Digestion in the mouth
Chewing food, a person moves it in the mouth with the help of the tongue (with the help of the receptors of which we feel the taste, mechanical properties and temperature of food). The oral cavity contains the teeth necessary for mechanical grinding of food during the chewing process. The more thoroughly the food is chopped in the mouth, the better it is prepared for processing with digestive enzymes.
In the mouth, food is moistened with saliva, which is secreted by the salivary glands. Saliva is 98–99% water.
- enzymes that break down complex carbohydrates to simple carbohydrates (for example, the ptyalin enzyme breaks down starch to an intermediate product that another enzyme, maltase, converts to glucose).
- mucin – a mucous substance that sticks food particles together into a lump;
- lysozyme is a disinfecting substance that destroys bacteria that enter the oral cavity and promotes the healing of minor injuries.
Food chewed and moistened with saliva from the oral cavity enters the pharynx, is swallowed and ends up in the esophagus.
Food moves along the esophagus due to its peristalsis – wave-like contractions of the muscles of the esophageal wall.
The mucus produced by the glands in the esophagus makes it easier for food to move.
Digestion in the stomach
Proteins and some fats (such as milk fat) begin to be digested in the stomach.
In a lump of food, for some time, under the action of the enzymes of saliva, the breakdown of carbohydrates continues, but soon the food is saturated with gastric juice, and its enzymes begin to break down proteins.
An important feature and condition for effective digestion in the stomach is an acidic environment (since the enzymes of the gastric juice act on proteins only at body temperature and in an acidic environment).
Gastric juice is acidic. Hydrochloric acid, which is part of it, activates the enzyme of gastric juice – pepsin, causes swelling and denaturation (destruction) of proteins and promotes their subsequent breakdown into amino acids.
Stirring food with gastric juice is promoted by the slow contraction of the stomach walls.
In the stomach, food lingers for some time (3 – 10 hours). The contents of the stomach then enter the duodenum (the initial section of the small intestine).
Digestion in the small intestine
The digestion of food in the duodenum continues. Digestion occurs both in the intestinal cavity (cavity) and on cell membranes (parietal), which form a huge number of villi lining the small intestine.
In the duodenum, food is affected by:
- intestinal juice enzymes (formed by the glands of the intestinal walls),
- pancreatic juice enzymes (pancreatic juice);
- bile (excreted by the liver).
In the small intestine, the breakdown of nutrients continues into simple compounds (proteins – to amino acids, fats – to glycerol and fatty acids, carbohydrates – to glucose) and their absorption into the blood and lymph.
The surface of the small intestine is covered with villi, of which there are so many (2500 villi per 1 cm²) that the surface appears velvety. The villi increase the total absorptive surface (the total surface of the villi in the intestine reaches 200 m²).
The walls of the villi consist of a monolayer epithelium, and in the center of each villi there is a lymphatic capillary and blood capillaries. The products of fat processing enter the lymph, and amino acids and simple carbohydrates enter the blood.
A feature and condition for effective digestion in the intestine is a weakly alkaline environment.
The intestinal walls are constantly contracting, mixing food masses and moving them to the large intestine. A sphincter is located between the small and large intestines, which allows the movement of intestinal contents in portions and prevents it from returning to the small intestine.
The large intestine serves primarily to remove undigested food debris. The feces that enter it contain up to 70% water and undigested residues (mainly fiber).
In the large intestine, water and fiber breakdown products are absorbed. The symbiont bacteria that live in the human large intestine perform a number of functions – fermentation of cellulose, synthesis of vitamins K and B.
The movement of food residues in the large intestine takes about 12 hours. Water and nutrients dissolved in it are absorbed into the intestinal walls from the residues. The glands of the large intestine produce juice that does not contain enzymes, but contains mucus, which is necessary for the formation of feces. Feces accumulate in the rectum and are removed through the anus.