolic products in the liver, bilirubin and cholesterol are of extrahepatic origin.
In hepatocytes, cholic and chenodeoxycholic acids (primary bile acids) are formed from cholesterol. Combining in the liver with the amino acids glycine or taurine, both of these acids are excreted in the form sodium salt taurocholic acid. In the distal small intestine, about 20% of primary bile acids are converted under the action of bacterial flora into secondary bile acids - deoxycholic and lithocholic. Here, approximately 90-85% of bile acids are actively reabsorbed, returned through the portal vessels to the liver and included in the bile. The remaining 10-15% of bile acids, mainly associated with undigested food, are excreted from the body, and their loss is replenished by hepatocytes.
Bile pigments - bilirubin and biliverdin - are excreted products of hemoglobin metabolism and give bile its characteristic color. The bile of humans and carnivores is dominated by bilirubin, which causes its golden yellow color, while the bile of herbivores contains biliverdin, which stains bile in green color. In hepatocytes, bilirubin forms water-soluble conjugates with glucuronic acid and, in a small amount, with sulfates. Bile pigments form urine pigments and kalaurobilin, urochrome and stercobilin.
The secret is secreted by hepatocytes into the lumen of the bile capillaries, from which, through the intralobular or interlobular bile ducts, bile enters the larger bile ducts, accompanied by
branching of the portal vein. The bile ducts gradually merge and form the hepatic duct, from which bile can enter either through the cystic duct into the gallbladder or into the common bile duct.
Liquid and transparent, golden yellow in color, hepatic bile, when moving through the ducts, begins to undergo some changes due to the absorption of water and the addition of biliary mucin, but this does not significantly change it. physical and chemical properties. The most significant changes in bile occur during the extradigestive period, when it is directed through the cystic duct to the gallbladder. Here the bile is concentrated, becomes dark, cystic mucin contributes to an increase in its viscosity, increases specific gravity, absorption of bicarbonates and the formation of bile salts leads to a decrease in the active reaction (pH 6.0-7.0). In the gallbladder, bile concentrates 7-10 times in 24 hours. Due to this concentrating ability, the human gallbladder, which has a volume of only 50-80 ml, can accommodate bile produced within 12 hours (table 9.2).
Table 9.2 Composition of hepatic and cystic bile
The main processes of food digestion take place in the small intestine. Particularly great is the role of its initial section - the duodenum. In the process of digestion, pancreatic, intestinal juices and bile are involved here. With the help of enzymes that are part of the pancreatic and intestinal juices, hydrolysis of proteins, fats and carbohydrates occurs.
Composition and properties of pancreatic juice
The exocrine activity of the pancreas consists in the formation and secretion of 1.5-2.0 liters of pancreatic juice into the duodenum. The composition of pancreatic juice includes water and dry residue (0.12%), which is represented by inorganic and organic substances. The juice contains Na+, Ca2+, K+, Mg+ cations and Cl-, SO32-, HPO42- anions. There are especially many bicarbonates in it, due to which the pH of the juice is 7.8-8.5. Pancreatic enzymes are active in a slightly alkaline environment.
Pancreatic juice is represented by proteolytic, lipolytic and amylolytic enzymes that digest proteins, fats, carbohydrates and nucleic acids. Alpha-amylase, lipase and nuclease are secreted in an active state; proteases - in the form of proenzymes. Pancreatic alpha-amylase breaks down polysaccharides into oligo-, di- and monosaccharides. Nucleic acids are cleaved by ribo- and deoxyribonucleases.
Pancreatic lipase, active in the presence of bile salts, acts on lipids, splitting them into monoglycerides and fatty acids. Lipids are also affected by phospholipase A and esterase. In the presence of calcium ions, the hydrolysis of fats is enhanced. Proteolytic enzymes are secreted in the form of proenzymes - trypsinogen, chymotrypsinogen, procarboxypeptidase A and B, proelastase. Under the influence of duodenal enterokinase, trypsinogen is converted to trypsin. Trypsin itself then acts autocatalytically on the remaining amount of trypsinogen and on other propeptidases, converting them into active enzymes. Trypsin, chymotrypsin, elastase mainly cleave the internal peptide bonds of food proteins, resulting in the formation of low molecular weight peptides and amino acids. Carboxypeptidases A and B cleave C-terminal bonds in proteins and peptides.
Regulation of pancreatic secretion
Regulation of pancreatic exocrine secretion is carried out by nervous and humoral mechanisms. The vagus nerve enhances the secretion of the pancreas. Sympathetic nerves reduce the amount of secretion, but increase the synthesis of organic substances (beta-adrenergic effect). A decrease in secretion also occurs due to a decrease in the blood supply to the pancreas by narrowing the blood vessels (alpha-adrenergic effect). Intense physical and mental work, pain, sleep cause inhibition of secretion. Gastrointestinal hormones, secretin and CCK-PZ increase the secretion of pancreatic juice. Secretin stimulates the secretion of juice rich in bicarbonates, CCK-PZ - rich in enzymes. Pancreatic secretion is increased by gastrin, serotonin, bombesin, insulin, and bile salts. Chymodenin stimulates the secretion of chymotrypsinogen. The inhibitory effect is exerted by GIP, PP, glucagon, calcitonin, somatostatin, enkephalin.
There are 3 phases of pancreatic secretion: complex reflex, gastric and intestinal. The separation of pancreatic juice is influenced by the nature of the food taken. These influences are mediated through the corresponding gastrointestinal hormones. So, food products that enhance the secretion of hydrochloric acid in the stomach (extractive meat substances, vegetables, protein digestion products), stimulate the production of secretin, and therefore lead to the release of pancreatic juice rich in bicarbonates. The products of the initial hydrolysis of proteins and fats stimulate the secretion of CCK-PZ, which, in turn, promotes the secretion of juice with a large number of enzymes. Thus, with a long-term predominance in diet only carbohydrates, or proteins, or fats occur and a corresponding change in the enzymatic composition of pancreatic juice occurs.
The pancreas also has intrasecretory activity, producing insulin, glucagon, somatostatin, pancreatic polypeptide, serotonin, VIP, gastrin, enkephalin, kallikrein, lipoxin, and vagotonin.
Composition and properties of intestinal juice
Intestinal juice is a secret of glands located in the mucous membrane along the entire small intestine (duodenal or Brunner glands, intestinal crypts or Lieberkün glands, intestinal epitheliocytes, goblet cells, Paneth cells). In an adult, 2-3 liters of intestinal juice is separated per day, pH is from 7.2 to 9.0. Juice consists of water and dry residue, which is represented by inorganic and organic substances. Of the inorganic substances in the juice contains a lot of bicarbonates, chlorides, phosphates of sodium, calcium, potassium. The composition of organic substances includes proteins, amino acids, mucus. In the intestinal juice there are more than 20 enzymes that provide the final stages of digestion of all nutrients. These are enterokinase, peptidases, alkaline phosphatase, nuclease, lipase, phospholipase, amylase, lactase, sucrase. There are hereditary and acquired deficiencies of intestinal enzymes that break down carbohydrates (disaccharidases), which leads to intolerance to the corresponding disaccharides. For example, many people, especially the peoples of Asia and Africa, have lactase deficiency. The main part of the enzymes enters the intestinal juice when the cells of the intestinal mucosa are rejected. A significant amount of enzymes is adsorbed on the surface of intestinal epithelial cells, carrying out parietal digestion.
Regulation of intestinal secretion
The regulation of the activity of the glands of the small intestine is carried out by local neuro-reflex mechanisms, as well as by humoral influences and chyme ingredients. Mechanical irritation of the mucous membrane of the small intestine causes the release of liquid secretion with a low content of enzymes. Local irritation of the intestinal mucosa by the products of digestion of proteins, fats, hydrochloric acid, pancreatic juice causes the separation of intestinal juice rich in enzymes. Enhance intestinal juice secretion GIP, VIP, motilin. The hormones enterocrinin and duocrinin, secreted by the mucous membrane of the small intestine, stimulate the secretion of the Lieberkühn and Brunner glands, respectively. Somatostatin has an inhibitory effect.
Abdominal and parietal digestion in the small intestine
There are two types of digestion in the small intestine: abdominal and parietal. Cavitary digestion occurs with the help of enzymes of digestive secrets entering the cavity of the small intestine (pancreatic juice, bile, intestinal juice). As a result of cavity digestion, large molecular substances (polymers) are hydrolyzed mainly to the stage of oligomers. Their further hydrolysis takes place in the area adjacent to the mucous membrane and directly on it.
Parietal digestion in a broad sense occurs in the layer of mucous overlays located above the glycocalyx, the glycocalyx zone and on the surface of the microvilli. The layer of mucous overlays consists of mucus produced by the mucous membrane of the small intestine and desquamated intestinal epithelium. This layer contains many pancreatic enzymes and intestinal juice.
Nutrients passing through the mucus layer are exposed to these enzymes. Glycocalyx adsorbs digestive juice enzymes from the small intestine cavity, which carry out intermediate stages of hydrolysis of all essential nutrients. The products of hydrolysis enter the apical membranes of enterocytes, into which intestinal enzymes are embedded, which carry out their own membrane digestion, which results in the formation of absorbable monomers. Due to the close location of the own intestinal enzymes embedded in the membrane and transport systems that provide absorption, conditions are created for conjugation of the processes of final hydrolysis of nutrients and the beginning of their absorption.
Membrane digestion is characterized by the following dependence: the secretory activity of epitheliocytes decreases from the crypt to the top of the intestinal villus. In the upper part of the villus, hydrolysis of dipeptides occurs mainly, at the base - of disaccharides. Parietal digestion depends on the enzyme composition of enterocyte membranes, the sorption properties of the membrane, the motility of the small intestine, the intensity of abdominal digestion, and diet. Membrane digestion is influenced by adrenal hormones (synthesis and translocation of enzymes).
