Glucose is the primary source of energy for the body. In fact, it is normally the only fuel used by the brain’s nerve cells, called neurons. Neurons can’t store excess glucose for back-up energy, so a constant supply must be available in the blood. However the supply must be kept in tight balance because too much sugar in the blood causes damage to cells throughout the body. Control of the amount of glucose in the blood depends on two hormones that are produced and secreted by the pancreas.
The pancreas is an unusual organ because it serves two functions. One part of the pancreas is an endocrine gland that produces and secretes hormones. It’s also an exocrine (or digestive) gland that produces enzymes needed by the small intestine to break down and absorb proteins, fats and carbohydrates.
Endocrine Function of the Pancreas
The endocrine function of the pancreas is responsible for regulating the amount of glucose (sugar) in the blood. Throughout the pancreas are structures called islets of Langerhans. Two types of cells in the islets are alpha and beta cells. The alpha cells comprise about 25 percent of the islets. They’re responsible for secreting a hormone known as glucagon. The beta cells account for about 75 percent of the islets. They produce and secrete a hormone known as insulin. Capillaries surrounding the islets allow the hormones to be secreted directly into the blood.
Glucagon and Insulin
Glucagon increases the amount of glucose in the blood by accelerating the rate at which the liver converts stored glycogen into glucose and releases it into the blood. Insulin decreases the amount of glucose in the blood by transporting glucose from the blood and into the muscle cells. It also stimulates the conversion of glucose back into glycogen so that it can be stored.
Receptors in the pancreas detect the amount of glucose in the blood, and this in turn stimulates either the secretion of glucagon or insulin. Control is based on a negative feedback loop. Glucagon causes an increase in blood sugar, and that in turn stimulates the beta cells to secrete insulin as the levels rise too much. In the opposite manner, insulin causes a decrease in blood sugar that stimulates alpha cells to release glucagon to counteract levels that may be too low. This tight control maintains a balance of blood sugar that protects the body from the damaging effects of widely fluctuating levels.
Other cells within the pancreatic islets secret somatostatin, which inhibits several different hormones in the body, including human growth hormone, insulin and glucagon. It plays an important role because hormones such as human growth hormone indirectly stimulate the release of insulin.