Digestion in the Mouth and Stomach

    Food is ingested in the mouth. Here, the teeth cut and grind food into smaller pieces. Chemical digestion also begins; saliva contains an enzyme called pytalin which acts on cooked starch, converting it into complex sugars. Saliva is a watery, slightly alkaline substance secreted by the salivary glands. The tongue mixes the food with saliva and rolls it into a ball or bolus. The bolus is then swallowed. 

 The mouth opens into a short , wide and muscular pharynx. The nasal cavity also opens into the pharynx. The pharynx leads into two tubes. The one in front is the trachea or windpipe which leads to the lungs. Behind the trachea is the oesophagus or gullet through which food food passes down to the stomach. During the swallowing of food, the entrance to the trachea must be covered to prevent choking. The epiglottis flaps down to cover the trachea during swallowing. The food thus goes into the oesophagus.

 The wall of the oesophagus is muscular. Bands of muscles contract and relax alternatively yo push each bolus of food downwards slowly. This process is known as peristalsis. In fact, the Wales of the entire alimentary canal are muscular, and peristalsis occurs in all regions of the canal. Sometimes food moves backwards. This is known ad anti peristalsis.

 Food enters the stomach when the ring of muscles or sphincter at the entrance of the stomach relaxes. The muscular walls of the stomach contract and relax forcefully. Thus churning the food. Small pieces of food are broken down even more and gastric juice is mixed into the food. Gastric juice contains two important enzymes, pepsin and rennin, as well as , dilute hydrochloric acid. These are produces by special cells lining the wall of the stomach. Mucus is also secreted by certain cells in the stomach wall. 

 Pepsin is the protease; it digests proteins to polypeptides, which are intermediate products in protein digestion. Pepsin works best in an acidic medium. The acid also helps to kill bacteria present in food. Rennin causes the coagulation of milk into thick curds. It does this by acting on the soluble milk protein, caseinogen, and converting it into insoluble casein. This action is important in infants whose main diet is milk. The casein is then broken down by pepsin. Food is retained in the stomach for three to four hours. By this time , it is a thick, creamy fluid called chyme. It moves out through the sphincter at the bottom of the stomach into the duodenum.