Digestion is closely related to all processes and functions of our body, without exception. The normal functioning of the digestive system ensures a stable composition of the internal environment (homeostasis), supports metabolism at an optimal level, and this is an important condition for the well-being of the body, its health.

Violation of any digestive link inevitably affects the state of the body as a whole. Often, the causes of the violation are not recognized, only the consequences are felt, often associated with the activities of other physiological systems, and especially the cardiovascular system, for example, when atherosclerotic changes in blood vessels develop as a result of malnutrition.

Digestion is a complex set of enzymatic and physico-chemical processes of food digestion, due to which food substances that enter the oral cavity and gastrointestinal tract are broken down into simple water-soluble compounds, absorbed into the bloodstream and transferred to cells and tissues.

On the structure and functions of the digestive system.

Food processing begins already in the oral cavity . Receptors of the oral cavity (taste, tactile, temperature) “evaluate” the quality of food, its taste, texture, temperature. Although the food is in the mouth for a short time, it has a great influence on the digestion process in the stomach and small intestine. Nutrients irritate the receptors of the tongue, the impulses from which reach the food center located at different levels of the brain: in the medulla oblongata, hypothalamus and in the cerebral cortex. At the same time, the excitability of the food center increases, as a result of which the secretion of salivary, gastric and pancreas is reflexively activated.

Saliva is the first digestive fluid, the first “reagent” to process nutrients. It performs a variety of functions, moisturizes and impregnates dry food. The mucous substance (mucin) contained in it envelops the food lump, and this creates favorable conditions for food to slip along the esophagus. Saliva contains a significant amount of amylase – an enzyme involved in the digestion of carbohydrates, as well as enzymes that break down proteins. All of them are active only with an alkaline or neutral reaction of the medium. Therefore, acidic gastric juice stops their action. But, nevertheless, their effect lasts for some time in the stomach, because the food lump is not immediately saturated with gastric juice.

Saliva has bactericidal (killing microbes) properties. It prevents the development of dental caries due to the presence of the lysozyme enzyme in it. In humans, salivation is also associated with speech function: saliva during speech moisturizes the mucous membrane of the oral cavity (it is established that salivation even accompanies the act of writing). During the day, from 0.5 l to 2 l of saliva is released.

To ensure complete chemical processing of food substances in the stomach, it is necessary to grind the food well with your teeth. Chewing stimulates salivary and gastric secretion. Thanks to him, food is mixed with saliva, which facilitates not only the swallowing of the food lump, but also the digestion of carbohydrates and proteins. Thus, chewing movements increase the effect of saliva and contribute to the rapid formation of a lump, ready to be swallowed. The ingestion of poorly chewed food into the stomach negatively affects its processing and digestibility and may be one of the causes of the development of diseases of the gastrointestinal tract.

As soon as the food lump reaches the entrance to the pharynx, swallowing movements occur reflexively. A large number of muscles (tongue, soft palate, pharynx, etc.) are involved in the act of swallowing. This complex and coordinated process is a semi-arbitrary , semi-reflex act, it is regulated by a swallowing center located in the medulla oblongata.

When food enters the esophagus , a wave-like contraction of its muscles occurs, pushing the food lump into the stomach. Outside the food intake, the entrance to the stomach from the esophagus is closed, but when the food passes through the esophagus and stretches it, the entrance to the stomach reflexively opens. Under normal conditions, after food enters the stomach, the entrance to it immediately closes, and therefore the contents of the stomach cannot enter the esophagus. However, with some diseases of the digestive tract, the entrance to the stomach during the period of digestion of food may periodically open, and in such cases, the acidic contents of the stomach are thrown back into the esophagus. It causes a feeling of heartburn. More often this phenomenon is observed in chronic gastritis, but it also happens with neurosis.

The stomach of an adult is located directly below the diaphragm and has an average of 15-18 cm in length and 12-14 cm in diameter. The maximum volume of the cavity of a healthy stomach is about 3 liters, with an empty stomach it is reduced to 50 ml.

Gastric juice is the second “reagent” poured onto the food mass. About 2–2.5 liters of gastric juice, a colorless liquid without any odor, is released per day in a person eating ordinary mixed food. The glands of the stomach differ from other digestive glands in their unique ability to produce hydrochloric acid, which is the main component of gastric juice. It performs numerous functions: the acid reaction of the gastric juice causes the swelling of proteins, promotes curdling of milk. In addition to hydrochloric acid, numerous organic and inorganic components (acidic, phosphates, carbonic acid, chlorides, calcium, sodium, magnesium ions) are in the gastric juice.

Due to the high concentration of hydrochloric acid, as well as special substances produced by the gastric mucosa, gastric juice has the ability to destroy pathogenic microbes that have entered the stomach, or to inhibit their growth.

Digestion of the proteins occurs in the stomach using enzymes – pepsin and gastriksina , cleaving proteins into simpler compounds.

Gastric juice also contains a small amount of enzymes that promote the digestion of fats. In addition to hydrochloric acid and enzymes, gastric cells produce a special mucus called mucin, which is very important for the normal functioning of the stomach. Mucin performs an important protective role – it is formed of a dual layer mucosal barrier lining the conductive inner surface zheludka.Etot barrier has high tackiness, viscosity, and prevents direct contact of gastric contents from its shell. The mucous barrier inhibits the action of pepsin and neutralizes hydrochloric acid. All this protects the gastric mucosa from self-digestion , as well as its mechanical and chemical damage.

Natural pathogens of the gastric glands are nutrients .

When a person eats, a stable stereotype of a secretory reaction is developed. Therefore, a sharp change in the diet, as well as improper nutrition (erratic eating, overeating, hasty food, poor chewing of food, abuse of alcohol, nicotine, drugs, etc.) can lead to the development of pathological conditions of the stomach, initially functional (in the form excessive increase or decrease in secretion and changes in its composition), then organic, manifested by various changes in the gastric mucosa up to the development of ulcers. 

The composition and quality of the gastric juice reflect the usual type of human nutrition .

So, when eating predominantly plant foods, the gastric juice acquires a lower acidity than with mixed nutrition. On the contrary, mainly a meat diet leads to increased gastric secretion and a significant increase in the acidity of gastric juice.

IP Pavlov in experiments on dogs studied the nature of gastric secretion caused by various nutrients. He found that gastric juice begins to be produced even before food enters the mouth. This is the so-called mouth-watering or “firing” juice; the entry into the cavity of the stomach of this juice is caused by the look and smell of food, as well as the sounds associated with its preparations. In this case, nerve impulses coming from the eyes, nose, and ears play the role of a trigger for the secretion of gastric juice, preparing the stomach in advance for digesting food.

According to I.P. Pavlov, a strong appetite always means an abundant separation of digestive juices from the very beginning of the meal. “Appetite is juice,” he wrote in “Lectures on the physiology of digestion.” “There is no appetite, there is no such initial juice; to restore a person’s appetite means to give him a large portion of good juice at the beginning of the meal. ” When the food mass enters the stomach, it further enhances the formation of gastric juice.

The most effective pathogen for juice removal is protein foods of animal and vegetable origin.

Protein digestion processes occur mainly in the zone of contact of the stomach wall with food, i.e., in the surface layers of the food lump. These layers, as they are digested and liquefied, move to the exit from the stomach, where they undergo thorough mixing and final grinding before leaving the duodenum. Regular servings of food entering the stomach are arranged in layers in the form of funnels or cones, nested one in the other. Thus, previously eaten portions of food are located closer to the walls of the stomach, so they are digested in the first place and quickly pass into the duodenum.

Since the food does not mix in the center of the stomach, a neutral or even slightly alkaline reaction persists for quite a long time inside the food lump. This creates the conditions for the continued digestion of carbohydrates, which began in the oral cavity.

In those cases when only carbohydrates are consumed during meals, due to the layered arrangement of food portions, half of them are digested already in the stomach. The intake of carbohydrates together with proteins further improves the absorption of carbohydrates, since proteins partially neutralize the hydrochloric acid of gastric juice, which contributes to a more complete digestion of carbohydrates in the stomach. At the same time, during the first two hours after eating in the stomach, only 10% of the protein from its original amount is digested. This is due to the fact that protein digestion occurs only in a thin surface layer of the food lump, while carbohydrate digestion continues in almost its entire volume.

The secretion of the gastric glands is also well adapted to the quantity and consistency of nutrients. As the amount of food entering the stomach increases, gastric secretion increases. However, this is observed only to a certain limit, beyond which a further increase in the volume of food no longer affects the amount of juice, since the maximum secretory capacity of the stomach is reached. In such cases, food lingers in the stomach, part of it, which has not had time to digest, begins to decompose.

The total amount of gastric juice, its acidity and other indicators depend on the degree of grinding of food _; The better the food is chewed, the sooner it is digested in the stomach and the faster it passes into the duodenum, thereby freeing the stomach from doing unnecessary work. Liquid food under normal conditions almost does not linger in the stomach, but immediately enters the duodenum. Evacuation of solid food from the stomach depends on the speed of its dilution under the influence of gastric juice and on the speed of its digestion.

The residence time of food in the stomach is of great importance for the subsequent absorption of nutrients in the small intestine, since the stomach is a kind of reservoir in which food gruel is diluted to the required consistency. The stomach protects the small intestine from an excessive flow of substances that can disrupt its normal activity and change the composition of the blood. In addition, the stomach regulates the flow of water into the small intestine, preventing the dilution of blood due to the excessively fast absorption of water in the intestine.

Periodic movements (familiar to everyone by the feeling of hunger) are characteristic of an empty stomach. During eating, they stop, the muscles of the stomach relax. The movements of the stomach during digestion depend on the type of food. Rough foods (rye bread, meat) cause stronger and longer stomach contractions than crushed ones (semi-liquid cereals, minced meat). The first phase of motility lasts from 20 minutes to 1.5-2 hours. In the second phase, contractions of the stomach become more rare. Due to peristaltic contraction of the muscles of the stomach, mechanical processing and displacement of the surface layers of the crushed and chemically processed contents of the stomach to the entrance to the duodenum occur.

Food leaves the stomach after 3.5-4.5 hours, so that with 3-4 meals a person’s stomach is almost or completely empty at the time of the next meal.

After exiting the stomach, the food slurry is exposed to the action of enzymes of the pancreatic juice , bile and intestinal juice produced by the glands of the duodenum and small intestine. Pancreatic digestive juice is rich in enzymes that help digest proteins, fats, and carbohydrates. The pancreas begins to function 1-3 minutes after the start of the meal. Unlike gastric secretion, the largest amount of pancreatic juice is secreted when taking bread, and slightly less – meat. The pancreas, like the stomach, reacts to milk with minimal juice secretion .

The enzymatic composition of pancreatic juice ( pancreas – the Latin name for the pancreas) “harmoniously” (as expressed by I. P. Pavlov) with the quantity and quality of nutrients entering the small intestine. Special studies, in which subjects received food with a high content of fats, or proteins, or carbohydrates for 1-3 weeks , showed that the concentration and ratio of enzymes in the juice of the pancreas change according to the predominant nutrient in the diet. The active causative agents of pancreatic secretion are diluted vegetable juices, broths, various organic acids (citric, malic, acetic). 

Activities of the pancreas is not limited to the elaboration of the components of the digestive juice. Its functions are much wider. Various hormones are formed in it, including insulin, which regulates the concentration of sugar in the blood. The secretion of the pancreas is influenced by the hormones of the pituitary gland, thyroid gland, adrenal glands and the cerebral cortex. So, in a person who is in an excited state, a decrease in the enzymatic activity of the pancreatic juice is observed, and at rest, its increase.

In some diseases of the gastrointestinal tract, as well as when the diet is overloaded with fats, “artistic harmony” disappears: the ability of the pancreas to secrete juice corresponding to the nutrients entering the small intestine is impaired. The lack of protein in the diet has the same effect.

The liver has a very special position among all organs of the digestive system. All the blood flowing from the stomach, spleen, pancreas, small and large intestines flows to the liver through the portal vein (one of the largest). Thus, all digestion products from the stomach and intestines enter primarily the liver, the body’s main chemical laboratory, where they undergo complex processing, and then pass through the hepatic vein to the inferior vena cava.

In the liver there is a neutralization ( detoxification ) of toxic products of protein breakdown and many medicinal compounds, as well as waste products of microbes that live in the colon.

The product of the secretory activity of the liver – bile – is actively involved in the digestion process. Bile contains bile, fatty acids, cholesterol, pigments, water and various minerals. Bile enters the duodenum 5-10 minutes after eating. Bile secretion continues for several hours and stops with the last portion of food leaving the stomach. The diet affects the quantity and quality of bile: it is formed most of all with mixed nutrition, and the most powerful physiological pathogens for the release of bile into the duodenum are yolks, milk, meat, fats and bread.

“The main role of bile is to replace gastric digestion with intestinal digestion, destroying the action of pepsin as an agent dangerous for pancreatic juice enzymes and extremely favorable for pancreatic juice enzymes, especially fatty”; (I.P. Pavlov).

Bile plays an important role in the absorption of carotene, vitamins D, E, K and amino acids. It increases tone and enhances intestinal motility, mainly the duodenum and colon, has an inhibitory effect on the intestinal microbial flora, preventing the development of putrefactive processes.

The liver is involved in almost all types of metabolism: protein, fat, carbohydrate, pigment, water. Its participation in protein metabolism is expressed in the synthesis of albumin (blood protein) and maintaining its constant amount in the blood, as well as in the synthesis of protein factors of the blood coagulation and anticoagulation systems ( fibrinogen, prothrombin, heparin , etc.). Urea, the end product of protein metabolism, is formed in the liver, followed by its excretion from the body by the kidneys.

Cholesterol and some hormones are formed in the liver . Excess cholesterol is excreted mainly with bile. In addition, complex compounds consisting of phosphorus and fat-like substances – phospholipids – are synthesized in the liver . Subsequently, they are included in the composition of nerve fibers and neurons. The liver is the main site of glycogen (animal starch) formation and the site of accumulation of its reserves. Usually liver contains ² / total amount of glycogen ( ¹ / h is contained in the muscle). Together with the pancreas, the liver supports and regulates the concentration of glucose in the blood.

From the stomach, food passes into the duodenum, which is the initial section of the small intestine (its total length is about 7 m).

The duodenum, together with the pancreas and liver, is the central node of the secretory, motor and evacuation activities of the digestive system. In the cavity of the duodenum, the main processes of digestion of proteins, fats and carbohydrates continue. It absorbs almost all products obtained as a result of the breakdown of nutrients, as well as vitamins, most of the water and salts.

In the small intestine , the final breakdown of nutrients occurs. Food gruel is processed under the influence of pancreatic juice and bile, impregnating it in the duodenum, as well as under the influence of numerous enzymes produced by the small intestine glands. The absorption process occurs on a very large surface, since the mucous membrane of the small intestine forms many folds. Furthermore, it is densely dotted villi – peculiar fingerlike protrusions (very large number of villi: the adult it reaches 4 m). In addition, on the epithelial cells of the mucous membrane there are microvilli. All this increases the absorption surface of the small intestine hundreds of times.

From the small intestine, nutrients pass into the portal vein blood and enter the liver, where they are processed and rendered harmless, after which some of them are carried with blood flow throughout the body, penetrate through the walls of the capillaries into the intercellular spaces and further into the cells. Another part (e.g. glycogen) is deposited in the liver.

In the colon , absorption of water is completed and feces are formed . The juice of the colon is characterized by the presence of mucus, in its dense part some enzymes ( alkaline phosphatase, lipase, amylase , etc.) are contained .

The large intestine is a place of abundant reproduction of microorganisms. 1 g of feces contains several billion microbial cells. Intestinal microflora is involved in the final decomposition of the components of digestive juices and undigested food residues, synthesizes enzymes, vitamins (groups B and vitamin K), as well as other physiologically active substances that are absorbed in the colon. The intestinal microflora creates an immunological barrier against pathogenic microbes. So, animals grown in sterile conditions without germs in the intestines are much more sensitive to infection than animals grown under ordinary conditions (intestinal microflora contributes to the development of immunity).

In a healthy intestine, microbes perform another protective function: they have a pronounced antagonism towards “alien” bacteria, including pathogenic bacteria, and thereby protect the host organism from their introduction and reproduction.

The protective functions of normal intestinal microflora to a large extent suffer from the introduction of antibacterial drugs into the gastrointestinal tract. Suppression of normal microflora with antibiotics can cause abundant growth in the colon of yeast-like fungi. Too long-term use of antibiotics often causes severe complications caused by rapid proliferation of antibiotic-resistant forms of staphylococcus and Escherichia coli, which are no longer held back by competing microorganisms.

Intestinal microflora decomposes an excess of pancreatic juice enzymes ( trypsin and amylase ) and bile , contributes to the breakdown of cholesterol.

In humans, about 4 kg of food masses passes from the small intestine to the colon per day.

In the cecum, the gruel continues to be digested. Here, with the help of enzymes produced by microbes, fiber is broken down and water is absorbed, after which the food masses gradually turn into feces. This is facilitated by the movement of the colon, mixing the food gruel and favoring the absorption of water. An average of 150-250 g of formed feces is produced per day, approximately one third of them are bacteria.

The intestinal liberation from feces is ensured by active motility, which occurs when feces irritate the intestinal wall receptors.

When eating foods that contain enough fiber, its coarse undigested fibers irritate the nerve endings in the muscles of the small and especially large intestines and thereby cause peristaltic movements that accelerate the movement of food gruel. A lack of fiber makes it difficult to empty the intestines, since weak motility, and even less its absence, causes a long delay in the intestines of food residues, which can cause various diseases of the digestive system (for example, impaired gallbladder function, hemorrhoids ).

In chronic constipation, feces are greatly dehydrated, since in the colon there is excessive absorption of water, which under normal conditions must be removed with feces. In addition, too long stools in the colon (ie, chronic constipation ) breaks the intestinal “barrier”, and the intestinal walls begin to pass into the blood not only water with small molecules of nutrients, but also large molecules of rotting products that are harmful to the body and fermentation – self-poisoning of the body occurs.