Why are villi present in the intestine and not in the stomach?

(a) Villi are present in the intestine and not in the stomach because it increases the surface area for absorption of food in the small intestine more efficiently.

(b) Pepsinogen is a precursor of pepsin and it remains stored in the stomach walls. By the action of hydrochloric acid, pepsinogen is converted into pepsin. Activation of pepsin is done in the form of pepsinogen.

(c) The four basic layers of walls of the alimentary canal are as follows:

(i) The outermost layer of the human alimentary canal is named serosa. A thin layer of secretory epithelial cells, with some connective tissues underneath together, constitute the serosa.

(ii) A thin layer of smooth muscles arranged into an outer longitudinal layer and an inner circular layer is muscularis.

(iii) A layer of loose connective tissues, containing nerves, blood, and lymph vessels is a submucosa layer that supports the mucosa.

(iv) The innermost lining of the lumen of the alimentary canal is mucosa which is mainly involved in absorption and secretion.

(d) A digestive juice secreted by the liver and stored in the gallbladder is bile juice. The bile salts such as bilirubin and biliverdin remain present in bile juice. For the breakdown of large fat globules into smaller globules, the bile juice is responsible. This breakdown is essential so that the pancreatic enzymes can easily act on fat globules. The process of breakdown of fat is known as the emulsification of fats. Bile juice also activates lipase and makes the medium alkaline.