The digestive system performs. Test your knowledge. Hygienic conditions for normal digestion

The given sequence of digestion processes provides the most complete mechanical and chemical processing of the food bolus in order to extract all essential substances. The stages of the digestion process are discussed in this article. You can learn about the process of digestion in the human body, starting with oral cavity and ending with the large intestine. It is very difficult to overestimate the importance of the digestion process, in fact it is a factor in maintaining the organic life of the body. normal process human digestion provides all the needs for proteins, fats and carbohydrates. FROM energy point view, the process of digestion in the body is necessary to extract calories in order to direct them to the work of the muscles and internal organs. The work of the brain and the entire central nervous system, including its function of thermoregulation, is based on the same principle.

Fundamentals of the physiology of digestion

Nutrition is a complex process of intake, digestion and absorption of nutrients. In recent decades, a special science of nutrition, nutriciology, has begun to actively develop. Consider the basics of the physiology of digestion in the human oral cavity, stomach and intestines.

Digestive system- a set of organs that ensure the absorption of the nutrients the body needs as an energy source for cell renewal and growth. Distinguish between cavity and membrane digestion. Abdominal is carried out in the oral cavity, stomach, small and large intestines. Membrane - at the level of the surface of the cell membrane and the intercellular space, characteristic of the small intestine.

Proteins, fats, carbohydrates, vitamins, minerals that come with food cannot be absorbed by the body, its tissues and cells unchanged. Complex food substances are broken down by hydrolase enzymes that are released into the cavity of the digestive tract in certain parts of it. In the process of digestion, from high-molecular compounds, they gradually turn into low-molecular, soluble in water. Proteins are broken down by proteases into amino acids, fats by lipases into glycerol and fatty acids, carbohydrates - amylases to monosaccharides.

All these substances are absorbed in the digestive tract and enter the blood and lymph, i.e., into the liquid media of the body, from where they are extracted by tissue cells. The end products of digestion that are absorbed into the blood are simple sugars, amino acids, fatty acids, and glycerol.

Vitamins, macro- and microelements in the digestive system can be released from bound state in which they are in food products, but the molecules themselves do not split.

The digestive system consists of several parts: the mouth, pharynx, esophagus, stomach, small intestine, large intestine, and rectum.

The essence, physiology and features of the processes of digestion in the human oral cavity

The essence of digestion in the oral cavity is that food is crushed. In the oral cavity, the processes of digestion conclude that there is an active processing of food with saliva (0.5-2 l is formed per day), the interaction of microorganisms and enzymes (amylases, proteinases, lipases). In saliva, some substances dissolve and their taste begins to appear. The physiology of digestion in the oral cavity is based on the fact that saliva contains the enzyme amylase, which breaks down starch into sugars.

So, the action of amylase is easy to trace: if you chew bread for 1 minute, you feel a sweet taste. Proteins and fats do not break down in the mouth. Average duration digestion in the oral cavity is minimal and is only 15-20 s.

Features of digestion in the oral cavity are that further food bolus(usually 5-15 cm3) moves into the stomach. The act of swallowing includes the oral (voluntary), pharyngeal (fast involuntary), esophageal (slow involuntary) phases. On this, the process of digestion in the human oral cavity is considered to be actually completed. The average duration of the passage of the food bolus through the esophagus is 2-9 s and depends on the density of the food. The digestive tract is provided with special valves to prevent backflow, as well as to differentiate the effects of digestive enzymes.

The processes of digestion that occur in the human stomach

The stomach is the widest part of the digestive tract, it can grow in size and contain a large number of food. Due to the rhythmic contraction of the muscles of the walls, digestion in the stomach begins with the fact that the food is thoroughly mixed with acidic gastric juice.

The food bolus, once in the stomach, stays in it for 3-5 hours and is subjected to mechanical and chemical processing. The processes of digestion in the stomach begin with the fact that food is exposed to gastric juice(2-2.5 liters are released per day) and hydrochloric acid present in it (provides an acidic environment), pepsin (digests proteins) and other acidic proteases such as rennin (chymosin).

Pepsinogens (precursors of pepsin) are divided into two groups. The first, after activation with hydrochloric acid and transformation into pepsins, hydrolyzes certain types of proteins for the processes of digestion occurring in the stomach with the formation of large peptides at pH 1.5-2.0. The second fraction, after activation with hydrochloric acid, turns into gastrixin, which hydrolyzes food proteins at pH 3.2-3.5.

Enzymes in the process of digestion in the human stomach digest proteins to low molecular weight peptides and amino acids. The digestion of carbohydrates, which began in the mouth, stops in the stomach, because in acidic environment amylase loses its activity.

Features of the physiology of digestion in the cavity of the human stomach

Digestion in the human stomach is based on the action of gastric juice, which contains lipase, which breaks down fats. In digestion in the stomach cavity, hydrochloric acid of gastric juice plays an important role. Hydrochloric acid increases the activity of enzymes, causes denaturation and swelling of proteins, and has a bactericidal effect.

Normally, the acidity of gastric juice ranges from pH 1.6 to 1.8. Deviation of gastric juice from the norm is used in the diagnosis of stomach ulcers, anemia, tumors. Features of digestion in the stomach is that under the action of hydrochloric acid, many pathogens are deactivated.

The physiology of digestion in the stomach is such that food rich in carbohydrates stays in the stomach for about two hours, is evacuated faster than protein or fatty food, which lingers in the stomach for 8-10 hours.

Mixed with gastric juice and partially digested food in small portions, at certain intervals, when its consistency becomes liquid or semi-liquid, it passes into the small intestine.

Functions and features of the digestion process in the human small intestine

From the stomach, the food bolus enters the small intestine, the length of which in an adult reaches 6.5 meters. Digestion in small intestine is the most important from the biochemical point of view of the assimilation of substances.

Intestinal juice in this section of the digestive tract has an alkaline environment due to the entry into the small intestine of bile, pancreatic juice and secretions of the intestinal walls. Some people have a slow digestion process in the small intestine, due to a deficiency of the lactase enzyme, which hydrolyzes milk sugar (lactose), which is associated with indigestion whole milk. In total, more than 20 enzymes are used in digestion in the human small intestine (enterokinases, peptidases, phosphatases, nucleases, lipase, amylase, lactase, sucrase, etc.).

The functions of digestion in the small intestine depend on its departments. The small intestine has three sections that pass into each other - the duodenum, jejunum and ileum. AT duodenum bile is secreted, which is formed in the liver. In the duodenum, food is exposed to the action of pancreatic juice, bile. The juice secreted by the pancreas is a colorless clear liquid with pH 7.8-8.4. Pancreatic (pancreatic) juice contains enzymes that break down proteins and polypeptides: trypsin, chymotrypsin, elastase, carboxypeptidases and aminopeptidases.

The pancreatic juice contains: lipase, which breaks down fats; amylase, which completes the complete breakdown of starch to a disaccharide - maltose; ribonuclease and deoxyribonuclease, splitting ribonucleic and deoxyribonucleic acids. The secretion of pancreatic juice, depending on the composition of the food, lasts 6-14 hours, it is the longest when taking fatty foods.

An important role in the process of digestion is played by the liver, where bile is formed (0.5-1.5 liters per day). Features of digestion in the small intestine are that bile promotes fat emulsification, absorption of triglycerides, activates lipase, stimulates peristalsis, inactivates pepsin in the duodenum, has a bactericidal and bacteriostatic effect, enhances hydrolysis and absorption of proteins and carbohydrates.

Bile does not contain digestive enzymes, but is necessary for the dissolution and absorption of fats and fat soluble vitamins. With insufficient production of bile or its release into the intestine, the digestion and absorption of fats are disturbed, and their excretion increases unchanged with feces.

The final digestion of carbohydrates, protein residues, fats occurs in the jejunum and ileum with the help of enzymes that are produced by the cells of the mucous membrane of the intestine itself. Outgrowths of the wall of the small intestine are covered with enterocytes - villi. Through many villi from its surface, the breakdown products of proteins and carbohydrates enter the blood, and the breakdown products of fats enter the lymph. Due to the large number of special folds and villi, the total suction surface of the intestine is about 500 m2.

In the small intestine, the majority of simple chemical fragments of food are absorbed.

Physiology, functions and processes of digestion in the large intestine

Undigested food remains are then taken to the colon, in which they can be from 10 to 15 hours. In this section of the digestive tract, such processes of digestion in the intestine as the absorption of water and the microbial metabolism of nutrients are carried out.

The length of the large intestine in an adult is on average 1.5 m. It consists of three parts - the blind, transverse colon and rectum.

Digestion in the large intestine is dominated by reabsorption mechanisms. It absorbs glucose, vitamins and amino acids produced by bacteria in the intestinal cavity.

An important role in the processes of digestion in the large intestine is played by dietary ballast substances. These include indigestible biochemical components: fiber, hemicellulose, lignin, gums, resins, waxes.

The basis of ballast components are substances plant origin, included in the structure of the walls of plants and contained in wood, seed husks, bran. Most of the ballast substances are cellulose and branched polysaccharides based on xylose, arabinose, mannose, galactose. The ballast ingredients of animal origin include elements of the connective tissue of animals that are not utilized by the human body.

Collagen protein, resistant to the action of proteolytic enzymes, performs physiological functions digestion in the large intestine, similar to dietary fiber. The mucopolysaccharides that are not hydrolyzed in the intestine and contained in intercellular substance animal tissues. The largest number of these structural polysaccharides is found in the connective tissue, lungs, blood.

The structuring of food affects the rate of absorption in the small intestine and the duration of transit through the gastrointestinal tract.

Dietary fibers and products of collagen thermohydrolysis have the ability to retain a significant amount of water, which significantly affects the pressure, weight and electrolyte composition of feces, contributing to the formation of soft feces.

Dietary fiber and indigestible connective tissue proteins are among the main components that make up the environment in which beneficial intestinal bacteria live.

Dietary fibers and connective tissue elements have great importance for electrolyte metabolism in the gastrointestinal tract. This is due to the fact that collagen, like polysaccharides, has cation exchange properties and helps to eliminate various harmful compounds from the body.

Dietary fiber in human diets reduces the risk of developing neoplastic diseases, peptic ulcer, duodenal diseases, diabetes, cardiovascular diseases, have beneficial effect on the body of people overweight bodies suffering from atherosclerosis, hypertension and other diseases.

Dietary fiber not broken down by enzymes gastrointestinal tract, partially destroyed under the influence of microflora.

In the colon, fecal masses are formed, consisting of undigested food debris, mucus, dead cells of the mucous membrane and microbes, which continuously multiply in the intestine, causing fermentation and gas formation processes.

total weight intestinal microflora a person is 1.5-2.0 kg. The flora of the large intestine contains anaerobic species microorganisms: bifidobacteria (108-1010 cfu/g in adults, 109-10sh cfu/g in children), bacteroids (109-1010 cfu/g in adults, 106-108 cfu/g in children), lactobacilli (106-107 cfu/g /g in adults, 106-10 CFU/g in children), peptostreptococci, clostridia, which is up to 99% of the total composition. About 1% of the microflora of the large intestine is represented by aerobes: E. coli, enterobacteria (Proteus, Enterobacter, etc.), enterococci, staphylococci, yeast-like fungi. The amount of each species ranges from 104-108 CFU/g.

The process of splitting and absorption of substances in digestion

The process of absorption in digestion is the passage of nutrients from the cavity of the digestive tube into the cells intestinal epithelium and then into the blood. Preliminary breakdown of substances in the process of digestion is necessary to obtain products at the cellular and molecular level.

Absorption is carried out throughout the digestive tract, the surface of which is covered with villi. There are 30-40 villi per 1 mm2 of the mucosa. At the same time, 50-60% of the products of protein metabolism are absorbed in the duodenum; 30% - in the small intestine and 10% - in the large intestine. Carbohydrates are absorbed only in the form of monosaccharides. The products of fat metabolism, as well as most of the water- and fat-soluble vitamins that come with food, are absorbed in the small intestine.

The digestive system performs a number of functions:

-mechanical function, or crushing of food, is carried out with the help of teeth in the oral cavity and due to mixing in the stomach and small intestine, as well as transporting the food bolus through the digestive tract due to contraction of the muscular membrane (peristalsis);

-secretory function consists in the synthesis and secretion of digestive enzymes by the digestive glands;

-chemical function It consists in the chemical processing of food (digestion) with the help of digestive enzymes. The primary chemical processing of food begins in the oral cavity and ends in the small intestine, where the final chemical processing takes place. In the large intestine and at the junction of the large and small intestines inhabited by intestinal microflora- symbiotic microorganisms that help us digest plant and dairy foods;

- suction function ensures the absorption of digestion products into the blood and lymph. Partial absorption of carbohydrates begins in the oral cavity, continues in the stomach, where protein breakdown products begin to be absorbed. The main absorption occurs in the small intestine. It should be noted that the products of lipid digestion are absorbed into the lymph;

-excretory function- excretion of undigested food residues and waste products;

-endocrine- secretion of digestive hormones.

Oral cavity, or oral cavity(Fig. 1)

Rice. one.The oral cavity and pharynx: 1 - upper and 2 - lower lip; 3 - pharynx; 4 - language; 5 - palatoglossal and 6 - palatopharyngeal arches; 7 - palatine tonsil; 8 - tongue; 9 - soft and 10 - hard palate; 11 - gums

Teeth(Fig. 2). Main function- capture and primary mechanical processing of food (grinding).

In humans, there are two types of teeth, depending on the time of appearance:

-baby teeth(temporary). A child has 20 milk teeth, which function until they are replaced by permanent teeth at the age of 7 to 13-14 years. On each half of the jaw, 2 incisors, 1 canine, 2 large molars are distinguished;

-permanent teeth. A person has 32 permanent tooth: in each half of the jaw 2 incisors, 1 canine, 2 small molars and 3 large molars.

Rice. 2.Scheme of the structure of the tooth: I - enamel; 2 - dentin; 3 - tooth pulp; 4 - gum; 5 - cement; 6 - periodontal; 7-bone; I - tooth crown; II - the neck of the tooth; III - tooth root; IV - root canal

Language. A mobile muscular organ, dressed in a mucous membrane, richly supplied with blood vessels and nerves.

The mucosa is rich in taste buds - papillae(Fig. 3). Distinguish: filiform and fungiform papillae- scattered over the entire upper surface of the tongue; papillae, rolled, - in the amount of 7-11 are located on the border of the body and the root of the tongue; foliate papillae - clearly visible along the edges of the tongue. There are no papillae on the underside of the tongue.

The tongue is involved in the process of sucking, swallowing, speech articulation, is an organ of taste (fungiform and foliate papillae perceive sour, sweet and salty tastes, and papillae with a roller - bitter).

Rice. 3.Language: 1 - the root of the language; 2 - filiform, 3 - mushroom-shaped, 4 - surrounded by a roller and 5 - foliate papillae; 6 - blind fossa; 7 - palatine-lingual fold; 8 - palatine and 9 - lingual tonsils; 10 - epiglottis

Pharynx

Muscular organ that connects the mouth to the esophagus nasal cavity with the larynx, i.e. in the pharynx the digestive and Airways . The pharynx is divided into three parts: nasopharynx, oropharynx and guttural part. Located in the throat six tonsils. Nasopharynx through choanae communicated with nasal cavity. On the side walls are openings of the auditory (Eustachian) tubes, which connect it with the cavity middle ear, helping to equalize pressure in the middle ear with external pressure. tonsils perform important protective and partly hematopoietic functions. Sharp increase tonsils - the first sign of angina, scarlet fever, diphtheria.

Esophagus

It is a muscular tube about 25 cm long (Fig. 4). Begins without sharp borders from the pharynx at level VI cervical vertebra and at level XI thoracic vertebra opens into the stomach. The muscular layer has the following features: in the upper third it consists of striated muscles, a in the lower third - only from smooth muscles. The main function of the esophagus is to carry the food bolus to the stomach. Partially the esophagus performs protective function with the help of three constrictions (it is in these constrictions that accidentally swallowed foreign objects very often get stuck). It does not have its own digestive glands, digestion is carried out by saliva enzymes. It has an alkaline environment.

Rice. four.The structure of the wall of the esophagus. Mucosa (I), muscular (II) and serous (III) membranes: 1 - multilayer squamous epithelium; 2 - own and 3 - muscular layers of the mucous membrane; 4 - submucosal layer; 5 - mucous gland; 6 - layer of circular and longitudinal (7) muscles

Stomach

The only expanded part of the digestive tube up to 5 liters (Fig. 5). Distinguish inlet (cardia), bottom, body and exit (gatekeeper). At the entrance and exit there are circular muscles-contactors (sphincters). The muscular layer has three types of muscles: longitudinal, ring and oblique.

The stomach performs several functions: mechanical processing of food due to mixing, temporary storage and chemical processing of food, and partial absorption. Chemical processing of food is carried out by gastric juice secreted by own glands. Gastric juice It has acidic environment(pH 2). glands are made up of three types of cells: main, highlighting digestive enzymes, lining, highlighting hydrochloric acid, and additional that secrete mucus.

Rice. 5.Stomach with an open anterior wall (A) and its muscular membrane (B): 1 - cardial part; 2 - cardiac opening; 3 - bottom of the stomach; 4 - the body of the stomach; 5 - small and 6 - large curvature of the stomach; 7 - pyloric (pyloric) part; 8 - gatekeeper; 9 - pylorus hole; 10 - muscular membrane; 11 - longitudinal (outer) layer; 12 - circular layer; 13 - pyloric sphincter; 14 - oblique fibers

Small intestine

The longest part of the digestive tract (up to 5 m) is divided into three parts: duodenum, skinny and ileum. A characteristic feature is the presence villi formed by the mucous membrane (Fig. 6, 7). villi have microvilli, formed villus epithelium. On the border with the stomach and large intestine there are sphincters. Ducts open into the duodenum pancreas and gallbladder.

Rice. 6.The mucous membrane of the small intestines. A - skinny; B - iliac: 1 - muscular membrane; 2 - mesentery; 3 - serous membrane; 4 - single follicles; 5 - circular folds; 6 - mucous membrane; 7 - group follicles

Rice. 7.Scheme of the structure of the villi of the small intestine: 1 - intestinal epitheliocytes; 2 - goblet cells; 3 - central lymphatic sinus; 4 - arteriole; 5 - venule; 6 - blood capillaries

The small intestine is the organ in which the breakdown of proteins, fats and carbohydrates is finally completed and digestion products are absorbed as well as salts and water. Digestion takes place under the influence intestinal juice allocated intestinal glands, pancreatic juice secreted by the pancreas, and bile. Available abdominal and parietal digestion.

Colon

It has a length of up to 2 m and a diameter of up to 5-7 cm. It consists of three sections: the caecum with appendix(Fig. 8), colon and rectum. There is a large number of symbiotic bacteria here. The main functions of the large intestine are the absorption of water and the formation of feces. Due to the presence of bacteria, fiber fermentation and protein putrefaction, a number of bacteria synthesize vitamins.

Rice. eight.Caecum with appendix (appendix): 1 - appendix (appendix); 2 - opening of the appendix; 3 - caecum; 4 - opening of the small intestine; 5 - large intestine; 6 - colon

digestive glands

Salivary glands . The salivary glands secrete saliva, which is protein secretion(serous) and mucous component. Protein secretion is isolated parotid glands , slimy - palatine and back lingual; submandibular and sublingual- mixed secret. The main components of saliva are: mucin- mucous protein substance, lysozyme- bactericidal agent, amylase enzymes and maltase.

Distinguish small and large salivary glands . The small ones are labial, buccal, dental, lingual, palatine. These glands are located in the corresponding parts of the oral mucosa. There are three pairs of major salivary glands: parotid, submandibular and sublingual; they lie outside the oral mucosa, but excretory ducts open into the mouth.

Liver - the largest gland (weight up to 1.5 kg). Most of it is in the right hypochondrium, the smaller one goes into left side abdominal cavity. The main secret that the liver secretes into the digestive system is bile. Bile emulsifies fats, activates fat-splitting enzymes of the pancreas, but does not contain enzymes itself. Carbohydrates are converted to glycogen in the liver. The liver also performs a barrier function, neutralizing toxic substances that appear in the body in the process of metabolism. Outside of the digestion process, bile is collected in gallbladder.

Pancreas - digestive gland 20 cm long and 4 cm wide, located behind the stomach. The pancreas is related to mixed glands. The exocrine part produces pancreatic juice, containing trypsinogen, amylase, maltase, lactase, lipase, nuclease. The endocrine part produces hormones: insulin and glucagon.

Digestive enzymes

main function digestive system- digestive - performed by specialized proteins - digestive enzymes. In each section of the digestive tract, specific enzymes function that help digest certain substances.

Digestive enzymes

vitamins

vitamins called a group of biologically active organic compounds of various chemical nature, entering the body with food of plant and animal origin. Some vitamins are synthesized microbial flora of the intestine. Vitamins are present in food in negligible amounts, and the body also needs them in small quantities, but at the same time they play a very important role in metabolic processes, often being an integral part of enzymes. In the absence of any vitamin or its precursor in the body, disease occurs - avitaminosis. But, although vitamins are important for the body, their overdose (intoxication) due to intake higher doses also leads to painful manifestations and is called hypervitaminosis.

Vitamins are divided into two groups depending on the solvents in which they dissolve: fat-soluble(vitamins A, D, E, K) and water soluble(vitamins of group B, PP, C, etc.).

The main functions of the digestive system are:

secretory - consists in the synthesis and secretion of digestive juices (saliva, gastric, pancreatic, intestinal juices, bile) by glandular cells;

motor or motor: chewing, swallowing, advancing and mixing with digestive juices, and excretion of residues - carried out by smooth muscles, and only the oral cavity, initial department the esophagus and external sphincter of the rectum have striated muscles;

suction- penetration through the mucous membrane into the blood or lymph of the breakdown products of proteins, fats and carbohydrates, water, salts and vitamins.

The processes of secretion, motility and absorption are interconnected and are subject to complex neuro-humoral mechanisms of regulation. In addition to digestive functions, the digestive system has: endocrine function associated with the secretion of hormones and biologically active substances into the blood; excretory, associated with the removal of toxins and food debris into the external environment; protective function.

Protective systems of the digestive tract

The theory of adequate nutrition considers the intake of food into the body not only as a way to restore plastic and energy costs, but also as an allergic and toxic aggression. Nutrition is associated with the danger of penetration into the body of exogenous food antigens (food proteins and peptides), autoantigens of desquamated intestinal cells. With food through digestive tract many bacteria, viruses and various toxic substances enter the body. It is safe to say that at present, environmentally friendly food and natural water Hardly ever. In the second half of the 20th century, there was widespread pollution of the environment by industrial, in some regions, radioactive waste. In plant growing and animal husbandry, chemical and biological technologies are widely used without appropriate strict sanitary and epidemic control of the products produced.

Currently, food additives (preservatives, dyes, flavoring agents) are widely used in the manufacture of food products. This is usually chemical substances, the use of which in food production must be scientifically substantiated, and their content in the product must not exceed the permissible limits. Many of these substances can cause not only allergic reactions, but also have a carcinogenic effect. Plant foods can contain excessive amounts of nitrates and pesticides (chemicals used to protect plants from pests), many of which are poisonous to humans. Products of animal origin may contain drugs used to treat animals, growth stimulants used in their cultivation. The presence of these drugs in food can change the sensitivity to antibiotics and cause endocrine disorders. The above negative aspects of nutrition in a healthy body are neutralized due to the complex system of protection of the digestive tract. There are non-specific and specific (immune) defense mechanisms.

Types of non-specific protection:

Mechanical or passive protection is associated with the limited permeability of the mucous membrane of the digestive tract for macromolecular substances (with the exception of newborns).

The mucous membrane is lined with a layer of mucus, which protects it not only from mechanical, but also chemical influences. The outer layer of mucus adsorbs viruses, toxic substances, salts heavy metals(mercury, lead) and, being rejected into the cavity of the stomach and intestines, contributes to their excretion from the body.

Saliva, gastric juice, bile have antibacterial activity. Hydrochloric acid creates an acidic environment in the stomach, has a bacteriostatic effect, preventing the development of putrefactive processes.

The nonspecific protective barrier is associated with the preliminary enzymatic hydrolysis of antigenic molecules, which lose their antigenic properties.

Specific protection in the digestive tract is carried out by immunocompetent lymphoid tissue. In the mucous membrane of the mouth and tonsils there are a large number of cellular elements: macrophages, neutrophils, lymphocytes that carry out phagocytosis of bacteria and antigenic proteins. In the mucous membrane of the small intestine there is a powerful leukocyte layer that separates the enteric and internal environments of the body. It consists of a large number of plasma cells, macrophages, eosinophils, lymphocytes. The intestinal immune system is part of immune system organism. The lymphatic tissue of the small intestine (25% of the entire mucosa) consists of Peyer's patches, individual lymphatic nodules localized in the region of the lamina propria of the villi and T- and B-lymphocytes scattered in the epithelium (see Fig. 3). Designations in the figure, description in the text. There are also intraepithelial lymphocytes.

Fig. 3 Cross section of the intestinal villus.

In the epithelium above the plaques, special M-cells are localized, which transport antigens to the lymph nodes. Thus, lymphocytes carry out both cellular and humoral immunity.. They produce immunoglobulins adsorbed on the surface of the epithelium in the area of ​​the glycocalyx and create an additional protective layer. In addition to these tissues, the defense system includes mesenteric The lymph nodes and the reticuloendothelial system of the liver. The detoxification and barrier functions of the liver are essential in neutralizing the products of protein decay (indole, skatole, phenol) formed in the intestine, as well as toxic substances and drugs that come with food, and are considered in detail by biological chemistry.

General principles of regulation of digestive functions

Central nervous regulation is carried out by the digestive centers of the brain and spinal cord with the help of conditioned and unconditioned reflexes. The type and smell of food, the time and environment of its intake, a reminder of food excite the digestive glands (salivary, gastric, pancreas) in a conditioned reflex way.

Eating, irritating the receptors of the mouth and stomach, causes unconditioned reflexes. Afferent pathways of unconditioned reflexes are represented by sensitive fibers of the cranial nerves: lingual, glossopharyngeal, upper laryngeal, vagus. Efferent pathways common to conditioned and unconditioned reflexes are formed by parasympathetic and sympathetic fibers.

As the distance from the proximal part increases, the participation of central reflexes in the regulation of functions decreases. The main importance in the small and large intestines is acquired by local nervous and humoral regulation. local nervous regulation is based on "short" reflex arcs. In the wall of the stomach and intestines there is a developed network of nerve cells that form two main plexuses: intermuscular (Auerbach) and submucosal (Meissner). Among nerve cells there are sensory neurons, intercalary and effector. The latter innervate smooth muscles, secretory epithelium and endocrine cells.

Figure 4. Metasympathetic system of the small intestine

A is local reflex arc regulation of motility, B - local reflex arc regulation of the secretion of exocrine and endocrine cells: 1. vagus nerve; 2. mucous membrane; 3. exocrine cell; 4. Meisner's plexus; 5.circular muscle; 6. Auerbach's plexus; 7. longitudinal muscle; 8.endocrine cell

In addition to acetylcholine and norepinephrine, more than ten neuropeptides take part in the transmission of regulatory effects on target cells: cholecystokinin, somatostatin, neurotensin, substance P, enkephalin, etc. There are neurons whose mediators are serotonin and purine bases. The set of nerve cells lying inside the organ and forming local reflex arcs was called the metasympathetic nervous system (A.D. Nozdrachev). This system interacts with the central nervous system, but is more independent of it than the autonomic nervous system, because it has its own sensory link (receptive field). Various receptors respond to the initial composition of food and the changes that occur during hydrolysis. The metasympathetic nervous system (Fig. 4) programs and coordinates motor activity, regulates secretion and carries out the relationship between these processes, regulates the secretion of endocrine cells, local blood flow.

Digestion of food is a gradual and continuous process, therefore Humoral mechanisms are of great importance in the regulation of secretion, motility and absorption. In the epithelial layer of the mucous membrane of the stomach and small intestine, the pancreas there are diffusely scattered endocrine cells (the mass of these cells is greater than the mass of all endocrine glands), which secrete hormones and peptides. Some hormones are secreted into the blood and through it have a distant effect on target cells (gastrin  parietal cell), others have a local or paracrine effect, being released into the intercellular fluid, others (neuropeptides) are released in nerve endings along with mediators. The secretion of hormones can activate the CNS (for example, nervus vagus), but many endocrine cells have receptors in the enteric environment that are directly affected by food hydrolysis products. Since all textbooks give a detailed description of gastrointestinal hormones and their influences, let us only note that hormones have both synergism of varying severity and antagonism. They can activate or inhibit secretion, motility, absorption.

Thus, in the digestive tract there is gradient distribution of regulatory mechanisms. In the initial sections, the central ones predominate. reflex mechanisms. In the middle sections (stomach, duodenum, jejunum, pancreas) - central reflexes have a starting value, and hormonal regulation complements it and becomes dominant. In the small and especially in the large intestine, the role of local (nervous and humoral) regulatory mechanisms is important. However, all mechanisms can regulate the activity of the same organ (stomach, pancreas).

The cells and tissues of the human body need constant replenishment. nutrients. The body receives them as part of food containing proteins, fats, carbohydrates, which are used as a building material when digging and recreating new cells to replace dying ones. Food also serves as a source of energy, which is consumed in the process of vital activity of the body.

Vitamins are of great importance for normal life. mineral salts and water from food. Vitamins are part of a variety of enzyme systems, and water is needed as a solvent. Before being absorbed by the body, food undergoes mechanical and chemical processing. These processes are carried out in the digestive organs, which consist of the esophagus, stomach, intestines, glands. Digestion of food is impossible without enzymes produced by the digestive glands. All enzymes in living organisms are of a protein nature; in small quantities they enter into the reaction and at the end of it come out unchanged. Enzymes differ in specificity: for example, an enzyme that breaks down proteins does not act on a starch molecule, and vice versa. All digestive enzymes contribute to the dissolution of the original substance in water, preparing it for further splitting.

Each enzyme acts on certain conditions, best at a temperature of 38-40 ° C. Its increase inhibits activity, and sometimes destroys the enzyme. Enzymes are also affected by the chemical environment: some of them are active only in an acidic environment (for example, pepsin), while others are active in an alkaline environment (ptyalin and pancreatic juice enzymes).

The alimentary canal has a length of about 8-10 m, along its length it forms expansions - cavities and narrowings. The wall of the alimentary canal consists of three layers: inner, middle, outer. The internal is represented by the mucous and submucosal layers. The cells of the mucous layer are the most superficial, facing the lumen of the canal and produce mucus, and the digestive glands lie in the submucosal layer located under it. The inner layer is rich in blood and lymphatic vessels. middle layer includes smooth muscle, which, by contracting, moves food through the alimentary canal. The outer layer consists of connective tissue that forms the serous membrane, to which the mesentery is attached throughout the small intestine.

The alimentary canal is divided into following departments: oral cavity, pharynx, esophagus, stomach, small and large intestines.

Oral cavity from below it is limited by the bottom formed by muscles, in front and outside - by teeth and gums, from above - by hard and soft palate. Back department soft palate protrudes, forming a tongue. Behind and on the sides of the oral cavity, the soft palate forms folds - the palatine arches, between which lie the palatine tonsils. There are tonsils at the root of the tongue and in the nasopharynx, together they form lymphoid pharyngeal ring, in which microbes penetrating with food are partially retained. In the oral cavity is the tongue, which consists of a striated muscle tissue covered with mucous membrane. In this organ, the root, body and tip are distinguished. The tongue is involved in the mixing of food and the formation of a food bolus. On its surface are filiform, mushroom-shaped and leaf-shaped papillae, in which taste buds end; receptors on the root of the tongue perceive bitter taste, receptors on the tip for sweet, and receptors on the lateral surfaces for sour and salty. In humans, the tongue, together with the lips and jaws, performs the function of oral speech.

In the cells of the jaws there are teeth that mechanically process food. A person has 32 teeth, they are differentiated: in each half of the jaw there are two incisors, one canine, two small molars and three large molars. A crown, neck and root are distinguished in the tooth. The part of the tooth that protrudes from the surface of the jaw is called the crown. It consists of dentin, a substance close to bone, and is covered with enamel, which has a much greater density than dentin. The narrowed part of the tooth, lying on the border between the crown and the root, is called the neck. The part of the tooth that is in the hole is called the root. The root, like the neck, consists of dentin and is covered with cement from the surface. Inside the tooth there is a cavity filled with loose connective tissue with nerves and blood vessels forming the pulp.

The mucous membrane of the mouth is rich in glands that secrete mucus. The ducts of three pairs of large salivary glands open into the oral cavity: parotid, sublingual, submandibular and many small ones. Saliva is 98-99% water; from organic substances, it contains the protein mucin and the enzymes ptyalin and maltase.

The oral cavity behind passes into a funnel-shaped pharynx, connecting the mouth to the esophagus. The digestive and respiratory tracts cross in the pharynx. The act of swallowing occurs as a result of the contraction of striated muscles, and food enters the esophagus - a muscular tube about 25 cm long. The esophagus passes through the diaphragm and opens into the stomach at the level of the 11th thoracic vertebra.

Stomach- This is a greatly expanded section of the alimentary canal, located in the upper part of the abdominal cavity under the diaphragm. It distinguishes the input and output parts, the bottom, the body, as well as the greater and lesser curvature. The mucous membrane is folded, which, when filled with food, allows the stomach to stretch. In the middle part of the stomach (in its body) are the glands. They are formed by three types of cells that secrete either enzymes, or hydrochloric acid, or mucus. There are no acid-secreting glands at the outlet of the stomach. The outlet is closed by a strong obturator muscle - the sphincter. Food from the stomach enters the small intestine 5-7 m long. Its initial section is the duodenum, followed by the jejunum and ileum. The duodenum (about 25 cm) has the shape of a horseshoe, the ducts of the liver and pancreas open into it.

Liver- the largest gland of the digestive tract. It consists of two unequal lobes and is located in the abdominal cavity, on the right under the diaphragm; left lobe liver covers most stomach. The outside of the liver is covered serosa, under which lies a dense connective tissue capsule; at the gates of the liver, the capsule forms a thickening and, together with the blood vessels, is introduced into the liver, dividing it into lobes. Vessels, nerves, bile duct pass through the gates of the liver. All deoxygenated blood from the intestines, stomach, spleen and from the pancreas enters the liver through portal vein. Here the blood is released from harmful products. Located on the lower surface of the liver gallbladder - a reservoir that stores bile produced by the liver.

The bulk of the liver is made up of epithelial (glandular) cells that produce bile. The bile enters hepatic duct, which, connecting with the duct of the gallbladder, forms the common bile duct, which opens into the duodenum. Bile is produced continuously, but when digestion does not occur, it accumulates in the gallbladder. At the time of digestion, it enters the duodenum. The color of bile is yellow-brown and is due to the pigment bilirubin, which is formed as a result of the breakdown of hemoglobin. Bile is bitter in taste, contains 90% water and 10% organic and mineral substances.

In addition to epithelial cells in the liver, there are stellate cells with phagocytic properties. The liver is involved in the process of carbohydrate metabolism, accumulating in its cells glycogen(animal starch), which can be broken down here to glucose. The liver regulates the flow of glucose into the blood, thereby maintaining the concentration of sugar at a constant level. It synthesizes the proteins fibrinogen and prothrombin, which are involved in blood coagulation. At the same time, it neutralizes some toxic substances that are formed as a result of the decay of proteins and come from the large intestine with the bloodstream. In the liver, amino acids are broken down, resulting in the formation of ammonia, which is converted here into urea. The work of the liver to neutralize toxic products of absorption and metabolism is its barrier function.

Pancreas divided by partitions into a number of lobules. It distinguishes head, enclosed by the flexure of the duodenum, body and tail, adjacent to the left kidney and spleen. Its duct runs along the entire length of the gland, opening into the duodenum. The glandular cells of the lobules produce pancreatic, or pancreatic, juice. Juice It has a pronounced alkalinity and contains several enzymes involved in the breakdown of proteins, fats and carbohydrates.

Small intestine begins with the duodenum, which passes into the lean, continuing into the ileum. The mucous wall of the small intestine contains many tubular glands that secrete intestinal juice, and is covered with the thinnest outgrowths - villi. Them total reaches 4 million, the height of the villi is about 1 mm, the joint suction surface is 4-5 m 2. The surface of the villus is covered with a single-layered epithelium; in the center of it pass lymphatic vessel and an artery disintegrating into capillaries. Thanks to muscle fibers and nerve ramifications, the villus is able to contract. This is carried out reflexively in response to contact with food gruel and enhances the circulation of lymph and blood during digestion and absorption. skinny and ileum with their villi, the main site of absorption of nutrients.

Colon has a relatively short length - about 1.5-2 m and combines the blind (with appendix), colon and rectum. The caecum is continued by the colon, into which the ileum flows. The mucous membrane of the large intestine has semilunar folds, but there are no villi in it. The peritoneum, which covers the large intestine, has fatty annular folds. The final section of the digestive tube is the rectum, ending in the anus.

Digestion of food. In the oral cavity, food is crushed by the teeth and wetted by saliva. Saliva coats food and makes it easier to swallow. The ptyalin enzyme breaks down starch into an intermediate product - the disaccharide maltose, and the maltase enzyme converts it into a simple sugar - glucose. They only operate in alkaline environment, but their work also continues in a neutral and slightly acidic environment in the stomach until the food bolus is saturated with acidic gastric juice.

In the study of salivation, great merit belongs to the Soviet physiologist Acad. who first applied fistula method. This method was also used in the study of digestion in the stomach and intestines, and made it possible to obtain exceptionally valuable information on the physiology of digestion throughout the body.

Further digestion of food takes place in the stomach. Gastric juice contains the enzymes pepsin, lipase and hydrochloric acid. Pepsin acts only in an acidic environment, breaking down proteins to peptides. Lipase gastric juice decomposes only emulsified fat (milk fat).

Gastric juice released in two phases. The first begins as a result of food irritation of the receptors of the oral cavity and pharynx, as well as visual and olfactory receptors (the appearance, smell of food). The excitation that has arisen in the receptors along the centripetal nerves enters the digestive center located in medulla oblongata, and from there - along the centrifugal nerves to salivary glands and glands of the stomach. Juice secretion in response to irritation of the receptors of the pharynx and mouth is without conditioned reflex, and the secretion of juice in response to irritation of the olfactory and taste receptors is a conditioned reflex. The second phase of secretion is caused by mechanical and chemical stimuli. In this case, meat, fish and vegetable decoctions, water, salt, fruit juice serve as irritants.

Food from the stomach moves in small portions into the duodenum, where bile, pancreas and intestinal juices. The speed of food intake from the stomach to the underlying sections is not the same: fatty foods linger in the stomach for a long time, dairy and carbohydrate-containing foods pass into the intestines quickly.

pancreatic juice - colorless liquid alkaline reaction. It contains protein enzymes trypsin and others that break down peptides into amino acids. Amylase, maltase and lactase act on carbohydrates, turning them into glucose, lactose and fructose. Lipase breaks down fats into glycerol and fatty acids. The duration of the secretion of juice by the pancreas, its quantity and digestive power depend on the nature of the food.

Suction. After mechanical and chemical (enzymatic) food processing, the breakdown products - amino acids, glucose, glycerol and fatty acids - are absorbed into the blood and lymph. Suction - difficult physiological process carried out by villi thin department intestines and going only in one direction - from the intestines to the villi. The epithelium of the intestinal walls does not just carry out diffusion: it actively passes only certain substances into the cavity of the villi, for example, glucose, amino acids, glycerol; unsplit fatty acids are insoluble and cannot be absorbed by the villi. Bile plays an important role in the absorption of fats: fatty acids, combining with alkalis and bile acids, are saponified and form soluble salts of fatty acids (soaps), which easily pass through the walls of the villi. In the future, their cells synthesize fat from glycerol and fatty acids, which is characteristic of human body. Droplets of this fat, unlike glucose and amino acids that enter the blood vessels, are absorbed by the lymphatic capillaries of the villi and carried by the lymph.

Slight absorption of some substances begins in the stomach (sugar, dissolved salts, alcohol, some pharmaceuticals). Digestion mainly ends in the small intestine; the glands of the large intestine secrete mainly mucus. In the large intestine, water is mainly absorbed (about 4 liters per day), feces are formed here. This section of the intestine contains great amount bacteria, with their participation, the cellulose of plant cells (fiber) is broken down, which passes through the entire digestive tract unchanged. Bacteria synthesize some B vitamins and vitamin K , necessary for the body person. putrefactive bacteria large intestines cause decay of protein residues with the release of a number of substances toxic to the body. Their absorption into the blood could lead to poisoning, but they are neutralized in the liver. In the final section of the large intestine - the rectum - the feces are compacted and excreted through the anus.

Food hygiene. Food poisoning occurs as a result of the consumption of food containing toxic substances. These poisonings can cause poisonous mushrooms and berries, roots mistaken for edible, and products prepared from cereals, where the seeds of some weeds fall poisonous plants and spores or hyphae of fungi. For example, the presence of ergot in bread causes an "evil cramp", the admixture of cockle seeds - the destruction of red blood cells. To prevent these food poisonings, a thorough cleaning of the grain from poisonous seeds and ergot is necessary. Poisoning can also be caused by metal compounds (copper, zinc, lead) if they are ingested. Of particular danger is poisoning with stale food, in which microorganisms have multiplied and accumulated poisonous products of their vital activity - toxins. Such products can be minced meat products, jelly, sausage, meat, fish. They spoil quickly, so they cannot be stored for a long time.