1.4.1 Digestion in the Oral Cavity
Here food is tasted, smelled and felt. The food is chewed by crushing and grinding. The teeth, cheeks and tongue take part in these actions. As a result, food is physically and quickly broken down into smaller pieces. Food in the oral cavity stimulates the salivary glands to secrete saliva. The water and mucous in saliva moistens, softens and lubricate food. Saliva contains salivary amylase. This enzyme digests starch into maltose. Another enzyme, lipase, is produced by the cells in the tongue. It breaks down triglycerides, to mostly diglyceride and free fatty acids. The food is rolled by the tongue into small, slippery, spherical mass called bolus. The bolus is swallowed and enters the oesophagus through the pharynx.
1.4.2 Digestion in Stomach
On the peristaltic reflex the cardiac sphincter relaxes to allow food in the stomach.
Gastric Juice: The food in the stomach stimulates the gastric glands to secrete gastric juice. It consists of mucous, HCl, pepsin and rennin.
Mucous: Mucous forms a coating over the stomach. It prevents the digestion of stomach wall by the enzyme pepsin.
Hydrochloric Acid: HCl performs the following functions: (a) Stops the action of salivary amylase. (b) It converts the inactive form of enzyme (pepsinogen) to active form (pepsin). It provides acidic medium suitable for the action of enzymes. (c) It kills many microorganisms.
Pepsin: It is secreted as inactive pepsinogen. HCl or already activated pepsin converts pepsinogen to pepsin. Pepsin digests proteins to polypeptides (chain of many amino acids) and peptides (chain of few amino acids).
Renin: Rennin is secreted only in infants. It helps in the digestion of milk protein.