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. Cells of the pancreatic islet make up the endocrine portion and secrete insulin and glucagon into the blood stream as needed. The exocrine portion participates in digestion by the secretion of the natural antacid , bicarbonate, electrolytes, fluid and digestive enzymes that are added to the contents of the duodenum. Secretion by the exocrine pancreas is regulated mostly by GI hormones with some minor influence of the nervous system.