Digestion is the process of physical and chemical processing in the gastrointestinal tract nutrients to elementary particles, suitable for suction and participate in the metabolism of the body. Such elementary particles are: to fat (see) - glycerol and fatty acids, proteins (see) - amino acids, carbohydrates (see) - monosaccharides. Digestion occurs in different sections of the gastro-intestinal tract with digestive juices and allocating large glands with the excretory ducts (the salivary glands, pancreas), and glandular cells of the mucous membranes of the oral cavity, stomach and intestines.
Motorized activities that ensure the promotion of food, best wetting her digestive juices, and so on, at the expense of reduction of circular and longitudinal smooth muscles, located in the wall of the digestive tract throughout its duration, starting from the upper third of the esophagus to the anus.
The oral cavity, the initial part of the esophagus and external sphincter anal holes have striated muscles, reduce and relax arbitrarily. The rest of secretory and motor activity of the digestive tract is not subject to arbitrary control and regulated by subcortical centers of the autonomic nervous system, intramural nerve plexus and humoral influences.
The body is constantly consumes nutrients, so you need constant replenishment. The meal is regulated by the food center, a view which was first put forward by I. P. Pavlov, who considered that the centre does not have a specific location, and located in different parts of the brain, from oblong to the cerebral cortex. In the future this viewpoint I. P. Pavlov received experimental confirmation. In particular, it was established that the center of hunger and saturation is located in the hypothalamus (see). In the lateral hypothalamus is located in the centre of hunger (its destruction leads to afakii - the rejection of food), and in the medial center of saturation (its destruction leads to giperfagia - insatiable hunger, and irritation to afakii); these centers are connected so that the decreased activity of the lateral hypothalamus depends on the excitation of media. Excitation of medial hypothalamus in turn depends on the presence in the body of nutrients. With the decrease of their content in the blood ("hungry" blood), liberation of the stomach and the shortage of glycogen in the liver lowers the tone of the medial hypothalamus, reduced its impact on the lateral, increased activity of the latter; at the same time the feeling of hunger and the activation of feeding behavior (see Motivation).
Digestive activity begins from the moment of receipt of food in the mouth. In this process actively involved not only the chewing muscles and salivary glands, but also the entire digestive system. In experiments I. P. Pavlova alleged feeding it has been proven that, when esophageal transection food in the stomach did not fall, stomach began to secrete after 6-8 minutes from the start of a meal, and before he began to secrete the pancreas. Received the oral cavity food is machined and moistened with saliva. Released saliva not only moisturizes the food and creates conditions for formation easily swallow the food bolus, but also helps digestion make up the food carbohydrates, as the person and omnivorous in the composition of saliva include enzymes amylase (ptyalin) and Maltese.
The saliva occurs as a result of contact stimulus to receptors mucous membranes of the oral cavity, and under the influence of conditional - distant - irritants. The stimulation of receptors mucous membrane extends to sljunootdelitelnoe centre oblong brain, and thence by the sympathetic and parasympathetic fibers to the salivary glands, causing them secretion. Under the influence of parasympathetic nerve (drum string) is the separation of liquid saliva, and sympathetic - small amounts of saliva with high content of organic substances (see Salivation). Saliva removes unwanted and harmful substances, by accident, in the oral cavity. Therefore, the saliva is separated not only food, but also to reject substance.
The quantity and quality of saliva depend on the quality of the stimulus, its chemical composition and humidity; for solids saliva significantly more than in the wet.
Enough spoon-fed and hydrated food is served in the form of a lump at the root of the tongue and in the result of swallowing movements, accompanied by the rise of the larynx and closing her epiglottis, translated from the mouth to the esophagus (see Swallowing). Here arose peristaltic wave it moves further and enters the stomach. Peristaltic wave occurs because of the pervasive process of excitation and reduce groups fibers circular and longitudinal muscles higher food clumps and braking muscle fibers located below it. The result is a food lump as if pressed from the esophagus to the stomach.
Food is delayed in the mouth no more than 10-15 sec.; therefore, the digestion of carbohydrates under the action of enzymes saliva occurs in the stomach and continues until caught in the stomach food supply does not become acidic gastric juice. Digestion in the stomach and intestines have been studied in detail, I. P. Pavlov, staff, and students in experiments with imaginary feeding, education isolated Pavlovsky ventricle, and with the imposition of various experimental fistula. Gastric juice (see) starts to separate after 6-8 minutes from the beginning of the meal in the reflex influences caused by the irritation of the receptors mucous membranes of the oral cavity and pharynx. These effects are passed on sensitive fibers through the nucleus of the vagus nerve, he efferent fibres to the main (secreting the enzyme pepsin) and tuck (secreting hydrochloric acid) cells, located on the small and the greater curvature of the stomach. The content of acids in the gastric juice does not exceed 0.5%.
In addition to the first phase of gastric secretion caused by the influence of the vagus nerve, there is a second phase, resulting from irritation of the mucous membrane of the pyloric portion of the stomach and income in the blood of germs of gastric secretion. These viruses can come from the outside or formed during the digestion of protein, but mainly they are formed in the mucosa of the pyloric portion of the stomach in the form of specific pathogen of gastric secretion of the hormone gastrin. By the end of digestion in the stomach, he secretion fades out as the result of education in the pyloric portion of the stomach is another hormone - gastrulation, which is absorbed into the blood and humoral inhibits gastric secretion (see Stomach, physiology). Secretory response of the stomach may occur when hitting him not only food, but also foreign substances. To reject substance stomach can respond antiperistaltic reductions that causes vomiting (see).

Motor activity of the stomach for the first time after a meal weakened, but as soaking the food mass gastric juice she grows that promotes mixing the contents of the stomach. If allowed to accumulate in the chyme products of digestion of protein motility of the stomach is even stronger, there peristaltic waves, in some cases ending with the opening of the pyloric sphincter and the transfer of a small mass of chyme from the stomach into the intestines. Of great importance in the progressive gastric emptying belongs to the influence of the pyloric sphincter hydrochloric acid. As it was found in the laboratory I. P. Pavlov, the pyloric sphincter opens at the irritation of hydrochloric acid on the surface from the side of the stomach and immediately closes when annoyed by it from the duodenum. The sphincter is closed up until caught with a portion of chyme acid is not neutralized by the alkaline intestinal juice, pancreatic juice and bile.
When eating in the stomach often have large volumes of water or water can be separated from food products, as, for example, occurs when stvorazhivanii milk. The presence of water may hinder the process of digestion, since it leads to a dilution of gastric juice. To prevent this, in the stomach, where the absorption is limited, there is, apparently, a special device in the form of a groove - water chute, located on small curvature, through which excess water is removed from the stomach into the intestines.
The chyme coming from the stomach portions in the intestines, is exposed here further digestion (see Intestinal physiology). Fragments of the protein in the form peptone are then digested under the influence of the enzyme pancreatic trypsin, activated by enterokinase intestinal juice (see), as well as enzyme erepsin intestinal juice and brought to amino acids. Carbohydrates, partially-digested under the influence of amylase and Maltese saliva, continue to digest under the influence of amylase, Maltese, lactase and other enzymes intestinal juice and pancreatic juice. Fats are digested in the duodenum (see) under the influence of the enzyme lipase intestinal juice and pancreatic juice with the participation of bile. The value of bile is that it emulgelooe fats, activates the lipase and helps dissolve fatty acids.
Now put forward the idea of the existence of a special, so-called wall, digestion in the small intestine. A. M. Ugolev that expressed this point of view, justifies its experiences with the hydrolysis of starch amylase in the presence of small pieces of cancer, pre-inactivated against amylase 10% solution of trichloroacetic acid. In these conditions, the hydrolysis increases significantly. On this basis the conclusion is drawn that the mucous membrane of intestine, the surface of which is due to the presence of microvilli great, can adsorb enzymes and thereby to influence the kinetics of enzymatic reactions.
The mechanism of secretion of the pancreas (see) close to that of the stomach. The first phase of secretion begins in 3-5 minutes after the start feeding occurs and, later, under the influence of the vagus nerve. The second phase is under the influence of humoral mechanism in education in the intestinal mucosa of the hormone secretin, which is formed from the inactive prosecretin under the influence coming from the stomach hydrochloric acid.
Bile (see) of the bile duct begins to stand out in the intestines and in 20 minutes after administration of milk, in 30-40 minutes after meat and 40-50 minutes after bread. Biliary excretion (see) is closely connected with the receipt of chyme from the stomach into the intestines. First bile comes from the gallbladder, and then it starts mixed with lighter hepatic bile. The flow of bile into the intestine, caused by the reflex influences from the surface of the colon, followed by the opening of the sphincter of the common bile duct and the reduction of the gall bladder.
In the small intestine ends digestion and absorption occurs simultaneously (see) nutrients. This is facilitated by the permanent pendulum-like bowel movements. As digestion and nutrient utilisation of waste, slag, substances move peristaltic movements of the small intestine to the side of the large intestine. Here is the final use of the rests of nutrients. Reabsorption of water and the formation of Cala completed in the rectum, filling the capsules which leads to the act of defecation.