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Digestion begins in mouth and completes in small Intestine.

Digestion process takes place in two ways

Mechanical digestion and chemical digestion

In the humans , food enters the mouth and mechanical digestion of the food starts by the action of mastication (chewing), a form of mechanical digestion, and the wetting contact of saliva  . Saliva, a liquid secreted by the salivary glands ,contains salivary amylase, an enzyme which starts the digestion of starch in the food; the saliva also contains mucus, which lubricates the food, and hydrogen carbonate  , which provides the ideal conditions of alkalin pH for amylase to work. After undergoing mastication and starch digestion, the food will be in the form of a small, round slurry mass called a bolus. It will then travel down the esophagus and into the stomach by the action of peristalsis. Gastric juice in the stomach starts protein digestion. Gastric juice mainly contains hydrochloric  acid and pepsin. In infants and toddlers gastric juice also contains rennin . As the first two chemicals may damage the stomach wall, mucus  is secreted by the stomach, providing a slimy layer that acts as a shield against the damaging effects of the chemicals. At the same time protein digestion is occurring, mechanical mixing occurs by peristalsis,