<P> In the human digestive system , 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 pH ( alkaline ) 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 . As these 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 , which is waves of muscular contractions that move along the stomach wall . This allows the mass of food to further mix with the digestive enzymes . </P>
<P> In the human digestive system , 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 pH ( alkaline ) 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 . As these 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 , which is waves of muscular contractions that move along the stomach wall . This allows the mass of food to further mix with the digestive enzymes . </P>
where does protein breakdown begin in the human body
where does protein breakdown begin in the human body
stomach
stomach