Storage of food occurs as a result of relaxation of smooth muscles of stomach. Food is temporarily accommodated in stomach, because intestines, being smaller in diameter, are unable to hold large amount of food at a time. During storage, the salivary digestion of carbohydrates is continued.
Gastric juice is a mixture of hydrochloric acid, pepsin and rennin enzymes and is synthesized and secreted from gastric (oxyntic) glands situated in the wall of stomach. Gastric juice is highly acidic with a pH value of pure secretion being 1.0 due to very high concentration of hydrochloric acid (about 150 milli moles per liter). After mixing with food the pH of gastric contents becomes approximately 2.5 which is suitable for digestion of proteins in the stomach. Following are the major functions of gastric juice.
Hydrochloric acid lowers down the pH of gastric contents for better action of pepsin enzyme. It is also bactericidal in nature and kills possible infections entering through food.
Pepsinogen is activated to pepsin by the acid. Pepsin converts tougher proteins (specifically animal proteins of milk, meat and egg) into peptides and peptones. Food stays in stomach for 1-3 hours. Prolong stay and acidic pH favours the breakdown of proteins.
Rennin enzyme cause curdling of milk by converting casein (milk protein) into calcium paracaseinate. This insoluble protein stays for longer period in stomach, thus undergoes better digestion.
The gastric juice also contains mucus, a glycoprotein and water soluble compound of high viscosity and lubricating property. Mucus serves as a protective barrier between food and stomach wall and avoid direct contact of gastric juice with stomach tissue. Through this, mucus prevents auto digestive action of gastric enzymes on stomach tissue, thus protects from development of peptic ulcers in the stomach wall. Decreased secretion of mucus under stressful conditions leads to development of peptic ulcers. Food after mixing with gastric juice is now termed as chyme. Acidic chyme with active pepsin is highly injurious to the stomach/pyloric tissue. Auto digestion is a probable threat to stomach tissue from the active pepsin enzyme of the gastric juice. Protection against autodigestion is provided by the mucus secretion from the pyloric glands of stomach.
Another important function of stomach is to regulate the outflow of chyme into the duodenum. This is achieved through controlled contractions of smooth muscles of antrum (last segment of stomach) and relaxation of pyloric sphincter (a kind of valve present between stomach and duodenum). The chyme is delivered into duodenum through pyloric sphincter as a bolus of 15-20ml at a time at regular interval of about 3-5 seconds.