The pancreas contains exocrine cells that secrete digestive enzymes into the , and clusters of endocrine cells called pancreatic islets, which secrete insulin and , to regulate blood glucose levels. When blood glucose levels are high, the pancreas secretes the hormone insulin, which causes the liver to remove excess glucose from the blood and to store it in the liver as . When this stored form of glucose is low, the liver converts from fats and amino acids to glucose molecules. Most pancreatic cells produce pancreatic juice, which contains , to help neutralize the stomach acid. Pancreatic amylase digests , trypsin digests protein, and lipase digests fat. The major functions of the liver include: removing or detoxifying harmful substances from the blood; storing glycogen, , and several vitamins; synthesizing many proteins found in the blood ; and regulating blood cholesterol. Another function of the liver is to produce bile, which is a yellowish-green color because it contains bilirubin, a breakdown product of . Bile also contains bile salts, which serve to emulsify in the small intestine.