Sunday, March 6, 2016

HUMAN DIGESTIVE SYSTEM-PART-3 TEXT





Stomach as a muscular bag it is present on the left side in the abdominal cavity, below the diaphragm. Part of the stomach into which oesophagus opens is called cardiac  stomach.
Part of the stomach that opens into duodenum is called pyloric stomach. Opening of the pyloric stomach into duodenum is protected by pyloric sphincter.
Muscles in the walls of the stomach are involuntary muscles. These are arranged longitudinally, diagonally and circularly. These muscles, contract in different directions. As a result food is churned in the stomach.
Stomach has three important roles:
1. it stores the food temporarily.
2. mixing of various components in the food thoroughly – this occurs due to contraction  and relaxation of muscles.
3. it brings about phusical and chemical changes in the food.
Internally stomach wall is lined by mucous membrane. A number of glands called gastric glands are present in this membrane. Each gastric gland opens by a small pore into the lumen of stomach. Gastric glands secrete gastric juice and mucin.

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Gastric juice is a thick, clear and straw coloured fluid. Gastric juice contains hydrochloric acid and enzymes. The food gets mixed with hydrochloric acid present in the gastric juice.
Hydrochloric acid kills bacteria present in food. It also destroys the structure of proteins, so that enzymes can digest them easily.  Mucous membrane protects stomach wall from the action of acid present in the gastric juice.
Pepsin and lipase are the enzymes present in the gastric juice. When pepsin is secreted, it is inactive and is called as pepsinogen. Acids converts inactive pepsinogen to pepsin which is the active form of the enzyme.
Pepsin breaks down proteins into peptones and proteases. Lipase converts fats into fatty acids and glycerol. In children, another enzyme called rennin is secreted into the stomach. It curdling of milk. This enzyme disappears as the child grows.
Food is retained in the stomach for two to four hours and is partially digested in the stomach. As the food is undergoing changes in stomach, the pyloric sphincter closes the opening of stomach into duodenum.
The pyloric sphincter allows only small quantities of food into duodenum at a time. The food that enters the duodenum is called chyme. This is acidic and very soft.

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