in digestive system the main and accessory organs and their main function.?
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DIGESTIVE SYSTEM OF HUMAN:
It is a system of alimentary canal and digestive glands that takes part in ingestion of food, its crushing , digestion , absoption of digested nutrient and egestion of undigested materials.
It is a tube of variable diameter having muscular wall and glandular epithelium. Alimentary canal starts at mouth and ends at anus. It is very long ( 6- 9) m long and consists of many specialised parts like mouth, vestibule , buccal cavity, pharynx , oesophagus , stomach, small intestine and large intestine and anus.
DIGESTIVE GLANDS : They include salivary glands , gastric glands, liver, pancreas , and intestinal glands. Their sectretions contain enzymes called hydrolases. The latter perform extracellular digestion.
DIGESTION: Conversion of complex insoluble constituents of food into simple, soluble forms is called digestion. It begins in buccal or oral cavity and is completed in small intestine. In buccal cavity the food is tasted and mixed with saliva which has a lot of water and mucin. Tongue manipulates the food and brings it in between the cheek teeth. Movement of lower jaw help in crushing the food into pulpy mass.
It is known as mastication. Saliva has lysozyme and other antibacterial ingredients for preventing growth of bacteria. Ptyalin or salivary amylase is the only enzyme contained in saliva. It changes atarch and glycogen into limit dextrins, maltose and isomaltose. About 30% of starch is hydrolysed by ptyalin. Tongues rolls up the masticated moistened food into small ball called bolus. Bolus is pushed to pharynx. A number of involuntary actions occur. Soft palate is raised to close the internal nares. The larynx also rises to come in contact with epiglottis. Walls of pharynx now contracts. Contraction of pharynx pushes the bolus into oesophagus. The phenomenon is known as swallowing. .In oesophagus, the food is mixed with mucus and passed on into stomach through peristalsis by the opening of guirding sphincter. Peristalsis is accomplished by sequential alternate contraction and relaxatoon of circular and longitudnal muscles of the gut walls. Ptyaline action continues in oesophagus.
Food is kept in stomach for 1 - 5 hrs. Here , it is mixed with gastric juice. Action of saliva stops. Stomach undergoes churning movements. Food doesnot pass back into oesophagus due to cardiac sphincter. Gastric pepsin further softens the foods. The pulpy food is now called chyme. Chyme passes into duodenum through the opening of pyloric sphincter. Burner's glands of duodenum is activated by vagus and secretin hormone. They secrete a lot of alkaline watery and mucoid fluid for protection against acidity of chyme. Thereafter, the food mixes with bile and pancreatic juices. Action of pepsin stops. Food becomes alkaline. Food continues to move forward by peristalsis. It is mixed with intestinal juice and churned further through pendular movements. In small intestine all constituents of food except cellulose are broken and hydrolysed into soluble state. The soluble food is absorbed by large intestine. Water is absorbed in the region of colon. Colon has bacteria that feed on undigested food. Some of them produce B- vitamins. Small quantities of vitamin K and odoriferous gases like phenols and hydrogen sulphide. In colon the undigested matter is changed into faeces. With the accumulation of faecal matter , colon develops a strong peristaltic movement for pushing it into rectum. This develops an urge for defaecation. Mass constraction of colon - rectal complex occurs. Internal anal sphincter relaxes. The external anal sphincter is relaxed through volantary action to pass out faeces or egestion.