Digestion

Digestion – a combination of physical and chemical processes in the conversion of food substances which may be absorbed by the cells of the body and used for energy purposes or plastic.
Throughout his life a person must consume foods. As a result of the conversion of nutrients the body receives the material is used in the plastic and energy metabolism. During plastic exchange nutrients used in cell division and growth of the body, and in the processes of oxidation of energy metabolism of nutrients is the source of all energy needed for normal functioning of the body.

Food components

The food composition should include proteins, fats, carbohydrates, vitamins, mineral salts and water.
Proteins are polymers of amino acids 20-25. In a normal person gets their power mainly from animal food. However, when a vegetarian diet can pick up a number of vegetable protein containing all the essential amino acids for humans. Fats are glyceryl esters of fatty acids. They enter the body both with animals and with plant food. Carbohydrates such as polysaccharides (e. g., starch) and monosaccharides (e. g., glucose and fructose), enter the body mainly with plant food. A number of carbohydrates enters the body by eating animal liver (glycogen).

On the other components of food – vitamins, mineral salts and water.
The functions and structure of the digestive system
The main functions of the digestive system:

    1) mechanical and chemical processing of food; 2) the absorption of nutrients into the body (blood, lymph); 3) excretion of undigested food residue and nevsosavsheysya.

In part of the digestive system consists of digestive tract, and digestive gland. Human digestive tract having a length of 8-10 meters, and consists of the following departments: the mouth, pharynx, esophagus, stomach, small intestine, fat intestine. The wall of the digestive tract consists of three layers: the outer (connective tissue), middle (muscle), internal (epithelial). The muscular layer of the oral cavity, pharynx and upper esophagus is composed of striated muscle, and the muscle layer underlying smooth muscle sections presented.

Digestion in the mouth

The first section of the digestive system, which enters the food – it’s mouth. It is limited to the top palate, sides – the cheeks, below – mylohyoid muscle. In the mouth are teeth and tongue. It ducts open three pairs of large and many small salivary glands. The teeth are arranged in the cells of the upper and lower jaws. In adults, 32 teeth: in each jaw 4 incisor tusk 2, 4 and 6 small molars. The basis of the tooth dentin – a kind of bone tissue. Within the dentin have a cavity filled with pulp – connective tissue. In the pulp enter the blood vessels that feed the tooth tissue, and nerves. Outside the tooth is covered with enamel – the hardest substance in the body.
Language is formed by striated muscle and covered with a mucous membrane. He is involved in stirring the food during chewing and swallowing helps. Furthermore, in the mucosa of tongue are numerous taste receptors necessary for determining the taste of food.

Saliva is released into the oral cavity parotid, submandibular and sublingual glands pair, as well as thousands of small salivary glands scattered throughout the oral mucosa. All glands secrete per day about 1 liter of saliva, but this volume is heavily dependent on the quantity and nature of food eaten. In 98-99% of saliva is water and has a slightly alkaline reaction. Saliva contains enzymes: amylase and maltase. The amylase cleaves carbohydrate polymer – Starch – to disaccharides (maltose) and maltase continues maltose cleavage to two monosaccharide molecules – glucose. Also, included in the saliva protein substance – mucin, which makes the bolus slippery. Saliva also contains antibacterial agent lysozyme, partially disinfecting food.
For those 10-20 seconds, during which time the food is in the mouth, it crushed, soaked with saliva, the stomach content and starts the digestion of carbohydrates.

Chewed food by means of movements of the cheeks and tongue moves to its roots. When mechanical stimulation bolus receptors of the tongue signals from these receptors come in the swallowing center of the medulla oblongata and excite its neurons. As the nerve fibers coming from these neurons, the excitement flows to the muscles of the mouth, pharynx, larynx. Muscles contract, causing the soft palate rises and closes the way to the nasal cavity and closes the epiglottic cartilage food the way to the larynx. The muscles of the mouth, pharynx, and larynx are reduced, and the lump of food passes into the throat and into the esophagus. Thus, the swallowing reflex is a complex act that requires the coordinated participation of many muscles.
Throat – a muscular tube in the oral part that crisscross the food and the respiratory tract. Hoany connects the throat to the nose, Eustachian tubes connecting the throat to the middle ear. The hypopharynx constricted throat portion enters the esophagus.

Esophagus – a muscular tube about 30 cm long, the upper third of which is formed by striated muscle, and the remaining two-thirds – smooth muscle. With undulating muscle contractions of the esophagus bolus of food goes into the stomach.

Digestion in the stomach

Stomach – expansion of the digestive tube of about 2 liters. Stomach walls are composed of a connective tissue sheath, inner muscular layer and the mucosa. The mucosa is up to 14 million small glands producing gastric juice (about 2 liters per day). The glands distinguish the main cells that produce pepsinogen, parietal cells that produce 0.5% hydrochloric acid, and the additional cells that produce mucous substance that protects the stomach wall from damage. Under the action of hydrochloric acid formed from pepsinogen enzyme – pepsin, decomposing food proteins into peptides. Furthermore, in the gastric juice include such enzymes as lipase milk decomposing milk fats to glycerol and fatty acids, and gelatinase, cleaving gelatin.
The walls of the stomach slowly shrinking, slowly mixing food with gastric juice. The resulting mass called chyme. Since the mixing of food is pretty slow, in the center of the bolus environment remains slightly alkaline saliva and enzymes for a long time (over an hour) continue the digestion of carbohydrates. They cease when the pH in the stomach content is below 5.0. Depending on the composition and amount of food eaten her stay in the stomach lasts for 3 to 10 hours. After enzyme treatment portions gastric chyme passes from the stomach into the duodenum through an opening closed by a special annular muscles – sphincter.

Digestion in the small intestine

The small intestine has a length of about 5-6 meters and divided into duodenum, jejunum and ileum. The walls of the intestine is constantly shrinking, moving mass along the food through the intestines, and mixing them. The most important process of digestion occurs in the duodenum, having a length of about 30 cm. Here in the chyme enzymes affect the juice of the intestinal wall, pancreatic enzymes and bile. Wednesday in the duodenum has a slightly alkaline reaction of pH = 8.0-8.5. By ductless pancreatic digestive enzymes located in the intestinal lumen as precursors, i. e. inactive and is only converted to the active form. Trypsinogen under the influence of the enzyme intestinal glands – enterokinase – is transformed into trypsin – peptidase, continuing the degradation of proteins, which began in the stomach. Under the influence of his major break polypeptides into smaller oligopeptides, and those in turn – to the individual amino acids. The pancreatic juice also includes lipase – an enzyme that breaks down fats into glycerol and fatty acids. For activation of lipase require the presence of bile, which is produced by the liver. Amylase, maltase and lactase contained in pancreatic juice, break down carbohydrates. Nuclease cleaves DNA and RNA to single nucleotides.

An important role in digestion in the small intestine belongs liver. The liver cells – hepatocytes – continuously form bile consisting of bile acids and bile pigments (bilirubin and biliverdin). Because the liver bile enters the gallbladder, where it is the accumulation. As required sphincter closing the common bile duct, opens and enters the bile into the intestinal lumen. In the duodenum bile promotes fat mechanically crushed into fine droplets (emulsified fats), thereby repeatedly increases the contact surface fat with an enzyme lipase. In addition, bile activates digestive enzymes, especially lipase enhances secretion of pancreatic enzymes and the intestinal wall, participates in the absorption of fat breakdown products.

The walls of the small intestine contains a huge number of small glands that produce enzymes that complete digestion of nutrients. The structure of these enzymes include: multiple peptidase disrupting peptides to individual amino acids; amylase, maltase, lactase, invertase – finishing the digestion of sugars; special kind of lipases and some other enzymes.
Part of digestive enzymes acts directly in the intestinal lumen, and this type is called cavitary digestion. However, there is also a membrane digestion. The fact that the mucous membrane of the small intestine formed special cells – enterocytes. The surface of enterocytes, facing the lumen of the intestine is covered with a huge number of small villi (up to 2000 per 1 cm2). The outer membrane of intestinal villi of enterocytes, in turn, covered with the finest microvilli, forming a “brush border”. These microvilli adsorbed on a huge amount of digestive enzymes. When food intake their molecules pass through the brush border, where they are finally digestion. This process is called membrane digestion.

The vast majority of nutrients absorbed through the small intestine, namely, jejunum and ileum. The epithelium of the intestine villi forms, which have the branching of blood capillaries and lymph begin blindly closed capillaries. The total surface of the villi in the intestine is 200 m2. Thus, the process of absorption of digested nutrients is the passage of the enterocyte membranes and getting into the blood stream or lymph. Absorption of nutrients may proiskhodt passive or active means. Passively, by diffusion or osmosis, absorbed water, chlorides, some vitamins (B2, B6). Active, i. e. the costs of energy absorbed amino acids, sugars, calcium ions and others. The amino acids and dextrose are absorbed directly into the blood entering the blood capillaries in the villi. Glycerol is soluble in water and is also well absorbed. But fatty acids interacting with alkalis form soaps, which under the action of bile acids are also readily soluble. Products – glitserin digestion of dietary fat and fatty acids – are the walls of the villi, the newly formed fat, but already inherent in man. These fats are absorbed in the lymphatic capillaries. The water and salts dissolved therein may be absorbed throughout the gastrointestinal tract: the oral cavity, stomach, small and large intestine, but the most intensive in their absorption in the small intestine.
The absorbed through the intestinal wall dissolved in water digesting food products primarily to the liver, where they are detoxification. The function of the liver is called a barrier. If present in excess of food and its glucose contents in blood plasma exceeds 0.11%, the excess is deposited in the liver as a glucose polymer – glycogen.

Digestion in the large intestine

Because of the small intestine undigested food mass and nevsosavsheysya pass into the large intestine. The colon has a length of about 2 meters and includes three divisions: the blind, colon and rectum. At the boundary between the small and large intestine is a special valve, transmissive portions of food mass and only in one direction. Glands walls of the large intestine does not produce enzymes, but secrete mucus necessary for the formation of feces.

The contents of the colon is a huge amount of bacteria that cause decay glucose polymer – fiber and undigested protein molecules. Fermentation and decay processes occurring in the colon must be strictly balanced, otherwise develop the disease of the gastrointestinal tract. Bacteria living in the gut secrete a number of essential human vitamins: K, E, Group B. During fermentation of fiber is formed glucose, which can be absorbed into the blood, and by the decay of protein products formed a whole set of toxic substances, phenol, indole, skatole. All these substances are neutralized by passing the blood through the liver. Food residues from which water is absorbed, converted into faeces and removed from the body.

Regulation of digestion

Regulation of digestion is carried out and nervous and humoral mechanisms. Classic studies of digestion and its regulation have been carried out Pavlov and his staff. It is for this research, Pavlov was awarded the Nobel Prize. He has developed a number of experimental surgical techniques – implantation of fistulas in different portions of the gastrointestinal tract, transection, the formation of a “small ventricle” and so on. D. Operations Pavlov carried out in such a way as to preserve the blood supply and innervation of the gastrointestinal department investigated tract. Experiments were thus chronic animals had time to recover after surgery and were apparently healthy. Pavlovskaya school and subsequent researchers have established the principles of the regulation of digestion and absorption.

The sight and smell of food leads to the isolation of certain amounts of saliva, this process is enhanced by ingestion of food in the mouth. Once in the mouth, food and mechanical irritating taste buds, and the excitement of them goes to the center of the medulla oblongata salivation. From this central neurons the nerve impulses are transmitted to the sympathetic and parasympathetic neurons of the autonomic nervous system centers innervating the salivary gland. The pulses arriving on sympathetic fibers inhibit the secretion of saliva and parasympathetic impulses enhances its selection.
Gastric juice happens all the time, but it has intensified in the form of food. Immediately after ingestion of oral irritation receptors invokes the reflex of gastric juice. The effects of coming to the stomach through the main parasympathetic nerve – the vagus, stimulate the secretion of hydrochloric acid, parietal cells of the stomach wall. These same effects led to the isolation of the cells of the intestinal mucosa gastrin peptide which is the bloodstream, t. E. Humoral comes to the walls of the stomach, also stimulating the release of components of gastric juice.

When partially digested food portions starts flowing from the stomach into the duodenum, gastric secretion, even at first increases and then gradually decelerated. This is due to the fact that under the influence of chyme produced in the walls of the intestine and enters the blood gastrin antagonist – secretin. In order to hit chyme into the intestine, is necessary to the disclosure of the sphincter at the edge of the stomach and duodenum. This occurs during stimulation of receptors chyme having acidic. When a portion of the chyme enters the duodenum and alkaline reaction it temporarily changes from acidic to alkaline, reflex sphincter closes.

When you receive a food mass into the duodenum reflex intensifies pancreatic juice and bile into the intestinal lumen. The composition of pancreatic juice varies depending on the quantity and composition of food. Further, the mass of food, mixing with the help of pendulum movements circular and longitudinal muscles of the intestinal wall, are moving through the small intestine by peristaltic movements, which provide a reduction of ring muscles. The regulation of bowel movements is mainly due to the influence coming of the vagus nerve – the vagus. However, the intestinal wall have a certain automaton if vagus cut, the peristaltic movement persist.


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Digestion