Explain the process of human digestion from the mouth to the stomach.

Food enters the mouth and is digested mechanically and chemically. Mechanical digestion is chewing (mastication) by the teeth and breaks the food into smaller pieces. Chemical digestion is by salivary amylase to break down starch. The food then moves down the oesophagus via peristalsis using circular and longitudinal muscles. The food bolus then enters the stomach where it is held for 4 hours while digestion occurs. The stomach muscles churn and break down the food as well as releasing gastric juices. The main component of gastric juice is hydrochloric acid which lowers the pH and kills bacteria. Low pH is also required for pepsin to work which is another component of gastric juice. Pepsin breaks down proteins. Goblet cells in the stomach secrete mucus to protect the interior of the stomach from acid and enzymes. In the stomach the food becomes a semi liquid called chyme