Physiology

1. Digestive system Physiology For XII standard EXIT

2. Digestive system comprises the alimentary canal, associated glands and regions of absorption of food.

3. Food provides us with fuel to live, energy to work and play, and the raw materials to build new cells. All the different varieties of food we eat are broken down by our digestive system into simple components and transported to every part of our body by our circulatory system. Digestive system serves to transfer organic molecules, salts and water from the external environment to the body’s internal environment. Digestion is accomplished by the enzymes produced from the glands associated with the digestive system.

4. Digestion in Mouth In mouth digestion starts with chewing. Chewing breaks up large pieces of food into smaller particles that can be swallowed without choking. It is accompanied by teeth, tongue, jaws and saliva. Chewing is controlled by the somatic nerves to the skeletal muscles of mouth and tongue. Saliva is secreted by 3 pairs of salivary glands, namely Parotid gland Sub lingual gland Sub mandibular gland

5. Daily secretion of saliva ranges from 1000 ml to 1500 ml. Saliva contains the salivary amylase and Mucin. Salivary amylase or ptyalin is an enzyme which converts cooked starch into maltose. Mucin is a glycoprotein helps in the lubrication of food. The lubricated, swallowable form of food is called the bolus. Salivary secretion is controlled by reflex activities.

6. Swallowing Oesophageal phase begins with relaxation of upper oesophageal sphincter. In oesophagus the food is moved towards stomach by a progressive wave of muscle contraction proceed downward to the stomach. Such waves are known as Peristaltive waves. One peristaltive wave takes 9 seconds to reach the stomach. Due to peristaltive waves, swallowing can occureven when a person is upside down. It is a complex reflex process controlled by the medulla oblongata. During swallowing; the soft palate is elevated, larynx gets raised, tongue forces food back into the pharynx, the epiglottis closes the glottis and the food slowly passes into the osesophagus.

7. Digestion in Stomach During feeding the volume of stomach may increase up to 1.5 litre. Stomach’s contractile action will produce peristaltic waves proceed towards the pyloric region. In the wall of stomach there are 40 million glands producing the gastric juice. The parietal (Oxyntic) cells produce HCl to create the acidic medium for enzymes.

8. Digestion in Stomach Enzymes of stomach are pepsin and renin. Pepsin is secreted as inactive pepsinogen, which is converted as active pepsin by the Hydrochloric acid. Pepsin hydrolyses the proteins into short polypeptides chains and peptones. Renin acts on soluble milk protein caesinogen and converts it into insoluble caesin. In the presence of ca+ ions caesin is precipitated to insoluble calcium-caesin compound (curd).

9. Digestion in Stomach (Inactive) pepsinogen + HCl → Pepsin (Active) Proteins pepsin→ polypeptides + peptones. Caesiongen Renin → caesin. caesin + ca+→ calcium-caesin Repeated peristaltic waves in the stomach help to soften the food. The food leaves the stomach in the form of chyme and enters the upper small intestine at periodic intervals

11. Digestion in Small intestine The food in the small intestine is mixed with 3 juices namely bile juice, pancreatic juice, intestinal juice. Bile juice: It is a brownish green, alkaline secretion of the liver and stored in the gall bladder and pored into duodenum via the bile duct. Bile salts emulsify fats and helps enzymes like lipase to act upon fats. Bile salts convert bigger fat particles into smaller fat globules called chilomicrons.

12. Digestion in Small intestine pancreatic juice: It is an alkaline [pH 7-8] fluid transported to duodenum through the pancreatic duct. It contains water, mineral salts and enzymes. Important enzymes are trypsin, chymotrypsin, amylase, lipase, carboxypeptidase and nuclease. Proteins + trypsin → polypeptides + peptones. Proteins chymopepsin→ large peptides. polypeptides Carboxypeptidase→ di peptides + tri peptides + fatty acids Emulsified fat (Tri glycerides) lipase→fatty acids + monoglycerides Starch Pancreatic Amylase→Maltose

14. Digestion in Small intestine • Changes take place in succus entericus • Erypsin converts poly peptide into aminoacids. • Maltase acts on maltose and converts into glucose. • Sucrase (Invertase) acts on sucrose and converts into glucose and fructose. • Lactase acts on lactose and converts into glucose and galactose. • Lipaseacts on fats and converts into fatty acids and glycerol.

15. Absorption and Assimilation Carbohydrates are broken into Glucose or fructose. Lipids are broken into glycerol and fatty acids. Proteins are broken into Amino acids. Along with this simpler molecules minerals, vitamins, water etc. are absorbed through the intestinal villi. Fatty acids absorbed through the lymph vessels. Vitamins by diffusion process. Glucose by active transport. From the lumens of the intestine absorbed food constituents are carried to the liver through the hepatic portal vein. From the liver food materials are transported to all other regions of body for utilization.

17. Absorption and Assimilation From the lumens of the intestine absorbed food constituents are carried to the liver through the hepatic portal vein. From the liver food materials are transported to all other regions of body for utilization. This conversion of food into energy and cellular organization is called assimilation.

18. Digestion in Large intestine Any useful substances in the leftovers, such as spare water and body minerals, are absorbed through the walls of the large intestine. The remains are formed into brown, semi-solid faeces, ready to be removed from the body. In Rectum and Anus The end of the large intestine and the next part of the tract, the rectum, store the faeces. These are finally squeezed through a ring of muscle, the anus, and out of the body.

19. Egestion The indigestible material is known as roughage. The removal of faecal material is known as defecation (or) egestion. Defaecation takes place through anus

20. Thank you End