<Tr> <Th> Latin </Th> <Td> systema digestorium </Td> </Tr> <Tr> <Td_colspan="2"> Anatomical terminology (edit on Wikidata) </Td> </Tr> <P> Digestion is the breakdown of large insoluble food molecules into small water - soluble food molecules so that they can be absorbed into the watery blood plasma . In certain organisms, these smaller substances are absorbed through the small intestine into the blood stream . Digestion is a form of catabolism that is often divided into two processes based on how food is broken down: mechanical and chemical digestion . The term mechanical digestion refers to the physical breakdown of large pieces of food into smaller pieces which can subsequently be accessed by digestive enzymes . In chemical digestion, enzymes break down food into the small molecules the body can use . </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>

An active protein enzyme in the stomach that begins the chemical digestion of protein is called