Notes for Class 11 Biology- Digestion and Absorption

Notes for Class 11 Biology- Digestion and Absorption – The Process of Egestion

Mechanism of Egestion:

Peristalsis (muscular contractions and relaxations that take place in the digestive tract), gradually pushes the indigestible materials of the small intestine into the large intestine or colon. Normally 1500 ml of chyme passes into the large intestine per day. The colon absorbs most of the water. It also absorbs electrolytes, including sodium and chloride from the chyme.

The epithelial cells of the colon also excrete certain salts such as iron and calcium from the blood. Escherichia coli(bacterium) live in the colon which feeds on the undigested matter. This bacterium, in turn, produces vitamin B12 (cobalamin), vitamin K, vitamin В1 (thiamine) and vitamin B2 (riboflavin) which are absorbed by the wall of the colon. Consequently, the chime converts into semisolid faeces.

The faecal matter is stored in the rectum, the lower part of the large intestine, as the pellets of faeces enter the rectum; distension of rectal wall induces the feeling of defecation due to a “defecation reflex”. This reflex initiates peristalsis in the last part of the colon (sigmoid colon) and the rectum, forcing the faeces towards the anus. As the faeces reach anus, involuntary relaxation of the internal anal sphincter and voluntary relaxation of external anal sphincter cause defecation. Defecation is the final act of digestion, by which organisms eliminate solid, semisolid, or liquid waste material from the digestive tract via the anus.

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