Digestion. The physical and chemical processing of food is a complex process that is carried out by the digestive system. Digestion in various parts of the digestive tract Digestive system and the digestion process in the oral cavity

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9.1. General characteristics of digestive processes

The human body in the process of life consumes various substances and a significant amount of energy. Nutrients, mineral salts, water and a number of vitamins that are necessary to maintain homeostasis and restore the body’s plastic and energy needs must be supplied from the external environment. At the same time, a person is not able to absorb carbohydrates, proteins, fats and some other substances from food without first processing it, which is carried out by the digestive organs.

Digestion is the process of physical and chemical processing of food, as a result of which it becomes possible to absorb nutrients from the digestive tract, enter them into the blood or lymph and be absorbed by the body. Complex physical and chemical transformations of food occur in the digestive apparatus, which are carried out thanks to motor, secretory and suction its functions. In addition, the organs of the digestive system also perform excretory function, removing from the body the remains of undigested food and some metabolic products.

Physical processing of food consists of crushing it, mixing and dissolving the substances it contains. Chemical changes in food occur under the influence of hydrolytic digestive enzymes produced by the secretory cells of the digestive glands. As a result of these processes, complex food substances are broken down into simpler ones, which are absorbed into the blood or lymph and participate in the body’s metabolism. During processing, food loses its species-specific properties, turning into simple constituent elements that can be used by the body. Thanks to the hydrolytic action of enzymes, amino acids and low molecular weight polypeptides are formed from food proteins, glycerol and fatty acids from fats, and monosaccharides from carbohydrates. These digestive products enter through the mucous membrane of the stomach, small and large intestines into the blood and lymphatic vessels. Thanks to this process, the body receives the nutrients necessary for life. Water, mineral salts and some

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amount of low molecular weight organic compounds can be absorbed into the blood without pre-treatment.

In order to digest food evenly and more completely, it requires mixing and movement through the gastrointestinal tract. This is ensured motor function of the digestive tract by contracting the smooth muscles of the walls of the stomach and intestines. Their motor activity is characterized by peristalsis, rhythmic segmentation, pendulum-like movements and tonic contraction.

Bolus transfer carried out at the expense of peristalsis, which occurs due to the contraction of circular muscle fibers and relaxation of longitudinal ones. The peristaltic wave allows the food bolus to move only in the distal direction.

Mixing of food masses with digestive juices is ensured rhythmic segmentation and pendulum-like movements intestinal wall.

The secretory function of the digestive tract is carried out by the corresponding cells that are part of the salivary glands of the oral cavity, proteases that break down proteins; 2) lipases, breaking down fats; 3) carbohydrase, breaking down carbohydrates.

The digestive glands are innervated mainly by the parasympathetic part of the autonomic nervous system and, to a lesser extent, by the sympathetic one. In addition, these glands are influenced by hormones of the gastrointestinal tract (gastrsh; secretsh and choleocystokt-pancreozymin).

Liquid moves through the walls of the human gastrointestinal tract in two directions. From the cavity of the digestive apparatus, digested substances are absorbed into the blood and lymph. At the same time, the internal environment of the body releases a number of dissolved substances into the lumen of the digestive organs.

The digestive system plays an important role in maintaining homeostasis due to its excretory functions. The digestive glands are capable of secreting into the cavity of the gastrointestinal tract a significant amount of nitrogenous compounds (urea, uric acid), salts, and various medicinal and toxic substances. The composition and quantity of digestive juices can be a regulator of the acid-base state and water-salt metabolism in the body. There is a close relationship between the

telial function of the digestive organs with the functional state of the kidneys.

9.2. Digestion in various parts of the gastrointestinal tract

Digestion processes in different parts of the gastrointestinal tract have their own characteristics. These are features of the physical and chemical processing of food, motor, secretory, absorption and excretory functions of different parts of the digestive tract.

Digestion in the oral cavity. Food processing begins in the oral cavity. Here it is crushed, wetted with saliva, the initial hydrolysis of some nutrients and the formation of a food bolus. Food is retained in the oral cavity for 15-18 s. Being in the oral cavity, it irritates the taste, tactile and temperature receptors of the mucous membrane and papillae of the tongue. Irritation of these receptors causes reflex acts of secretion of the salivary, gastric and pancreatic glands, the release of bile into the duodenum, and changes the motor activity of the stomach.

After grinding and grinding with teeth, food is chemically processed due to the action of hydrolytic enzymes in saliva. The ducts of three groups of salivary glands open into the oral cavity: slimy, se-pink and mixed.

Saliva - the first digestive juice, which contains hydrolytic enzymes that break down carbohydrates. Saliva enzyme amipase(ptialin) converts starch into disaccharides, and the enzyme maltaza - disaccharides to monosaccharides. The total amount of saliva secreted per day is 1-1.5 liters.

The activity of the salivary glands is regulated by reflex. Irritation of the receptors in the oral mucosa causes salivation the mechanism of unconditioned reflexes. The centripetal nerves in this case are the branches of the trigeminal and glossopharyngeal nerves, through which excitations from the receptors of the oral cavity are transmitted to the salivary centers located in the medulla oblongata. Effector functions are performed by parasympathetic and sympathetic nerves. The first of them provide a copious secretion of liquid saliva, while when the latter are irritated, thick saliva containing a lot of mucin is released. Salivation according to the mechanism of conditioned reflexes occurs even before food enters the mouth and occurs when

irritation of various receptors (visual, olfactory, auditory), accompanying food intake. In this case, the information enters the cerebral cortex, and the impulses coming from there excite the salivation centers of the medulla oblongata.

Digestion in the stomach. The digestive functions of the stomach include the deposition of food, its mechanical and chemical processing and the gradual evacuation of food contents through the pylorus into the duodenum. Chemical processing of food is carried out jelly-milk juice, of which a person produces 2.0-2.5 liters per day. Gastric juice is secreted by numerous glands of the body of the stomach, which consist of main, lining And additional cells. The main cells secrete digestive enzymes, the parietal cells secrete hydrochloric acid, and the accessory cells secrete mucus.

The main enzymes in gastric juice are proteases And whether-groove. Several proteases include pepsins, and gelatinase And hee-mozin. Pepsins are excreted as inactive pepsinogens. The conversion of pepsinogens into active pepsin is carried out under the influence of salt acids. Pepsins break down proteins into polypeptides. Their further breakdown to amino acids occurs in the intestines. Gelatinase promotes the digestion of connective tissue proteins. Chymosin curdles milk. Gastric juice lipase breaks down only emulsified fats (milk) into glycerol and fatty acids.

Gastric juice has an acidic reaction (pH during food digestion is 1.5-2.5), which is due to the content of 0.4-0.5% hydrochloric acid in it. Hydrochloric acid in gastric juice plays an important role in digestion. She calls denaturation and swelling of proteins^ thereby promoting their subsequent breakdown by pepsins, activates pepsinogens, promotes coagulation milk, participates in antibacterial action of gastric juice, activates the hormone gastrin ? formed in the mucous membrane of the pylorus and stimulates gastric secretion, and, depending on the pH value, enhances or inhibits the activity of the entire digestive tract. Entering the duodenum, hydrochloric acid stimulates the formation of the hormone there secretin, regulating the activity of the stomach, pancreas and liver.

Gastric mucus (muct) is a complex complex of glucoproteins and other proteins in the form of colloidal solutions. Mucin covers the entire surface of the gastric mucosa and protects it from both mechanical damage and self-digestion, since it has

pronounced antipeptic activity and is able to neutralize hydrochloric acid.

The whole process gastric secretion It is customary to divide it into three phases: complex reflex (cerebral), neurochemical (stomach) and intestinal (duodenal).

Complex reflex phase gastric secretion occurs when exposed to conditioned stimuli (the sight, smell of food) and unconditioned (mechanical and chemical irritation of food receptors of the mucous membrane of the mouth, pharynx and esophagus). The excitation that arises in the receptors is transmitted to the food center of the medulla oblongata, from where impulses travel along the centrifugal fibers of the vagus nerve to the glands of the stomach. In response to irritation of the above receptors, gastric secretion begins after 5-10 minutes, which lasts 2-3 hours (with imaginary feeding).

Neurochemical phase gastric secretion begins after food enters the stomach and is caused by the action of mechanical and chemical stimuli on its wall. Mechanical stimuli act on the mechanoreceptors of the gastric mucosa and reflexively cause secretion. Natural chemical stimulators of juice secretion in the second phase are salts, extractives of meat and vegetables, products of protein digestion, alcohol and, to a lesser extent, water.

The hormone plays a significant role in enhancing gastric secretion gastritis, which is formed in the wall of the pylorus. With the blood, gastrin enters the cells of the gastric glands, increasing their activity. In addition, it stimulates the activity of the pancreas and the secretion of bile.

Intestinal phase gastric juice secretion is associated with the transition of food from the stomach to the intestines. It develops when chyme irritates the receptors of the small intestine, as well as when nutrients enter the blood and is characterized by a long latent period (1-3 hours) and a long duration of secretion of gastric juice with a low content of hydrochloric acid. In this phase, the secretion of the gastric glands is also stimulated by the hormone enterogastrin, secreted by the mucous membrane of the duodenum.

Digestion of food in the stomach usually occurs within 6-8 hours. The duration of this process depends on the composition of the food, its volume and consistency, as well as on the amount of gastric juice released. Fatty foods stay in the stomach for an especially long time (8-10 hours).

Evacuation of food from the stomach to the intestines occurs unevenly, in separate portions. This is due to periodic contractions of the muscles of the entire stomach, and especially strong contractions of the sphincter at

gatekeeper The pyloric muscles reflexively contract (the release of food masses stops) when hydrochloric acid acts on the receptors of the mucous membrane of the duodenum. After neutralizing the hydrochloric acid, the pyloric muscles relax and the sphincter opens.

Digestion in the duodenum. In ensuring intestinal digestion, the processes occurring in the duodenum are of great importance. Here food masses are exposed to intestinal juice, bile and pancreatic juice. The length of the duodenum is small, so food is not retained here, and the main processes of digestion occur in the underlying sections of the intestine.

Intestinal juice is formed by the glands of the duodenal mucosa; it contains a large amount of mucus and enzyme peptide-zu, breaking down proteins. It also contains an enzyme enterokinase, which activates trypsinogen in pancreatic juice. The cells of the duodenum produce two hormones - secrett and cholecystokt-pancreozymin, enhancing pancreatic secretion.

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 breakdown of nutrients carried out in the duodenum, the role of pancreatic juice is especially important.

The role of the pancreas in digestion. The bulk of the pancreas tissue produces digestive juice, which is excreted through the duct into the cavity of the duodenum. A person secretes 1.5-2.0 liters of pancreatic juice 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 secreted by the pancreas in an active state and break down starch, milk sugar, nucleic acids and fats, respectively. Nucleases trypsin and chymotryp-syn are formed by gland cells in an inactive state in the form thripsto-gene and chymotrinsinogen. Trypsinogen in the duodenum under the action of its enzyme enteroctases turns into trypsin. In turn, trypsin converts chymotrypsinogen into active chymotrypsin. Under the influence of trypsin and chymotrypsin, proteins and high molecular weight polypeptides are broken down into low molecular weight peptides and free amino acids.

Secretion of pancreatic juice begins 2-3 minutes after eating and lasts from 6 to 10 hours, depending on the composition and volume of food.

cabbage soup It occurs under the influence of conditioned and unconditioned stimuli, as well as under the influence of humoral factors. In the latter case, duodenal hormones play an important role: secretin and cholecystokinin-pancreozymin, as well as gastrin, insulin, serotonin, etc.

The role of the liver in digestion. Liver cells continuously secrete bile, which is one of the most important digestive juices. A person produces about 500-1000 ml of bile per day. The process of bile formation is continuous, and its entry into the duodenum is periodic, mainly in connection with food intake. On an empty stomach, bile does not enter the intestine; it is sent to the gallbladder, where it is concentrated and slightly changes its composition.

Bile contains bile acids, bile pigments and other organic and inorganic substances. Bile acids take part in the process of food digestion. Bile pigment bilirubgsh is formed from hemoglobin during the destruction of red blood cells in the liver. The dark color of bile is due to the presence of this pigment in it. Bile increases the activity of enzymes in pancreatic and intestinal juices, especially lipase. It emulsifies fats and dissolves the products of their hydrolysis, thereby facilitating their absorption.

The formation and secretion of bile from the bladder into the duodenum occurs under the influence of nervous and humoral influences. Nervous influences on the biliary apparatus are carried out conditionally and unconditionally with the participation of numerous reflexogenic zones, and primarily - receptors of the oral cavity, stomach and duodenum. Activation of the vagus nerve increases the secretion of bile, the sympathetic nerve inhibits bile formation and stops the evacuation of bile from the sac. The hormone cholecystokinin-pancreozymin, which causes contraction of the gallbladder, plays an important role as a humoral stimulator of bile secretion. Gastrin and secretin have a similar, although weaker, effect. Glucagon and calciotonin inhibit the secretion of bile.

The liver, forming bile, performs not only secretory, but also ex-cretor(excretory) function. The main organic excrements of the liver are bile salts, bilirubin, cholesterol, fatty acids and lecithin, as well as calcium, sodium, chlorine, bicarbonates. Once in the intestines with bile, these substances are excreted from the body.

Along with the formation of bile and participation in digestion, the liver also performs a number of other important functions. The role of the liver is great in the exchange of goodssociety The products of food digestion are carried by the blood to the liver, and here

their further processing takes place. In particular, the synthesis of certain proteins (fibrinogen, albumin) is carried out; neutral fats and lipoids (cholesterol); Urea is synthesized from ammonia. Glycogen is deposited in the liver, and fats and lipoids in small quantities. Exchange takes place in it. vitamins, especially group A. One of the most important functions of the liver is barrier, which consists in neutralizing toxic substances and foreign proteins coming from the intestines with the blood.

Digestion in the small intestine. Food masses (chyme) from the duodenum move into the small intestine, where they continue to be digested by digestive juices released into the duodenum. At the same time, our own intestinal juice, produced by the Lieberkühn and Brunner glands of the mucous membrane of the small intestine. Intestinal juice contains enterokinase, as well as a full set of enzymes that break down proteins, fats and carbohydrates. These enzymes are only involved in wall digestion, since they are not excreted into the intestinal cavity. Cavity Digestion in the small intestine is carried out by enzymes supplied with food chyme. Cavity digestion is most effective for the hydrolysis of large molecular substances.

Parietal (membrane) digestion occurs on the surface of the microvilli of the small intestine. It completes the intermediate and final stages of digestion by hydrolysis of intermediate digestion products. Microvilli are cylindrical outgrowths of the intestinal epithelium 1-2 microns in height. Their number is huge - from 50 to 200 million per 1 mm 2 of intestinal surface, which increases the internal surface of the small intestine by 300-500 times. The extensive surface of 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 occur. The main enzymes involved in parietal digestion are amylase, lipase and prbtheases. Thanks to this digestion, 80-90% of peptide and glycolytic bonds and 55-60% of triglycerols are broken down.

The motor activity of the small intestine ensures the mixing of chyme with digestive secretions and its movement through the intestine due to the contraction of circular and longitudinal muscles. Contraction of the longitudinal fibers of intestinal smooth muscle is accompanied by a shortening of the intestinal section, while relaxation is accompanied by its lengthening.

The contraction of the longitudinal and circular muscles is regulated by the vagus and sympathetic nerves. The vagus nerve stimulates intestinal motor function. The sympathetic nerve transmits inhibitory signals that reduce muscle tone and inhibit mechanical movements of the intestines. Humoral factors also influence intestinal motor function: serotin, choline and enterokinin stimulate intestinal movements.

Digestion in the large intestine. Digestion of food ends mainly in the small intestine. The glands of the large intestine secrete a small amount of juice, rich in mucus and poor in enzymes. The low enzymatic activity of large intestinal juice is due to the small amount of undigested substances in the chyme coming from the small intestine.

A large role in the life of the body and the functions of the digestive tract is played by the microflora of the large intestine, where billions of different microorganisms live (anaerobic and lactic bacteria, E. coli, etc.). The normal microflora of the large intestine takes part in several functions: protects the body from pathogenic microbes: participates in the synthesis of a number of vitamins (B vitamins, vitamin K); inactivates and decomposes enzymes (trypsin, amylase, gelatinase, etc.) coming from the small intestine, and also ferments carbohydrates and causes rotting of proteins.

The movements of the large intestine are very slow, so about half of the time spent on the digestive process (1-2 days) is spent moving food debris in this section of the intestine.

In the large intestine, water is intensively absorbed, resulting in the formation of feces consisting of the remains of undigested food, mucus, bile pigments and bacteria. Emptying the rectum (defecation) is carried out reflexively. The reflex arc of the act of defecation closes in the lumbosacral part of the spinal cord and ensures involuntary emptying of the large intestine. The voluntary act of defecation occurs with the participation of the centers of the medulla oblongata, hypothalamus and cerebral cortex. Sympathetic nerve influences inhibit rectal motility, while parasympathetic influences stimulate.

9.3. Absorption of food digestion products

By suction is the process of entry into the blood and lymph of various substances from the digestive system. The intestinal epithelium is the most important barrier between the external environment, the role of which is played by the intestinal cavity, and the internal environment of the body (blood, lymph), where nutrients enter.

Absorption is a complex process and is provided by various mechanisms: filtration, associated with the difference in hydrostatic pressure in media separated by a semi-permeable membrane; differentialfusion substances along a concentration gradient; by osmosis. The amount of absorbed substances (with the exception of iron and copper) does not depend on the body's needs, it is proportional to food consumption. In addition, the mucous membrane of the digestive organs has the ability to selectively absorb some substances and limit the absorption of others.

The epithelium of the mucous membranes of the entire digestive tract has the ability to absorb. For example, the oral mucosa can absorb essential oils in small quantities, which is what the use of some medications is based on. The gastric mucosa is also capable of absorption to a small extent. Water, alcohol, monosaccharides, and mineral salts can pass through the gastric mucosa in both directions.

The absorption process is most intensive in the small intestine, especially in the jejunum and ileum, which is determined by their large surface, many times greater than the surface of the human body. The surface of the intestine is increased by the presence of villi, inside of which there are smooth muscle fibers and a well-developed circulatory and lymphatic network. The intensity of absorption in the small intestine is about 2-3 liters per hour.

Carbohydrates are absorbed into the blood mainly in the form of glucose, although other hexoses (galactose, fructose) can also be absorbed. Absorption occurs predominantly in the duodenum and the upper part of the jejunum, but can partially occur in the stomach and large intestine.

Squirrels absorbed in the form of amino acids and in small quantities in the form of polypeptides through the mucous membranes of the duodenum and jejunum. Some amino acids can be absorbed in the stomach and proximal colon. Amino acids are absorbed both by diffusion and active transport. After absorption through the portal vein, amino acids enter the liver, where they are deaminated and transaminated.
Fats Absorbed in the form of fatty acids and glycerol only in the upper part of the small intestine. Fatty acids are insoluble in water, therefore absorption, as well as the absorption of cholesterol and other lipoids, occurs only in the presence of bile. Only emulsified fats can be partially absorbed without preliminary breakdown into glycerol and fatty acids. Fat-soluble vitamins A, D, E and K also need emulsification to be absorbed. Most of the fat is absorbed into the lymph, then through the thoracic duct it enters the blood. No more than 150-160 g of fat is absorbed in the intestines per day.

Water and some electrolytes pass through the membranes of the mucous membrane of the digestive canal in both directions. Water passes through diffusion. The most intensive absorption occurs in the large intestine. Sodium, potassium and calcium salts dissolved in water are absorbed predominantly in the small intestine through the mechanism of active transport, against the concentration gradient.

9.4. The effect of muscle work on digestion

Muscular activity, depending on its intensity and duration, has a different effect on the digestive processes. Regular physical exercise and moderate work, increasing metabolism and energy, increase the body's need for nutrients and thereby stimulate the functions of various digestive glands and absorption processes. The development of the abdominal muscles and their moderate activity increase the motor function of the gastrointestinal tract, which is used in the practice of physical therapy.

However, the positive effect of physical activity on digestion is not always observed. Work performed immediately after eating slows down the digestion process. In this case, the complex reflex phase of secretion of the digestive glands is most inhibited. In this regard, it is advisable to perform physical activity no earlier than 1.5-2 hours after eating. At the same time, it is not recommended to work on an empty stomach. Under these conditions, especially during prolonged work, the body's energy resources quickly decrease, which leads to significant changes in body functions and a decrease in performance.

With intense muscle activity, as a rule, there is inhibition of the secretory and motor functions of the gastrointestinal tract. This manifests itself in inhibition of salivation, decreased secretory,

acid-forming and motor functions of the stomach. At the same time, hard work completely suppresses the complex-reflex phase of gastric secretion and significantly less inhibits the neurochemical and intestinal phases. This also indicates the need to take a certain break when performing muscle work after eating.

Significant physical activity reduces the secretion of digestive pancreatic juice and bile; less intestinal juice is secreted. All this leads to a deterioration in both cavity and parietal digestion, especially in the proximal parts of the small intestine. The depression of digestion is most pronounced after eating a meal rich in fats than after a protein-carbohydrate diet.

Inhibition of the secretory and motor functions of the gastrointestinal

tract during intense muscular work is due to inhibition of food-
centers as a result of negative induction from excited motors
body zones of the central nervous system. :

In addition, during physical work, the excitation of the centers of the autonomic nervous system changes with a predominance of the tone of the sympathetic department, which has an inhibitory effect on the digestive processes. Increased secretion of the adrenal hormone also has a depressing effect on these processes. adrenaline.

A significant factor influencing the functions of the digestive organs is the redistribution of blood during physical work. The bulk of it goes to the working muscles, while other systems, including the digestive organs, do not receive the required amount of blood. In particular, the volumetric blood flow rate of the abdominal organs decreases from 1.2-1.5 l/min at rest to 0.3-0.5 l/min during physical work. All this leads to a decrease in the secretion of digestive juices, a deterioration in the processes of digestion and absorption of nutrients. With many years of intense physical work, such changes can become persistent and serve as the basis for the emergence of a number of diseases of the gastrointestinal tract.

When playing sports, it should be taken into account that not only muscle work inhibits digestive processes, but digestion can also negatively affect physical activity. Excitation of food centers and blood outflow from skeletal muscles to the organs of the gastrointestinal tract reduce the effectiveness of physical work. In addition, a full stomach raises the diaphragm, which adversely affects the functioning of the respiratory and circulatory organs.

Complex physical and chemical transformations of food occur in the digestive apparatus, which are carried out thanks to its motor, secretory and absorption functions. In addition, the organs of the digestive system also perform an excretory function, removing the remains of undigested food and some metabolic products from the body.

Physical processing of food consists of crushing it, mixing it and dissolving the substances it contains. Chemical changes in food occur under the influence of hydrolytic digestive enzymes produced by the secretory cells of the digestive glands. As a result of these processes, complex food substances are broken down into simpler ones, which are absorbed into the blood or lymph and participate in metabolism

substances in the body. During processing, food loses its species-specific properties, turning into simple components that can be used by the body.

For the purpose of uniform and more complete digestion of food

it requires mixing and movement through the gastrointestinal tract. This is ensured by the motor function of the digestive tract due to the contraction of smooth muscles of the walls of the stomach and intestines. Their motor activity is characterized by peristalsis, rhythmic segmentation, pendulum-like movements and tonic contraction.

The secretory function of the digestive tract is carried out by the corresponding cells that are part of the salivary glands of the oral cavity, the glands of the stomach and intestines, as well as the pancreas and liver. Digestive secretions are a solution of electrolytes containing enzymes and other substances. There are three groups of enzymes involved in digestion: 1) proteases that break down proteins;

2) lipases that break down fats; 3) carbohydrases that break down carbohydrates. All digestive glands produce about 6-8 liters of secretion per day, a significant part of which is reabsorbed in the intestine.

The digestive system plays an important role in maintaining homeostasis through its excretory function. The digestive glands are capable of secreting into the cavity of the gastrointestinal tract a significant amount of nitrogenous compounds (urea, uric acid), water, salts, and various medicinal and toxic substances. The composition and quantity of digestive juices can be a regulator of the acid-base state and water-salt metabolism in the body. There is a close relationship between the excretory function of the digestive organs and the functional state of the kidneys.

The study of the physiology of digestion is primarily the merit of I. P. Pavlov and his students. They developed a new method for studying gastric secretion - they surgically cut out a part of the dog's stomach while preserving the autonomic innervation. A fistula was implanted into this small ventricle, making it possible to receive pure gastric juice (without food admixture) at any stage of digestion. This made it possible to characterize in detail the functions of the digestive organs and reveal the complex mechanisms of their activity. In recognition of I.P. Pavlov’s merits in the physiology of digestion, he was awarded the Nobel Prize on October 7, 1904. Further studies of digestive processes in the laboratory of I. P. Pavlov revealed the mechanisms of activity of the salivary and pancreas glands, liver and intestinal glands. It was found that the higher the glands are located in the digestive tract, the greater the importance of nervous mechanisms in the regulation of their functions. The activity of the glands located in the lower parts of the digestive tract is regulated primarily by humoral pathways.

DIGESTION IN DIFFERENT DEPARTMENTS OF THE GASTROINTESTINAL TRACT

Digestion processes in different parts of the gastrointestinal tract have their own characteristics. These differences relate to the physical and chemical processing of food, motor, secretory, absorption and excretory functions of the digestive organs.

DIGESTION IN THE ORAL CAVITY

Processing of ingested food begins in the oral cavity. Here it is crushed, moistened with saliva, the taste properties of food are analyzed, the initial hydrolysis of some nutrients and the formation of a food bolus. Food is retained in the oral cavity for 15-18 s. While in the oral cavity, food irritates the taste, tactile and temperature receptors of the mucous membrane and papillae of the tongue. Irritation of these receptors causes reflex acts of secretion of the salivary, gastric and pancreatic glands, the release of bile into the duodenum, changes the motor activity of the stomach, and also has an important effect on chewing, swallowing and taste assessment of food.

After grinding and grinding with the teeth, the food is chemically processed thanks to the action of hydrolytic enzymes of the spruce. The ducts of three groups of salivary glands open into the oral cavity: mucous, serous and mixed: Numerous glands of the oral cavity and tongue secrete mucous, mucin-rich saliva, the parotid glands secrete liquid, serous saliva, rich in enzymes, and the submandibular and sublingual glands secrete mixed saliva. The protein substance in saliva, mucin, makes the food bolus slippery, which makes it easier to swallow food and move it along the esophagus.

Saliva is the first digestive juice that contains hydrolytic enzymes that break down carbohydrates. The salivary enzyme amylase (ptialin) converts starch into disaccharides, and the enzyme maltase converts disaccharides into monosaccharides. Therefore, when chewing food containing starch for a long enough time, it acquires a sweet taste. The composition of saliva also includes acid and alkaline phosphatases, a small amount of proteolytic, lipolytic enzymes and nucleases. Saliva has pronounced bactericidal properties due to the presence of the enzyme lysozyme, which dissolves the bacterial membrane. The total amount of saliva secreted per day can be 1 -1.5 liters.

The food bolus formed in the oral cavity moves to the root of the tongue and then enters the pharynx.

Afferent impulses upon irritation of the receptors of the pharynx and soft palate are transmitted along the fibers of the trigeminal, glossopharyngeal and superior laryngeal nerves to the swallowing center located in the medulla oblongata. From here, efferent impulses travel to the muscles of the larynx and pharynx, causing coordinated contractions.

As a result of the sequential contraction of these muscles, the food bolus enters the esophagus and then moves to the stomach. Liquid food passes the esophagus in 1-2 s; hard - in 8-10 s. With the completion of the act of swallowing, gastric digestion begins.

DIGESTION IN THE STOMACH

The digestive functions of the stomach include the deposition of food, its mechanical and chemical processing and the gradual evacuation of food contents through the pylorus into the duodenum. Chemical processing of food is carried out by gastric juice, of which a person produces 2.0-2.5 liters per day. Gastric juice is secreted by numerous glands of the body of the stomach, which consist of main, parietal and accessory cells. The main cells secrete digestive enzymes, the parietal cells secrete hydrochloric acid and the accessory cells secrete mucus.

The main enzymes in gastric juice are proteases and lipase. Proteases include several pepsins, as well as gelatinase and chymosin. Pepsins are excreted as inactive pepsinogens. The conversion of pepsinogens and active pepsin is carried out under the influence of hydrochloric acid. Pepsins break down proteins into polypeptides. Their further breakdown to amino acids occurs in the intestines. Chymosin curdles milk. Gastric juice lipase breaks down only emulsified fats (milk) into glycerol and fatty acids.

Gastric juice has an acidic reaction (pH during food digestion is 1.5-2.5), which is due to the content of 0.4-0.5% hydrochloric acid in it. In healthy people, 40-60 ml of decinormal alkali solution is required to neutralize 100 ml of gastric juice. This indicator is called the total acidity of gastric juice. Taking into account the volume of secretion and the concentration of hydrogen ions, the flow rate of free hydrochloric acid is also determined.

Gastric mucus (mucin) is a complex complex of glucoproteins and other proteins in the form of colloidal solutions. Mucin covers the entire surface of the gastric mucosa and protects it from both mechanical damage and self-digestion, since it has pronounced antipeptic activity and is able to neutralize hydrochloric acid.

The entire process of gastric secretion is usually divided into three phases: complex-reflex (cerebral), neurochemical (gastric) and intestinal (duodenal).

The secretory activity of the stomach depends on the composition and quantity of incoming food. Meat food is a strong irritant of the gastric glands, the activity of which is stimulated for many hours. With carbohydrate foods, the maximum separation of gastric juice occurs in the complex-reflex phase, then secretion decreases. Fat and concentrated solutions of salts, acids and alkalis have an inhibitory effect on gastric secretion.

Digestion of food in the stomach usually occurs within 6-8 hours. The duration of this process depends on the composition of the food, its volume and consistency, as well as the amount of gastric juice released. Fatty foods linger especially long in the stomach (8-10 hours or more). Liquids pass into the intestines immediately after they enter the stomach.

The physical and chemical processing of food is a complex process that is carried out by the digestive system, which includes the oral cavity, esophagus, stomach, duodenum, small and large intestines, rectum, as well as the pancreas and liver with the gallbladder and bile ducts.

The study of the functional state of the digestive organs is important mainly for assessing the health status of athletes. Disturbances in the functions of the digestive system are observed in chronic gastritis, peptic ulcers, etc. Diseases such as peptic ulcers of the stomach and duodenum, chronic cholecystitis, occur quite often in athletes.

Diagnosis of the functional state of the digestive organs is based on the complex use of clinical (history, examination, palpation, percussion, auscultation), laboratory (chemical and microscopic examination of the contents of the stomach, duodenum, gallbladder, intestines) and instrumental (x-ray and endoscopic) research methods. Currently, intravital morphological studies using organ biopsies (for example, liver) are increasingly being carried out.

In the process of collecting anamnesis, athletes are asked to find out their complaints, state of appetite, clarify the diet and nature of nutrition, caloric content of food taken, etc. During the examination, pay attention to the condition of the teeth, gums and tongue (normally the tongue is moist, pink, without plaque), color skin, sclera of the eyes and soft palate (to identify jaundice), the shape of the abdomen (flatulence causes an enlargement of the abdomen in the area where the affected part of the intestine is located). Palpation reveals the presence of pain points in the area of ​​the stomach, liver and gallbladder, and intestines; determine the condition (dense or soft) and tenderness of the edge of the liver, if it is enlarged, even small tumors in the digestive organs are palpated. Using percussion, you can determine the size of the liver, identify inflammatory effusion caused by peritonitis, as well as sharp swelling of individual intestinal loops, etc. Auscultation, in the presence of gas and liquid in the stomach, reveals the “splash noise” syndrome; Auscultation of the abdomen is an indispensable method for identifying changes in peristalsis (increase or absence) of the intestine, etc.

The secretory function of the digestive organs is studied by examining the contents of the stomach, duodenum, gallbladder, etc., extracted using a probe, as well as using radiotelemetric and electrometric research methods. Radio capsules, swallowed by the test subject, are miniature (1.5 cm in size) radio transmitters. They allow you to obtain information directly from the stomach and intestines about the chemical properties of the contents, temperature and pressure in the digestive tract.

A common laboratory method for examining the intestines is the caprological method: description of the appearance of stool (color, consistency, pathological impurities), microscopy (detection of protozoa, worm eggs, determination of undigested food particles, blood cells) and chemical analysis (determination of pH, soluble protein enzymes and etc.).

Intravital morphological (fluoroscopy, endoscopy) and microscopic (cytological and histological) methods are currently gaining importance in the study of the digestive organs. The emergence of modern fibrogastroscopes has significantly expanded the possibilities of endoscopic studies (gastroscopy, sigmoidoscopy).

Dysfunction of the digestive system is one of the common causes of decreased athletic performance.

Acute gastritis usually develops as a result of food toxic infection. The disease is acute and is accompanied by severe pain in the epigastric region, nausea, vomiting, and diarrhea. Objectively: the tongue is coated, the abdomen is soft, diffuse pain in the epigastric region. The general condition worsens due to dehydration and loss of electrolytes through vomit and diarrhea.

Chronic gastritis is the most common disease of the digestive system. In athletes, it often develops as a result of intense training against a background of poor nutrition: irregular meals, consumption of unusual foods, spices, etc. Athletes complain of loss of appetite, sour belching, heartburn, a feeling of bloating, heaviness and pain in the epigastric region, usually worse after eating, occasional sour-tasting vomiting. Treatment is carried out using conventional methods; training and participation in competitions during treatment are prohibited.

Peptic ulcer of the stomach and duodenum is a chronic recurrent disease that develops in athletes as a result of disorders of the central nervous system and hyperfunction of the pituitary-adrenal cortex system under the influence of great psycho-emotional stress associated with competitive activity.

The leading place in gastric ulcers is occupied by epigastric pain that occurs directly during a meal or 20-30 minutes after a meal and calms down after 1.5-2 hours; pain depends on the volume and nature of food. In case of duodenal ulcer, “hungry” and night pains predominate. Dyspeptic symptoms include heartburn, nausea, vomiting, constipation; appetite is usually preserved. Patients often complain of increased irritability, emotional lability, and fatigue. The main objective sign of an ulcer is pain in the anterior abdominal wall. Sports activities with peptic ulcer disease are contraindicated.

Often, during examination, athletes complain of pain in the liver during physical activity, which is diagnosed as a manifestation of hepatic pain syndrome. Pain in the liver area usually occurs during prolonged and intense exercise, has no warning signs and is acute. They are often dull or constantly aching. Often there is irradiation of pain in the back and right shoulder blade, as well as a combination of pain with a feeling of heaviness in the right hypochondrium. Stopping physical activity or reducing its intensity helps reduce pain or eliminate it. However, in some cases, pain can persist for many hours and during the recovery period.

At first, pain appears randomly and infrequently, later it begins to bother the athlete in almost every training session or competition. The pain may be accompanied by dyspeptic disorders: loss of appetite, feeling of nausea and bitterness in the mouth, heartburn, belching of air, unstable stool, constipation. In some cases, athletes complain of headaches, dizziness, increased irritability, stabbing pain in the heart, and a feeling of weakness that worsens during physical activity.

Objectively, most athletes exhibit an increase in liver size. In this case, its edge protrudes from under the costal arch by 1-2.5 cm; it is compacted and painful on palpation.

The cause of this syndrome is still not clear enough. Some researchers associate the appearance of pain with overstretching of the liver capsule due to overfilling of the liver with blood, others, on the contrary, with a decrease in blood supply to the liver, with the phenomena of intrahepatic blood stagnation. There are indications of a connection between hepatic pain syndrome and pathology of the digestive organs, with hemodynamic disorders against the background of an irrational training regimen, etc. Electron microscopic studies (biopsy) of the liver in such athletes in some cases make it possible to identify morphological changes in it that can be associated with the history of the liver. previously viral hepatitis, as well as with the occurrence of hypoxic conditions when performing loads that do not correspond to the functional capabilities of the body.

Prevention of diseases of the liver, gall bladder and bile ducts is mainly associated with compliance with the diet, the basic provisions of the training regimen and a healthy lifestyle.

Treatment of athletes with hepatic pain syndrome should be aimed at eliminating diseases of the liver, gall bladder and biliary tract, as well as other concomitant diseases. Athletes should be excluded from training sessions and especially participation in competitions during the period of treatment.

The prognosis for increased athletic performance in the early stages of the syndrome is favorable. In cases of its persistent manifestation, athletes are usually forced to stop playing sports.

The concept of physiology can be interpreted as the science of the patterns of operation and regulation of a biological system in conditions of health and the presence of diseases. Physiology studies, among other things, the vital activity of individual systems and processes; in a particular case, this is, i.e. vital activity of the digestive process, patterns of its work and regulation.

The very concept of digestion means a complex of physical, chemical and physiological processes, as a result of which the food received in the process is broken down into simple chemical compounds - monomers. Passing through the wall of the gastrointestinal tract, they enter the bloodstream and are absorbed by the body.

Digestive system and oral digestion process

A group of organs is involved in the digestion process, which is divided into two large sections: the digestive glands (salivary glands, liver glands and pancreas) and the gastrointestinal tract. Digestive enzymes are divided into three main groups: proteases, lipases, and amylases.

Among the functions of the digestive tract are: promotion of food, absorption and removal of undigested food debris from the body.

The process begins. During chewing, food received during the process is crushed and moistened with saliva, which is produced by three pairs of large glands (sublingual, submandibular and parotid) and microscopic glands located in the mouth. Saliva contains enzymes amylase and maltase, which break down nutrients.

Thus, the process of digestion in the mouth consists of physically breaking up food, chemically attacking it, and moistening it with saliva to make it easier to swallow and continue the digestion process.

Digestion in the stomach

The process begins with food, crushed and moistened with saliva, passing through the esophagus and entering the organ. Over the course of several hours, the food bolus experiences mechanical (muscle contraction as it moves into the intestines) and chemical effects (stomach juice) inside the organ.

Gastric juice consists of enzymes, hydrochloric acid and mucus. The main role belongs to hydrochloric acid, which activates enzymes, promotes fragmentary breakdown, and has a bactericidal effect, destroying a lot of bacteria. The enzyme pepsin in the gastric juice is the main one, breaking down proteins. The action of mucus is aimed at preventing mechanical and chemical damage to the organ membrane.

What composition and amount of gastric juice will depend on the chemical composition and nature of the food. The sight and smell of food promotes the release of necessary digestive juices.

As the digestion process progresses, food gradually and portionwise moves into the duodenum.

Digestion in the small intestine

The process begins in the cavity of the duodenum, where the bolus is affected by pancreatic juice, bile and intestinal juice, since it contains the common bile duct and the main pancreatic duct. Inside this organ, proteins are digested into monomers (simple compounds), which are absorbed by the body. Learn more about the three components of chemical action in the small intestine.

The composition of pancreatic juice includes the enzyme trypsin, which breaks down proteins, which converts fats into fatty acids and glycerol, the enzyme lipase, as well as amylase and maltase, which break down starch into monosaccharides.

Bile is synthesized by the liver and accumulates in the gallbladder, from where it enters the duodenum. It activates the enzyme lipase, participates in the absorption of fatty acids, increases the synthesis of pancreatic juice, and activates intestinal motility.

Intestinal juice is produced by special glands in the inner lining of the small intestine. It contains more than 20 enzymes.

There are two types of digestion in the intestines and this is its peculiarity:

• cavitary - carried out by enzymes in the organ cavity;
• contact or membrane - performed by enzymes that are located on the mucous membrane of the inner surface of the small intestine.

Thus, nutrients in the small intestine are actually completely digested, and the final products - monomers - are absorbed into the blood. Upon completion of the digestion process, digested food remains pass from the small intestine to the large intestine.

Digestion in the large intestine

The process of enzymatic processing of food in the large intestine is quite minor. However, in addition to enzymes, the process involves obligate microorganisms (bifidobacteria, E. coli, streptococci, lactic acid bacteria).

Bifidobacteria and lactobacilli are extremely important for the body: they have a beneficial effect on intestinal function, participate in the breakdown of bacteria, ensure the quality of protein and mineral metabolism, increase the body's resistance, and have an antimutagenic and anticarcinogenic effect.

Intermediate products of carbohydrates, fats and proteins are broken down here into monomers. Microorganisms of the colon produce (groups B, PP, K, E, D, biotin, pantothenic and folic acids), a number of enzymes, amino acids and other substances.

The final stage of the digestion process is the formation of feces, which are 1/3 of bacteria, and also contain epithelium, insoluble salts, pigments, mucus, fiber, etc.

Nutrient Absorption

Let's take a closer look at the process. It represents the final goal of the digestion process, when food components are transported from the digestive tract into the internal environment of the body - blood and lymph. Absorption occurs in all parts of the gastrointestinal tract.

Absorption in the mouth is practically not carried out due to the short period (15 - 20 s) of food staying in the organ cavity, but not without exceptions. In the stomach, the absorption process partially involves glucose, a number of amino acids, dissolved alcohol, and alcohol. Absorption in the small intestine is most extensive, largely due to the structure of the small intestine, which is well adapted to absorption function. Absorption in the large intestine concerns water, salts, vitamins and monomers (fatty acids, monosaccharides, glycerol, amino acids, etc.).

The central nervous system coordinates all processes of nutrient absorption. Humoral regulation is also involved in this.

The process of protein absorption occurs in the form of amino acids and water solutions - 90% in the small intestine, 10% in the large intestine. Absorption of carbohydrates occurs in the form of various monosaccharides (galactose, fructose, glucose) at different rates. Sodium salts play a certain role in this. Fats are absorbed in the form of glycerol and fatty acids in the small intestine into the lymph. Water and mineral salts begin to be absorbed in the stomach, but this process occurs more intensely in the intestines.

Thus, it covers the process of digestion of nutrients in the oral cavity, in the stomach, in the small and large intestines, as well as the absorption process.

Nutrition is the most important factor aimed at maintaining and ensuring such basic processes as growth, development and the ability to be active. These processes can be maintained using only balanced nutrition. Before we begin to consider issues related to the basics, it is necessary to become familiar with the processes of digestion in the body.

Digestion- a complex physiological and biochemical process during which ingested food in the digestive tract undergoes physical and chemical changes.

Digestion is the most important physiological process, as a result of which complex nutritional substances in food, under the influence of mechanical and chemical processing, are transformed into simple, soluble and, therefore, digestible substances. Their further path is to be used as a building and energy material in the human body.

Physical changes in food consist of its crushing, swelling, and dissolution. Chemical - in the consistent degradation of nutrients as a result of the action on them of the components of digestive juices secreted into the cavity of the digestive tract by its glands. The most important role in this belongs to hydrolytic enzymes.

Types of digestion

Depending on the origin of hydrolytic enzymes, digestion is divided into three types: intrinsic, symbiont and autolytic.

Own digestion carried out by enzymes synthesized by the body, its glands, enzymes of saliva, stomach and pancreatic juices, and intestinal epithelium.

Symbiont digestion- hydrolysis of nutrients due to enzymes synthesized by symbionts of the macroorganism - bacteria and protozoa of the digestive tract. Symbiont digestion occurs in humans in the large intestine. Fiber in food in humans, due to the lack of the corresponding enzyme in the secretions of the glands, is not hydrolyzed (this has a certain physiological meaning - the preservation of dietary fiber, which plays an important role in intestinal digestion), therefore its digestion by the enzymes of symbionts in the large intestine is an important process.

As a result of symbiont digestion, secondary food substances are formed, in contrast to the primary ones, which are formed as a result of one’s own digestion.

Autolytic digestion carried out due to enzymes that are introduced into the body as part of the food consumed. The role of this digestion is essential when one’s own digestion is underdeveloped. Newborns have not yet developed their own digestion, so the nutrients in breast milk are digested by enzymes that enter the baby's digestive tract as part of breast milk.

Depending on the location of the process of nutrient hydrolysis, digestion is divided into intra- and extracellular.

Intracellular digestion consists in the fact that substances transported into the cell by phagocytosis are hydrolyzed by cellular enzymes.

Extracellular digestion is divided into cavitary, which is carried out in the cavities of the digestive tract by enzymes of saliva, gastric juice and pancreatic juice, and parietal. Parietal digestion occurs in the small intestine with the participation of a large number of intestinal and pancreatic enzymes on a colossal surface formed by folds, villi and microvilli of the mucous membrane.

Rice. Stages of Digestion

Currently, the digestion process is considered as a three-stage process: cavity digestion - parietal digestion - absorption. Cavitary digestion consists of the initial hydrolysis of polymers to the stage of oligomers, parietal digestion provides further enzymatic depolymerization of oligomers mainly to the stage of monomers, which are then absorbed.

The correct sequential operation of the elements of the digestive conveyor in time and space is ensured by regular processes at various levels.

Enzymatic activity is characteristic of each part of the digestive tract and is maximum at a certain pH value. For example, in the stomach, the digestive process takes place in an acidic environment. The acidic contents passing into the duodenum are neutralized, and intestinal digestion occurs in a neutral and slightly alkaline environment created by secretions released into the intestine - bile, pancreatic and intestinal juices, which inactivate gastric enzymes. Intestinal digestion occurs in a neutral and slightly alkaline environment, first according to the type of cavity and then parietal digestion, ending with the absorption of hydrolysis products - nutrients.

The degradation of nutrients according to the type of cavity and parietal digestion is carried out by hydrolytic enzymes, each of which has specificity expressed to one degree or another. The set of enzymes in the secretions of the digestive glands has specific and individual characteristics and is adapted to the digestion of the food that is characteristic of a given animal species and the nutrients that predominate in the diet.

Digestion process

The digestion process is carried out in the gastrointestinal tract, the length of which is 5-6 m. The digestive tract is a tube, expanded in some places. The structure of the gastrointestinal tract is the same throughout its entire length; it has three layers:

• outer - serous, dense membrane, which mainly has a protective function;
• medium - muscle tissue is involved in the contraction and relaxation of the organ wall;
• internal - a membrane covered with mucous epithelium that allows simple nutrients to be absorbed through its thickness; the mucous membrane often has glandular cells that produce digestive juices or enzymes.

Enzymes- substances of protein nature. In the gastrointestinal tract they have their own specificity: proteins are broken down only under the influence of proteases, fats - lipases, carbohydrates - carbohydrates. Each enzyme is active only at a certain pH environment.

Functions of the gastrointestinal tract:

• Motor, or motor - due to the middle (muscular) lining of the digestive tract, muscle contraction and relaxation carries out food capture, chewing, swallowing, mixing and moving food along the digestive canal.
• Secretory - due to digestive juices, which are produced by glandular cells located in the mucous (inner) lining of the canal. These secretions contain enzymes (reaction accelerators) that perform chemical processing of food (hydrolysis of nutrients).
• The excretory (excretory) function carries out the release of metabolic products into the gastrointestinal tract by the digestive glands.
• Absorption function is the process of assimilation of nutrients through the wall of the gastrointestinal tract into the blood and lymph.

Gastrointestinal tract begins in the oral cavity, then food enters the pharynx and esophagus, which perform only a transport function, the food bolus descends into the stomach, then into the small intestine, consisting of the duodenum, jejunum and ileum, where final hydrolysis (cleavage) mainly occurs ) nutrients and they are absorbed through the intestinal wall into the blood or lymph. The small intestine passes into the large intestine, where there is practically no digestion process, but the functions of the large intestine are also very important for the body.

Digestion in the mouth

Further digestion in other parts of the gastrointestinal tract depends on the process of digestion of food in the oral cavity.

The initial mechanical and chemical processing of food occurs in the oral cavity. It involves grinding the food, moistening it with saliva, analyzing the taste properties, the initial breakdown of food carbohydrates and the formation of the food bolus. The stay of the food bolus in the oral cavity is 15-18 s. Food in the oral cavity excites taste, tactile, and temperature receptors in the oral mucosa. This reflexively causes the activation of the secretion of not only the salivary glands, but also the glands located in the stomach and intestines, as well as the secretion of pancreatic juice and bile.

Mechanical processing of food in the oral cavity is carried out using chewing. The act of chewing involves the upper and lower jaws with teeth, masticatory muscles, oral mucosa, and soft palate. During the chewing process, the lower jaw moves in horizontal and vertical planes, the lower teeth come into contact with the upper teeth. In this case, the front teeth bite off food, and the molars crush and grind it. Contraction of the muscles of the tongue and cheeks ensures the supply of food between the teeth. Contraction of the lip muscles prevents food from falling out of the mouth. The act of chewing is carried out reflexively. Food irritates the receptors of the oral cavity, nerve impulses from which through the afferent nerve fibers of the trigeminal nerve enter the chewing center, located in the medulla oblongata, and excite it. Next, along the efferent nerve fibers of the trigeminal nerve, nerve impulses travel to the masticatory muscles.

During the chewing process, the taste of food is assessed and its edibility is determined. The more complete and intensive the chewing process is, the more active the secretory processes occur both in the oral cavity and in the underlying parts of the digestive tract.

The secretion of the salivary glands (saliva) is formed by three pairs of large salivary glands (submandibular, sublingual and parotid) and small glands located in the mucous membrane of the cheeks and tongue. 0.5-2 liters of saliva are produced per day.

The functions of saliva are as follows:

• Wetting food, dissolution of solids, impregnation with mucus and formation of food bolus. Saliva facilitates the swallowing process and contributes to the formation of taste sensations.
• Enzymatic breakdown of carbohydrates due to the presence of a-amylase and maltase. The enzyme a-amylase breaks down polysaccharides (starch, glycogen) into oligosaccharides and disaccharides (maltose). The action of amylase inside the bolus of food continues when it enters the stomach as long as it maintains a slightly alkaline or neutral environment.
• Protective function associated with the presence of antibacterial components in saliva (lysozyme, immunoglobulins of various classes, lactoferrin). Lysozyme, or muramidase, is an enzyme that breaks down the cell wall of bacteria. Lactoferrin binds iron ions necessary for the life of bacteria, and thus stops their growth. Mucin also performs a protective function, as it protects the oral mucosa from the damaging effects of foods (hot or sour drinks, spicy seasonings).
• Participation in the mineralization of tooth enamel - Calcium enters tooth enamel from saliva. It contains proteins that bind and transport Ca 2+ ions. Saliva protects teeth from the development of caries.

The properties of saliva depend on the diet and type of food. When eating solid and dry food, more viscous saliva is released. When inedible, bitter or sour substances enter the oral cavity, a large amount of liquid saliva is released. The enzyme composition of saliva can also change depending on the amount of carbohydrates contained in food.

Regulation of salivation. Swallowing. Regulation of salivation is carried out by autonomic nerves innervating the salivary glands: parasympathetic and sympathetic. When excited parasympathetic nerve The salivary gland produces a large amount of liquid saliva with a low content of organic substances (enzymes and mucus). When excited sympathetic nerve a small amount of viscous saliva is formed, containing a lot of mucin and enzymes. Activation of salivation when eating food first occurs according to the conditioned reflex mechanism when seeing food, preparing to eat it, inhaling food aromas. At the same time, from visual, olfactory, and auditory receptors, nerve impulses travel along afferent nerve pathways to the salivary nuclei of the medulla oblongata (salivation center), which send efferent nerve impulses along parasympathetic nerve fibers to the salivary glands. The entry of food into the oral cavity excites the receptors of the mucous membrane and this ensures the activation of the salivation process according to the mechanism of the unconditioned reflex. Inhibition of the activity of the salivary center and a decrease in the secretion of the salivary glands occurs during sleep, with fatigue, emotional arousal, as well as with fever and dehydration.

Digestion in the oral cavity ends with the act of swallowing and the entry of food into the stomach.

Swallowing is a reflex process and consists of three phases:

• 1st phase - oral - is arbitrary and consists in the entry of a food bolus formed during the chewing process onto the root of the tongue. Next, the muscles of the tongue contract and the bolus of food is pushed into the throat;
• 2nd phase - pharyngeal - is involuntary, occurs quickly (within approximately 1 s) and is under the control of the swallowing center of the medulla oblongata. At the beginning of this phase, contraction of the muscles of the pharynx and soft palate lifts the velum and closes the entrance to the nasal cavity. The larynx moves upward and forward, which is accompanied by lowering of the epiglottis and closing of the entrance to the larynx. At the same time, the muscles of the pharynx contract and the upper esophageal sphincter relaxes. As a result, food enters the esophagus;
• 3rd phase - esophageal - slow and involuntary, occurs due to peristaltic contractions of the esophageal muscles (contraction of the circular muscles of the esophageal wall above the food bolus and longitudinal muscles located below the food bolus) and is under the control of the vagus nerve. The speed of food movement through the esophagus is 2 - 5 cm/s. After the lower esophageal sphincter relaxes, food enters the stomach.

Digestion in the stomach

The stomach is a muscular organ where food is deposited, mixed with gastric juice and moved to the outlet of the stomach. The mucous membrane of the stomach has four types of glands that secrete gastric juice, hydrochloric acid, enzymes and mucus.

Rice. 3. Digestive tract

Hydrochloric acid imparts acidity to the gastric juice, which activates the enzyme pepsinogen, converting it into pepsin, participating in protein hydrolysis. The optimal acidity of gastric juice is 1.5-2.5. In the stomach, protein is broken down into intermediate products (albumoses and peptones). Fats are broken down by lipase only when they are in an emulsified state (milk, mayonnaise). Carbohydrates are practically not digested there, since carbohydrate enzymes are neutralized by the acidic contents of the stomach.

During the day, from 1.5 to 2.5 liters of gastric juice are released. Food in the stomach is digested from 4 to 8 hours, depending on the composition of the food.

Mechanism of gastric juice secretion- a complex process, it is divided into three phases:

• the cerebral phase, acting through the brain, involves both unconditioned and conditioned reflexes (sight, smell, taste, food entering the oral cavity);
• gastric phase - when food enters the stomach;
• intestinal phase, when certain types of food (meat broth, cabbage juice, etc.), entering the small intestine, cause the release of gastric juice.

Digestion in the duodenum

From the stomach, small portions of food gruel enter the initial section of the small intestine - the duodenum, where the food gruel is actively exposed to pancreatic juice and bile acids.

Pancreatic juice, which has an alkaline reaction (pH 7.8-8.4), enters the duodenum from the pancreas. The juice contains the enzymes trypsin and chymotrypsin, which break down proteins into polypeptides; amylase and maltase break down starch and maltose into glucose. Lipase only affects emulsified fats. The emulsification process occurs in the duodenum in the presence of bile acids.

Bile acids are a component of bile. Bile is produced by the cells of the largest organ - the liver, whose mass is from 1.5 to 2.0 kg. Liver cells constantly produce bile, which accumulates in the gallbladder. As soon as the food gruel reaches the duodenum, bile from the gallbladder enters the intestines through the ducts. Bile acids emulsify fats, activate fat enzymes, and enhance the motor and secretory functions of the small intestine.

Digestion in the small intestine (jejunum, ileum)

The small intestine is the longest section of the digestive tract, its length is 4.5-5 m, diameter is from 3 to 5 cm.

Intestinal juice is a secretion of the small intestine, the reaction is alkaline. Intestinal juice contains a large number of enzymes involved in digestion: peitidase, nuclease, enterokinase, lipase, lactase, sucrase, etc. The small intestine, due to the different structure of the muscle layer, has an active motor function (peristalsis). This allows food gruel to move into the true intestinal lumen. This is also facilitated by the chemical composition of food - the presence of fiber and dietary fiber.

According to the theory of intestinal digestion, the process of assimilation of nutrients is divided into cavity and parietal (membrane) digestion.

Cavity digestion is present in all cavities of the gastrointestinal tract due to digestive secretions - gastric juice, pancreatic and intestinal juice.

Parietal digestion is present only in a certain segment of the small intestine, where the mucous membrane has protrusions or villi and microvilli, increasing the internal surface of the intestine by 300-500 times.

Enzymes involved in the hydrolysis of nutrients are located on the surface of microvilli, which significantly increases the efficiency of the absorption of nutrients in this area.

The small intestine is the organ where most of the water-soluble nutrients pass through the intestinal wall and are absorbed into the blood; fats initially enter the lymph and then into the blood. All nutrients enter the liver through the portal vein, where, having been cleared of toxic digestive substances, they are used to nourish organs and tissues.

Digestion in the large intestine

The movement of intestinal contents in the large intestine takes up to 30-40 hours. Digestion in the large intestine is practically absent. Here glucose, vitamins, and minerals are absorbed that remain undigested due to the large number of microorganisms in the intestines.

In the initial segment of the large intestine, almost complete absorption of the liquid received there occurs (1.5-2 l).

The microflora of the large intestine is of great importance for human health. More than 90% are bifidobacteria, about 10% are lactic acid and E. coli, enterococci, etc. The composition of the microflora and its functions depend on the nature of the diet, the time of movement through the intestines and the use of various medications.

The main functions of normal intestinal microflora:

• protective function - creation of immunity;
• participation in the digestive process - final digestion of food; synthesis of vitamins and enzymes;
• maintaining a constant biochemical environment of the gastrointestinal tract.

One of the important functions of the large intestine is the formation and removal of feces from the body.