Parotid, submaxillary, sublingual glands add saliva to the food during mastication. Salivary amylase begins the breakdown of carbohydrates while the food is still in the mouth. Salivary lysozyme lyses bacterial cell walls. Mucopolysaccharides act as lubricants and the water acts to extract flavors from food. Salivary secretion is controlled by the nervous system.
Gastric glands are present throughout the mucosa of the stomach.
The mucous acts to protect the gastric lining from the acid and enzymes present. Parietal cells also produce a protein called intrinsic factor that is necessary for the absorption of vitamin B12 in the ileum. Pepsinogen is activated by the removal of a small fragment. Pepsin works optimally to digest proteins at a pH of 1.5-2.5. The enzyme rennin is most active in infants where it causes milk proteins to curdle.