Exocrine and Endocrine functions of the pancreas: digestive enzymes and zymogens, regulation of secretion; hormones produced by pancreatic endocrine cells.

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The pancreas is both an exocrine digestive gland and an endocrine gland. It secretes digestive enzymes into ducts that lead to the intestine, and hormones into the bloodstream to regulate blood sugar levels, among other functions.
Almost all of the pancreatic tissue is dedicated to exocrine function producing proteases, lipase and amylase; for digestion of proteins, fats, and carbohydrates, respectively.
The exocrine cells are organized in clusters around small ducts, in structures known as acini. These ducts collect secretion from the surrounding cells, then converge to form larger ducts which eventually open into the main pancreatic duct running along the length of the pancreas. The main pancreatic duct joins the bile duct before opening into the duodenum via the hepatopancreatic sphincter. There is also a small accessory pancreatic duct.
The pancreatic juice is a mixture of acinar secretion and sodium bicarbonate produced by the ducts. Bicarbonate serves to neutralize stomach acid that arrives in the duodenum.
Most enzymes, in particular proteases, are secreted as non-active pro-enzymes, called zymogens, among which trypsinogen plays a central role. Once in the intestine, trypsinogen is converted into its active form, trypsin, by the enzyme enterokinase produced by the intestinal epithelial cells. Trypsin then converts the other zymogens to their active form.
Amylase and lipase are not chemically altered after secretion, but they only become active upon exposure to the more favorable pH of the intestine.
Secretion of bile and pancreatic juice is stimulated by the parasympathetic arm, and inhibited by the sympathetic arm of the autonomic nervous system. In addition, several hormones are involved in the control of secretion:
- Cholecystokinin (CCK), produced in response to the arrival of fatty acids and proteins in the duodenum, stimulates secretion of pancreatic enzymes. CCK also triggers the contraction of the gallbladder and relaxation of the hepatopancreatic sphincter to release bile and pancreatic juice.
- Secretin, produced in response to the arrival of stomach acid in the duodenum, stimulates secretion of bicarbonate by the ducts.
The endocrine portion of the pancreas consists of scattered clusters of cells called the islets of Langerhans. There are five different endocrine cell types: alpha, beta, delta, epsilon, and upsilon; they produce glucagon, insulin, somatostatin, ghrelin, and pancreatic polypeptide, respectively.
- Insulin secretion is mainly regulated by blood glucose level. High plasma glucose stimulates beta-cells to secrete insulin. Insulin brings blood glucose level down by promoting glucose uptake and storage in various body tissues.
- Glucagon, on the other hand, is secreted mainly in response to low blood glucose. Glucagon brings blood glucose level up by promoting glucose production and inhibiting glucose breakdown in the liver, basically antagonizing the effect of insulin in the liver. Glucagon also stimulates energy production from non-carbohydrate sources.
- Beta-cells also produce amylin, which is co-secreted with insulin. Amylin inhibits food intake, delays gastric emptying, and suppresses glucagon secretion.
- Somatostatin is a broad-acting hormone. It inhibits the secretion of many other hormones, including growth hormone, thyroid-stimulating hormone and gastrointestinal hormones.
- Ghrelin, known as the “hunger hormone”, stimulates food intake, fat deposition, and growth hormone secretion. It also inhibits the secretion of insulin.