Pancreas

The pancreas is a composite gland, meaning it works as both an exocrine and an endocrine gland. Its endocrine part is made up of clusters of cells called the Islets of Langerhans. A normal human pancreas contains roughly one to two million of these islets, yet they make up only about one to two per cent of the total pancreatic tissue.

Cells of the Islets of Langerhans

The islets have two main cell types:

• Alpha (α) cells, which secrete the hormone glucagon.
• Beta (β) cells, which secrete the hormone insulin.

Both are peptide hormones, and together they regulate the level of glucose in the blood (glucose homeostasis), working in opposite directions.

Glucagon — the hyperglycemic hormone

Glucagon helps keep the blood glucose level normal by raising it when needed. It acts mainly on the liver cells (hepatocytes), where it:

• Stimulates glycogenolysis, the breakdown of stored glycogen, which pushes blood sugar up (hyperglycemia).
• Stimulates gluconeogenesis, the making of new glucose, which adds further to the rise in blood sugar.
• Reduces the uptake and use of glucose by cells.

Because its overall effect is to increase blood glucose, glucagon is a hyperglycemic hormone.

Insulin — the hypoglycemic hormone

Insulin plays the major role in keeping blood glucose in balance, and it works to lower it. Insulin acts mainly on hepatocytes (liver cells) and adipocytes (fat-storing cells of adipose tissue), where it:

• Enhances the uptake and use of glucose by cells, so glucose moves rapidly out of the blood and into these cells. This brings blood glucose down (hypoglycemia).
• Stimulates glycogenesis, the conversion of glucose into glycogen for storage in the target cells.

Joint control of blood glucose …