What provides intracellular digestion. Lecture: Digestion

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 supported using only rational nutrition. Before proceeding with the consideration of issues related to the basics, it is necessary to get acquainted with the processes of digestion in the body.

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

Digestion is the most important physiological process, as a result of which the complex nutritional substances of food under the influence of mechanical and chemical processing are converted 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 in its crushing, swelling, dissolution. Chemical - in the sequential 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: proper, symbiotic and autolytic.

own digestion carried out by enzymes synthesized by the body, its glands, enzymes of saliva, stomach and pancreatic juices, and the epithelium of the furnace intestine.

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

As a result of symbiotic digestion, secondary nutrients are formed, in contrast to the primary ones, which are formed as a result of their own digestion.

Autolytic digestion It is carried out due to enzymes that are introduced into the body as part of the food taken. The role of this digestion is essential in case of insufficiently developed own digestion. In newborns, their own digestion is not yet developed, so the nutrients in breast milk are digested by enzymes that enter the infant's digestive tract as part of breast milk.

Depending on the localization of the process of hydrolysis of nutrients, 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 process of digestion is considered as a three-stage: cavity digestion - parietal digestion - absorption. Cavitary digestion consists in 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 of various levels.

Enzymatic activity is characteristic of each section of the digestive tract and is maximum at a certain pH value of the medium. For example, in the stomach, the digestive process is carried out 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 juices and intestinal juices, which inactivate gastric enzymes. Intestinal digestion occurs in a neutral and slightly alkaline environment, first by the type of cavity, and then parietal digestion, culminating in the absorption of hydrolysis products - nutrients.

The degradation of nutrients by the type of cavity and parietal digestion is carried out by hydrolytic enzymes, each of which has a specificity expressed to some extent. The set of enzymes in the composition of the secrets of the digestive glands has species and individual characteristics, adapted to the digestion of the food that is characteristic of this type of animal, and those nutrients that prevail in the diet.

Digestion process

The process of digestion 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, it has three layers:

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

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

Functions of the gastrointestinal tract:

• Motor, or motor - due to the middle (muscular) membrane of the digestive tract, contraction-relaxation of the muscles captures food, chews, swallows, mixes and moves food along the digestive canal.
• Secretory - due to digestive juices, which are produced by glandular cells located in the mucous (inner) shell of the canal. These secrets contain enzymes (reaction accelerators) that carry out the chemical processing of food (hydrolysis of nutrients).
• The excretory (excretory) function carries out the excretion of metabolic products by the digestive glands into the gastrointestinal tract.
• Absorptive function - 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 12 duodenum, jejunum and ileum, where the final hydrolysis mainly occurs (splitting ) 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 takes place in the oral cavity. It includes grinding food, wetting it with saliva, analyzing taste properties, the initial breakdown of food carbohydrates and the formation of a food bolus. The stay of the food bolus in the oral cavity is 15-18 s. Food in the oral cavity excites taste, tactile, temperature receptors of the oral mucosa. This reflex causes the activation of the secretion of not only the salivary glands, but also the glands located in the stomach, intestines, as well as the secretion of pancreatic juice and bile.

Mechanical processing of food in the oral cavity is carried out with the help of chewing. The act of chewing involves the upper and lower jaws with teeth, chewing muscles, oral mucosa, soft palate. In the process of chewing, the lower jaw moves in the horizontal and vertical planes, the lower teeth are in contact with the upper ones. At the same time, the front teeth bite off food, and the molars crush and grind it. The contraction of the muscles of the tongue and cheeks ensures the supply of food between the dentition. The contraction of the muscles of the lips 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, along the afferent nerve fibers of the trigeminal nerve, enter the chewing center located in the medulla oblongata, and excite it. Further along the efferent nerve fibers of the trigeminal nerve, nerve impulses arrive at the masticatory muscles.

In the process of chewing, the taste of food is assessed and its edibility is determined. The more fully and intensively the chewing process is carried out, the more actively the secretory processes proceed both in the oral cavity and in the lower parts of the digestive tract.

The secret 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 is formed per day.

The functions of saliva are as follows:

• Wetting food, dissolution of solids, impregnation with mucus and the formation of a food bolus. Saliva facilitates the process of swallowing 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) to oligosaccharides and disaccharides (maltose). The action of amylase inside the food bolus continues when it enters the stomach until a slightly alkaline or neutral environment remains in it.
• 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 vital activity 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, hot spices).
• Participation in the mineralization of tooth enamel - calcium enters the 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 taking solid and dry food, more viscous saliva is secreted. When inedible, bitter or acidic 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. The regulation of salivation is carried out by autonomic nerves that innervate 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 containing a lot of mucin and enzymes is formed. The activation of salivation during food intake occurs first according to the conditioned reflex mechanism at the sight of food, preparation for its reception, inhalation of food aromas. At the same time, from visual, olfactory, auditory receptors, nerve impulses through afferent nerve pathways enter the salivary nuclei of the medulla oblongata (salivation center), which send efferent nerve impulses along the parasympathetic nerve fibers to the salivary glands. The entry of food into the oral cavity excites the mucosal receptors and this ensures the activation of the salivation process. by the mechanism of the unconditioned reflex. Inhibition of the activity of the center of salivation and a decrease in the secretion of the salivary glands occurs during sleep, with fatigue, emotional arousal, as well as with fever, 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 receipt of the food bolus formed during chewing on the root of the tongue. Next, there is a contraction of the muscles of the tongue and pushing the food bolus into the throat;
• 2nd phase - pharyngeal - is involuntary, carried out 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 raises the veil of the palate and closes the entrance to the nasal cavity. The larynx shifts upward and forward, which is accompanied by the descent of the epiglottis and the closure of the entrance to the larynx. At the same time, there is a contraction of the muscles of the pharynx and relaxation of the upper esophageal sphincter. As a result, food enters the esophagus;
• 3rd phase - esophageal - slow and involuntary, occurs due to peristaltic contractions of the muscles of the esophagus (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 movement of food through the esophagus is 2 - 5 cm / s. After relaxation of the lower esophageal sphincter, food enters the stomach.

Digestion in the stomach

The stomach is a muscular organ where food is deposited, mixed with gastric juice and promoted to the stomach outlet. 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, turning 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 is secreted. Food in the stomach is digested from 4 to 8 hours, depending on the composition of the food.

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

• the cerebral phase, acting through the brain, involves both the unconditioned and the conditioned reflex (sight, smell, taste, food entering the oral cavity);
• gastric phase - when food enters the stomach;
• the 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 the food slurry enter the initial section of the small intestine - the duodenum, where the food slurry 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. Juice contains the enzymes trypsin and chymotrypsin, which break down proteins - to polypeptides; amylase and maltase break down starch and maltose into glucose. Lipase acts only on 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, which weighs from 1.5 to 2.0 kg. Liver cells constantly produce bile, which is stored in the gallbladder. As soon as the food slurry reaches the duodenum, bile from the gallbladder through the ducts enters the intestines. Bile acids emulsify fats, activate fat enzymes, 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, its diameter is from 3 to 5 cm.

Intestinal juice is the secret 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 the food gruel to move into the true intestinal lumen. This is 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.

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

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

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

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

Digestion in the large intestine

The movement of intestinal contents in the large intestine is up to 30-40 hours. Digestion in the large intestine is practically absent. Glucose, vitamins, minerals are absorbed here, which remained unabsorbed due to the large number of microorganisms in the intestine.

In the initial segment of the large intestine, almost complete assimilation of the liquid that has entered there (1.5-2 liters) occurs.

Of great importance for human health is the microflora of the large intestine. More than 90% are bifidobacteria, about 10% are lactic acid and Escherichia 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 intake of various medications.

The main functions of normal intestinal microflora:

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

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

Plan

Introduction……………………………………………………….3

The essence of the processes occurring in the gastrointestinal tract……………………………………………...4

Types of digestion……………………………………………....5

Suction…………………………………………………….9

Suction regulation………………………………………….11

Conclusion…………………………………………………..14

References………………………………………….15

Introduction

All the substances necessary to perform physical and mental work, maintain body temperature, as well as the growth and restoration of deteriorating tissues and other functions, the body receives in the form of food and water. Food products consist of nutrients, the main of which are proteins, fats, carbohydrates, mineral salts, vitamins, water. These substances are part of the cells of the body. Most foods cannot be used by the body without prior processing. It consists in the mechanical processing of food and its chemical breakdown into simple soluble substances that enter the bloodstream and are absorbed from it by cells. This processing of food is called digestion.

The digestive system is a collection of digestive organs in animals and humans. In humans, the digestive system is represented by the oral cavity, pharynx, esophagus, stomach, intestines, liver and pancreas.

In the oral cavity, food is crushed (chewed), then subjected to complex chemical processing by digestive juices. The salivary glands secrete saliva, the glands of the stomach, the pancreas and intestinal glands secrete various juices, and the liver secretes bile. As a result of exposure to these juices, proteins, fats and carbohydrates are broken down into simpler soluble compounds. But this is possible only with the movement of food through the digestive canal and its thorough mixing. Moving and mixing food is carried out due to powerful contractions of the muscles of the walls of the digestive canal. The transition of nutrients into the blood and lymph is carried out by the mucous membrane of individual sections of the alimentary canal.

The essence of the processes taking place

in the gastrointestinal tract

Every day, an adult should receive about 80-100 g of protein, 80-100 g of fat and 400 g of carbohydrates. They come with food. Together with them, food contains mineral salts, trace elements, vitamins, as well as ballast substances, which are a valuable component of food.

The essence of digestion (Fig. 1) lies in the fact that after the necessary mechanical processing, i.e. grinding and rubbing food in the mouth, stomach and small intestine, hydrolysis of proteins, carbohydrates and fats occurs. It takes place in two stages - first, in the cavity of the digestive tract, the polymer is destroyed to oligomers, and then - in the region of the enterocyte membrane (parietal, or membrane digestion) - the final hydrolysis occurs to monomers - amino acids, monosaccharides, fatty acids, monoglycerides. Monomer molecules are absorbed with the help of special mechanisms, that is, they are reabsorbed through the apical surface of enterocytes and pass into the blood or lymph, from where they enter various organs, passing initially through the liver portal vein system. All "ballast" substances that could not be hydrolyzed by the enzymes of the gastrointestinal tract go to the large intestine, where, with the help of microorganisms, they undergo additional splitting (partial or complete), while some of the products of this splitting are absorbed into the blood of the macroorganism, and some goes to microflora nutrition. The microflora is also capable of producing biologically active substances and a number of vitamins, for example, B vitamins.

final stage digestion is the formation of feces and their evacuation (the act of defecation). On average, their mass reaches 150-250 g. Normally, the act of defecation occurs 1 time per day, in 30% of people - 2 times or more, and in 8% - less than 1 time per day. Due to aerophagy and the vital activity of microflora, about 100-500 ml of gas accumulates in the gastrointestinal tract, which is partially released during defecation or outside it.

Fig.1. The essence of the processes of digestion of food components.

Types of digestion

Depending on the origin of hydrolytic enzymes, there are:

1) own digestion - it comes at the expense of enzymes produced by a person or animal;

2) symbiotic - due to enzymes of symbionts, for example, enzymes of microorganisms inhabiting the large intestine;

3) autolytic - due to enzymes administered with food. This, for example, is typical for mother's milk, it contains enzymes necessary for curdling milk and hydrolyzing its components. In an adult, the main role in the processes of digestion has its own digestion.

Depending on the localization of the process of hydrolysis of nutrients, there are: intracellular and extracellular digestion, and extracellular is divided into distant (or cavity) and contact (or parietal) digestion.

intracellular digestion is a process that takes place inside the cell. Phagocytes are a prime example of the use of this hydrolysis method. As a rule, intracellular digestion is carried out with the help of hydrolases located in lysosomes. In the process of one's own (true) digestion in humans, the main role belongs to cavitary and parietal digestion.

cavity digestion occurs in various parts of the gastrointestinal tract, starting with the oral cavity, but its severity is different. Salivary glands, gastric glands, pancreatic glands, numerous intestinal glands produce the corresponding juices (saliva - in the oral cavity), which, in addition to various components, contain enzymes - hydrolases that hydrolyze the corresponding polymers - proteins, complex carbohydrates, fats. As a rule, hydrolysis occurs in the aqueous phase and is largely determined by the pH of the medium, temperature, and for lipases - by the content of fat emulsifier in the medium - bile acids. It ends with the formation of small molecules - disaccharides, dipeptides, fatty acids, monoglycerides.

Parietal (membrane) digestion- the idea of ​​its existence was expressed by A. M. Ugolev in 1963. While conducting experiments with a segment of the small intestine, he discovered that hydrolysis of starch under the influence of amylase in the presence of a segment of the small intestine of a rat treated in a special way (to remove its own amylase) occurs much faster than without it. A. M. Ugolev suggested that in the apical part of enterocytes, a process occurs that contributes to the final digestion of nutrients. The subsequent development of science confirmed the correctness of this hypothesis, which is now recognized as an axiom of the physiology of digestion.

Parietal digestion is carried out on the apical surface of the enterocyte. Here, in its membrane, hydrolase enzymes are built in, which perform the final hydrolysis of nutrients, for example, maltase, which breaks down maltose to two glucose molecules, invertase, which breaks down sucrose to glucose and fructose, dipeptidase. These enzymes consist of two parts - hydrophilic and hydrophobic. The hydrophilic part is located above the membrane, and the hydrophobic part is inside the membrane, it performs an "anchor" function. Enzymes that carry out parietal digestion are usually synthesized within the enterocyte itself, including maltase, invertase, isomaltase, gamma-amylase, lactase, trehalase, alkaline phosphatase, monoglyceride lipase, peptidases, aminopeptidases, carboxypeptidases, and others. After synthesis, these enzymes are incorporated into the membrane as typical integral proteins. The efficiency of parietal digestion largely increases due to the fact that this process is associated with the next stage - the transport of the molecule through the enterocyte into the blood or lymph, i.e., with the absorption process. As a rule, close to the hydrolase enzyme there is a transport mechanism (“transporter”, in the terminology of A. M. Ugolev), which, as in a relay race, takes over the formed monomer and transports it through the apical membrane of the enterocyte into the cell.

The enterocyte is covered with microvilli, on average up to 1700-3000 pieces per cell. There are about 50-200 million such villi per 1 mm2. Due to them, the area of ​​the membrane on which parietal digestion takes place increases by 14-39 times. In the membranes of these microvilli, enzymes - hydrolases are localized. Between the microvilli and on their surface there is a layer of glycocalyx - these are filaments located perpendicular to the surface of the enterocyte membrane (their diameter is from 2 to 5 nm, their height is 0.3-0.5 microns), which form a kind of porous reactor. Periodically, when the glycocalyx is excessively contaminated, it is rejected to clean the surface of the enterocyte. In pathology, situations are possible when the cell generally loses glycocalyx for a long time, and in this case, the process of parietal digestion is disrupted. The glycocalyx provides a peculiar environment above the apical membrane of the enterocyte. The glycocalyx is a molecular sieve and an ion exchanger - the distances between neighboring glycocalyx filaments are such that they do not let large particles into the glycocalyx, including "underdigested" products, microorganisms that inhabit the small intestine. Thanks to the presence electric charges(cations, anions) glycocalyx is an ion exchanger. In general, the glycocalyx provides sterility and selective permeability for the medium located above the enterocyte membrane. Between the filaments of the glycocalyx are enzymes - hydrolases, the main part of which comes from juices - intestinal and pancreatic, and here they complete the process of partial hydrolysis begun in the intestinal cavity.

Above the glycocalyx there is also another layer - the so-called layer of mucous overlays. It is formed by mucus produced by goblet cells and fragments of exfoliating intestinal epithelium. Many enzymes of pancreatic juice and intestinal juice are sorbed in this layer. This layer is the site of membrane digestion.

Thus, the transition from cavitary to parietal digestion is carried out gradually, through two functionally important layers - the layer of mucous overlays and the layer of glycocalyx. Then comes the actual layer of parietal (membrane) digestion, in which the final hydrolysis of nutrients takes place and their subsequent transport through the enterocyte into the blood or lymph.

Suction

The absorption of nutrients, i.e. nutrients, is the ultimate goal of the digestion process. This process is carried out throughout the gastrointestinal tract - from the oral cavity to the large intestine, but its intensity is different: in the oral cavity, monosaccharides are mainly absorbed, some medicinal substances, for example, nitroglycerin; in the stomach, water and alcohol are mainly absorbed; in the large intestine - water, chlorides, fatty acids; in the small intestine - all the main products of hydrolysis. Calcium, magnesium, and iron ions are absorbed in the duodenum; in this intestine and at the beginning of the jejunum, monosaccharides are predominantly absorbed, more distally, fatty acids and monoglycerides are absorbed, and in the ileum, protein and amino acids are absorbed. Fat-soluble and water-soluble vitamins are absorbed in the distal jejunum and proximal ileum (Fig. 2).

Fig.2. Absorption of cleavage products of proteins, carbohydrates and fats (probable options). Absorption into the blood (K).

A - amino acids, M - monosaccharides in conjugation with Na, G - glycerol, F - fatty acids - synthesis of likened triglycerides in epithelial cells - formation of Xm - chylomicrons and absorption into the lymph (LC). Zhel - bile acids are partially returned to the intestinal cavity, partially absorbed into the blood and returned to the liver.

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Not all areas of the small intestine are "occupied" by the absorption process, distal areas usually do not participate in this process. However, with the pathology of the proximal areas, the distal areas take on this function. Thus, there is a protective variant of absorption in the body.

The mechanisms of transport, i.e., the absorption of substances, are diverse. Some substances, such as water, can pass through intercellular (interenterocytic) spaces - this is the persorption mechanism. There is also a process of water reabsorption in the collecting ducts of the kidney. In a number of cases, the mechanism of endocytosis takes place, i.e., the absorption by the enterocyte of a large, undestroyed molecule into the cell, and then its release into the interstitium and into the blood due to the mechanism of exocytosis. Obviously, immunoglobulins are transported in this way in newborns and infants fed with human milk. It is possible that in adults a number of molecules are also transported by endo- and exocytosis.

An important place among the mechanisms of absorption is occupied by the mechanisms of passive transport - diffusion, osmosis, filtration, as well as facilitated diffusion (transport without energy expenditure along the concentration gradient, but using "transporters"). The osmosis mechanism allows you to reabsorb a large amount of water - an average of about 8 liters per day (2.5 - with food, the rest of the water is the water of digestive juices): together with osmotically active substances, for example, glucose, amino acids, sodium ions, calcium, potassium - enterocytes passively enter water. Partially, water enters the interstitium (and then into the blood) due to filtration processes - if the hydrostatic pressure in the intestinal cavity exceeds the osmotic pressure in this environment, then this creates an opportunity for water reabsorption using the filtration mechanism.

The main mechanism that ensures the reabsorption of various substances (glucose, amino acids, sodium, calcium, iron salts) is active transport, the implementation of which requires energy resulting from ATP hydrolysis. Sodium ions are transported through the mechanism of primary active transport, and glucose, amino acids and a number of other substances - due to secondary active transport, dependent on sodium transport.

A special position in transport is occupied by the products of lipolysis and the fats themselves. Being fat-soluble, they can pass through membrane barriers passively, along a concentration gradient. But for this it is necessary to "organize" such a flow, to make it real. Obviously, for this purpose, in the intestinal cavity, the products of lipid hydrolysis - fatty acids with long chains, 2-monoglycerides, cholesterol - are combined into micelles - the smallest droplets that can diffuse through the apical membrane of the enterocyte into it. The process of formation of micelles is associated with the action of bile acids. Inside the enterocyte, newly synthesized lipids form structures that are convenient for further transport - chylomicrons. It is possible that specific carriers exist in membranes to facilitate the transport of micelles and chylomicrons; facilitated diffusion takes place.

Suction regulation

It is carried out due to changes in the processes of blood flow through the intestinal mucosa, stomach, lymph flow, energy, as well as due to the synthesis of "transporters" (pumps and specific carriers).

The blood flow in the celiac region largely depends on the stage of digestion. It is known that under conditions of "food dormancy" 15-20% of the IOC enters the celiac circulation. With an increase in the functional activity of the gastrointestinal tract, it can increase by 8-10 times. This contributes not only to an increase in the production of digestive juices, motor activity, but also increases the absorption process, i.e., the blood flow through the villi of the intestinal mucosa increases, and favorable conditions are created for the outflow of blood rich in absorbed nutrient. The increase in blood flow occurs mainly due to the production of vasodilators, especially serotonin, the most powerful vasodilator of the gastrointestinal precapillaries. Other hormones, such as gastrin, histamine, cholecystokinin-pancreozymin, also contribute to this process. When, for some reason, systemic pressure changes, the blood flow through the villus is still preserved (in the range of systemic pressure changes from 100 to 30 mm Hg). This is ensured by a fairly pronounced mechanism of autoregulation, similar to what takes place in the vessels of the brain.

The intensity of blood flow and, especially, lymph flow can also be regulated due to the contractile activity of the villus: the MMC present in it, when intestinal hormones are released into the blood, are activated and cause periodic contraction of the villus, the contents of the blood and lymphatic vessels are squeezed out, which helps to remove nutrients from the enterocyte. It is believed that such a humoral substance is villikin, produced in the small intestine.

The activity of the longitudinal and circular muscles of the small intestine contributes to the mixing of chyme, the creation of optimal intra-intestinal pressure - all this also facilitates the absorption process. Therefore, all factors that positively affect the motor activity of the intestine increase the efficiency of absorption.

The regulation of the synthesis of "transporters" is carried out, as a rule, due to "classical" hormones - aldosterone, glucocorticoids, 1,25-dihydrooxycholecalciferol (1,25-vitamin D3) and other hormones. For example, an increase in the production of aldesterone is accompanied by an increase in the formation of sodium pumps in enterocytes, which contribute to the active transport of sodium. This indirectly affects the secondary active transport of amino acids and monosaccharides. The metabolite of vitamin D3-1,25-dihydrooxycholecalciferol increases the synthesis of calcium-binding protein in the intestine, promoting the absorption of calcium ions. Parathyroid hormone increases the rate of formation of this metabolite from vitamin D3 (cholecalciferol) and indirectly increases calcium absorption.

Hormones that change the process of reabsorption of a given substance in the intestines simultaneously and in the same direction change the processes of reabsorption of the same substance in the kidneys, since the mechanisms of reabsorption in the intestines and in the kidneys are largely common.

Conclusion

Digestion is a set of processes that provide mechanical grinding and chemical (mainly enzymatic) breakdown of nutrients into components that are devoid of species specificity and are suitable for absorption and participation in the metabolism of animals and humans. The food entering the body is comprehensively processed under the action of various digestive enzymes synthesized by specialized cells, and the breakdown of complex nutrients (proteins, fats and carbohydrates) into ever smaller fragments occurs with the addition of a water molecule to them. Proteins are ultimately broken down into amino acids, fats into glycerol and fatty acids, carbohydrates into monosaccharides. These relatively simple substances are absorbed, and complex organic compounds are again synthesized from them in organs and tissues. There are 3 main types of digestion: intracellular, distant (cavitary) and contact (parietal). Nutrient absorption is the ultimate goal of the digestion process. This process is carried out throughout the gastrointestinal tract.

Bibliography

Agadzhanyan N.A., Tel L.Z., Tsirkin V.I., Chesnokova S.A. Human Physiology (course of lectures) SPb., SOTIS, 1998.

Mamontov S.G. Biology (Textbook) M., Bustard, 1997.

Oke S. Fundamentals of neurophysiology M., 1969.

Sidorov E.P. General biology M., 1997.

Fomin N.A. Human Physiology M., 1992.

Intracellular digestion refers to all cases when an uncleaved or partially cleaved substrate penetrates into the cell, where it undergoes hydrolysis by enzymes that are not secreted outside it. Intracellular digestion can be divided into two subtypes - molecular and vesicular. Molecular intracellular digestion is characterized by the fact that enzymes located in the cytoplasm hydrolyze small substrate molecules penetrating the cell, mainly dimers and oligomers, and such molecules penetrate passively or actively. For example, using special transport systems, actively transported through the cell membrane of disaccharides and dipeptides in bacteria. It is assumed that in higher organisms, in particular in mammals, some dipeptides can be actively transported into intestinal cells - enterocytes. If intracellular digestion occurs in special vacuoles, or vesicles, which are formed as a result of endocytosis (pinocytosis or phagocytosis), then it is defined as vesicular or endocytic. In vesicular intracellular digestion of the endocytic type, a certain section (s) of the membrane is invaginated along with the absorbed substance. Further, this site is gradually separated from the membrane, and an intracellular vesicular structure is formed. As a rule, such a vesicle fuses with a lysosome containing a wide range of hydrolytic enzymes that act on all the main food components. In the resulting new structure - the phagosome, the hydrolysis of the incoming substrates and the subsequent absorption of the resulting products occur. Undigested phagosome residues are usually ejected outside the cell by exocytosis. Thus, intracellular digestion is a mechanism through which not only digestion is realized, but also the absorption of nutrients by the cell, including large molecules and supramolecular structures. Intracellular digestion is limited by membrane permeability and endocytosis processes. The latter are characterized by a low rate and, apparently, cannot play a significant role in meeting the nutritional needs of higher organisms. As we drew attention back in 1967 (Ugolev, 1967), from the point of view of enzymology, intracellular digestion of the vesicular type is a combination of microcavitary and membrane digestion. Vesicular intracellular digestion has been found in all types of animals - from protozoa to mammals (it plays a particularly important role in lower animals), and molecular digestion - in all groups of organisms.

This term refers to cases when non-split or partially split food substances penetrate into the cell, where they are hydrolyzed by cytoplasmic enzymes that are not released outside the cell. Intracellular digestion is common in the simplest and most primitive multicellular organisms, such as sponges and flatworms. As an additional mechanism for the hydrolysis of nutrients, it is found in nemerteans, echinoderms, some annelids, and many molluscs. In higher vertebrates and humans, it performs mainly protective functions, such as phagocytosis.

There are two types of intracellular digestion. The first is associated with the transport of small molecules across cell membranes and subsequent digestion by cytoplasmic enzymes. Intracellular digestion can also occur in special intracellular cavities - digestive vacuoles, which are constantly present or formed during phagocytosis and pinocytosis and disappear after the breakdown of the captured food. The second type of digestion in most cases is associated with the participation of lysosomes, which contain a wide range of hydrolytic enzymes (phosphatases, proteases, glucosidases, lipases, etc.) with an optimum action in an acidic environment (pH 3.5–5.5). Food structures or food solutions in the pericellular environment cause invaginations of the plasma membrane, which then lace up and sink into the cytoplasm, forming pinocytic and phagocytic vacuoles. Connecting with the latter, lysosomes form phagosomes, where the contact of enzymes with the corresponding substrates takes place. The resulting hydrolysis products are absorbed through the membranes of phagosomes. After the end of the digestive cycle, the remnants of phagosomes are thrown out of the cell by exocytosis. Lysosomes also play an important role in the breakdown of the cell's own structures, which are used as food material either by this cell or outside it.

According to its mechanisms, intracellular digestion can be considered as a combination of microcavitary and membrane hydrolysis within the cell. Indeed, during intracellular digestion, enzymes can exert their hydrolytic effect in the cytoplasm of the cell or in the phagosome, i.e. in the environment, which is characteristic of cavity digestion, as well as on the inner surface of the phagosomal membrane, which is characteristic of membrane digestion.

Intracellular digestion is limited by membrane permeability and epidocytosis processes, which are characterized by low speed and, apparently, cannot play a significant role in meeting the nutritional needs of higher organisms.