(2) Esophagus, also known as esophagus, is a long and straight muscle tube. Food is transported from pharynx to stomach by gravity and contraction of esophageal muscles. The esophagus is about 25 cm long and has three narrow places. It takes about 7 seconds for food to pass through the esophagus.
(3) The digestive stomach in the stomach is the digestive organ with the strongest swelling ability, and it has three parts: the L-shaped part protruding to the left is called the fundus of the stomach; The middle part is called stomach body; The contraction before the entrance of small intestine is called pylorus, and the entrance of esophagus is called cardia. The stomach secretes about 1.5 ~ 2.5L gastric juice every day, which mainly contains three components, namely pepsinogen, hydrochloric acid (gastric acid) and mucus. Among them, cells in the fundus of the stomach secrete hydrochloric acid, and gastrin cells in the stomach secrete pepsinogen. When pepsinogen is in acidic environment (pH 1.6), it can hydrolyze a part of protein. In addition, the stomach secretes chymosin, which can coagulate protein in milk and is very important for infant nutrition. If adults do not eat milk and its products for a long time, chymosin will be lacking in gastric juice secretion. The speed of food passing through the stomach mainly depends on the nutritional components of the diet. Carbohydrates pass through the stomach faster than protein and fat, while fat is the slowest. Water can reach the small intestine directly through the stomach and hardly stays in the stomach. The speed of food passing through the stomach is different.
(4) The small intestine digested in the intestine is connected with the pyloric end of the stomach and is about 5.5m long. It is divided into duodenum, jejunum and ileum. It is the main place for food digestion and absorption. In normal people, 90% ~ 95% of nutrients are absorbed in the upper part of small intestine. Intestinal mucosa has annular folds and a large number of villi, and surface cells have a large number of microvilli, which constitute a huge absorption area (200. Keep food for a long time. These microvilli form a rough interface and contain high concentration of digestive enzymes. The constant movement of the small intestine can mix food with secretions and make the villi of the small intestine absorb nutrients.
(5) Pancreas The pancreas is a lobular gland located in the small intestine and duodenum. The digestive juice secreted by the pancreas is alkaline and directly enters the small intestine through the pancreatic duct. Pancreatic juice is rich in bicarbonate, which can neutralize the highly acidic chyme produced in the stomach. Enzymes secreted by pancreas include proteolytic enzyme, lipase, amylolytic enzyme, nucleic acid hydrolase and some chemical buffers. Pancreatic amylolytic enzymes can decompose starch into maltose. In the case of maltase, trypsin, chymotrypsin and carboxypeptidase can digest protein into peptone, peptide and amino acid. Pancreatic lipase digests fat into fatty acids and glycerol.
(6) Liver and gallbladder liver include liver, gallbladder and bile duct. One of the main digestive functions of the liver is to secrete bile, which can dissolve and absorb dietary fat and help excrete some wastes, such as cholesterol and hemoglobin degradation products. The function of liver digestion and absorption is also manifested in storing and releasing glucose, storing vitamin A, vitamin D, vitamin E, vitamin K and vitamin B 1, etc. In addition, the liver has many physiological functions, including detoxification of harmful compounds, metabolism of productive nutrients, formation of plasma proteins, formation of urea, passivation of polypeptide hormones and so on.
(7) The colon and rectum are about 1.5m long and are divided into three parts: cecum, colon and rectum. It takes 30 ~ 90 min for food to travel from stomach to the end of small intestine. But it takes 65,438+0 ~ 7 days to pass through the large intestine. The large intestine contains a large number of bacteria, mainly Escherichia coli. These bacteria can affect the color and smell of feces. Food that does not react during digestion can be changed and digested by bacteria. In this way, some complex polysaccharides and a small amount of simple carbohydrates, such as lignin (four-carbon sugar) or raffinose (three-carbon sugar), are converted into hydrogen, carbon dioxide and short-chain fatty acids. Protein residue that can't be digested.
3. Absorption of nutrients When food is digested, macromolecules are transformed into micromolecules, in which polysaccharides are decomposed into monosaccharides, protein is decomposed into amino acids, and fats are decomposed into fatty acids and glycerol. Vitamins and minerals are released from the cells of food during digestion and enter the blood circulation through the digestive tract wall. These processes are called absorption. The way of absorption depends on the chemical properties of nutrients. Food is not absorbed before it enters the stomach. The stomach can only absorb a small amount of water and alcohol, while the large intestine mainly absorbs water and inorganic salts that are not fully absorbed by the small intestine, while the absorption of nutrients is mainly carried out in the small intestine. When nutrients are digested and absorbed, they are immediately transported to tissues that need or store them. Lymph and blood are the main transport media of nutrients. There is a lymphatic capillary network on the membrane of the intestine, and cholesterol, water, long-chain fat and some protein are finally transported from the lymphatic system to the venous system. Most low molecular weight nutrients are absorbed into the blood circulation.
(1) protein's absorption protein is decomposed into amino acids in the digestive tract, absorbed in the intestinal mucosa, and then enters the blood circulation through the capillaries in the villi of the small intestine, which is an active transport process. After natural protein is hydrolyzed by protease, its hydrolysis products are about 1/3 amino acids and 2/3 oligopeptides. The absorption rate of these products in the intestinal wall is much faster than that of pure mixed amino acids, and most of them enter the portal vein in the form of amino acids after absorption.
(2) Absorption of fat Fat is decomposed into glycerol and fatty acids through the digestive tract. Glycerol is easily soluble in water and can be directly absorbed into the blood; Fatty acids need to combine with bile salts in the digestive tract to form water-soluble complexes before they can be absorbed. After fatty acids are absorbed, a small part of them enter the villous capillaries of small intestine, enter the liver from portal vein, and most of them enter the lymphatic capillaries, and enter the blood circulation through large lymphatic vessels. Fat-soluble vitamins are also absorbed by fatty acids.
(III) Absorption of carbohydrates Carbohydrates are digested and decomposed into monosaccharides (mainly glucose and a small amount of fructose and galactose), which are absorbed through active transport, and then enter the liver through the portal vein, some of which are synthesized and stored in the liver, and the other part enters the human circulation through the hepatic vein for systemic use.
(4) Absorption of water, water-soluble vitamins and inorganic salts Water-soluble vitamins and inorganic salts can be absorbed directly in the small intestine without digestion. Water is absorbed in the intestine by osmotic pressure principle; Water-soluble vitamins are absorbed by diffusion. Among inorganic salts, sodium salt is absorbed by sodium pump, and negative ions such as chloride ion and bicarbonate ion are absorbed by potential difference. Fourth, biotransformation The liver is the main organ of biotransformation. The chemical transformation process of nutrients and non-nutrients in human liver and other tissues is called biotransformation. Small molecular active substances or poisons produced by substance metabolism in the body, as well as foreign substances such as various drugs, poisons and food additives. After entering the human body, its structure and properties can be changed by biotransformation in vivo, and then excreted through the liver or kidney. Many factors will affect the biotransformation reaction. Individual differences, ethnic factors and malnutrition (protein, phospholipids, vitamin A, vitamin C, vitamin E, etc. ) will affect biotransformation; The biological transformation ability of newborns is poor, and the transformation ability of the elderly tends to decline; Androgen and islet hormone in vivo can promote biotransformation in vivo; Severe liver disease can affect transformation. 5. After the excreted and ingested food is repeatedly absorbed by various digestive tracts, the residue, salt and a small amount of residual nutrients in the food finally enter the rectum. When feces containing a large number of intestinal microorganisms, gastrointestinal exfoliated cells and food residues enter the rectum, it will stimulate the intestinal wall and cause defecation reaction.