The pancreas works in two ways. As an exocrine gland, it secretes its products through ducts—in this case, enzymes and bicarbonate that promote digestion. As an endocrine gland, the pancreas secretes hormones directly into the bloodstream. These hormones help maintain glucose balance.
Exocrine function
Within the pancreas, groups of acinar cells produce precursor digestive enzymes that the pancreas secretes into the intestine through the main pancreatic duct. These enzymes play a crucial role in breaking down and metabolizing carbohydrates, proteins, and fats.
When these enzymes first come from the acinar cells, they aren't active; that doesn't happen until they combine with enzymes in the intestinal mucosa of the duodenum. The combined enzymes then convert partially digested food into substances the body can use for energy. The pancreas produces three main types of enzymes: amylase, which digests carbohydrates; trypsinogen and chymotrypsinogen, which digest protein; and pancreatic lipase, which combines with bile acids to digest triglycerides, a major component of dietary fats.
Pancreatic secretions also contain bicarbonate, which comes from the epithelial cells lining the smaller pancreatic ducts. The bicarbonate helps in digestion by neutralizing the acidic chyme that the stomach releases into the small intestine.
Endocrine function
The pancreas also secretes hormones directly into the bloodstream, where they're carried to different cells in the body. It produces these hormones in the islets of Langerhans, a specialized group of cells. Islet cells are dispersed throughout the pancreas, although the tail of the pancreas has the highest concentration.
Four different types of cells in the islets of Langerhans produce hormones. Beta cells, which compose 65% to 80% of islet cells, produce insulin, amylin, and C-peptide. Insulin regulates the use and storage of glucose. It acts by forcing many body cells to absorb and use glucose, which decreases blood glucose levels. When blood glucose levels rise again, the pancreas secretes more insulin. Amylin supplements insulin's actions by reducing blood glucose levels. It also inhibits the secretion of glucagon, slows the emptying of the stomach, and sends a satiety signal to the brain. C-peptide is a by-product of insulin production. Levels of this hormone help to identify viable beta cell mass.
Alpha cells—15% to 20% of islet cells—produce glucagon. This hormone helps maintain a steady blood glucose level. It does this by forcing many body cells to release or produce glucose. Low blood glucose levels stimulate its secretion; high glucose and amylin levels inhibit its secretion.
Delta cells, which make up 3% to 10% of islet cells, produce somatostatin. This hormone decreases the rate of nutrient absorption, inhibits the secretion of many other hormones—including growth hormone, insulin, glucagon, and other GI hormones—and suppresses pancreatic exocrine secretions. It also has various complex effects on the nervous system.
Gamma or pancreatic polypeptide cells make up a mere 1% of islet cells. These cells produce pancreatic polypeptide. Although its function is unknown, it's thought to inhibit appetite. Secretion of this hormone increases after a protein meal, fasting, exercise, and acute hypoglycemia. Somatostatin and IV glucose decrease its production.