The pancreas (pan’-kre-as) is an elongate organ that is located posterior to the stomach. It is both an exocrine gland and an endocrine gland. Its exocrine functions are performed by secretory cells that secrete digestive enzymes into tiny ducts within the gland. These ducts merge to form the pancreatic duct, which carries the secretions into the small intestine. Its endocrine functions are performed by secretory cells that are arranged in clusters or clumps called the pancreatic islets. Their secretions diffuse into the blood. The islets contain alpha cells and beta cells. Alpha cells produce the hormone glucagon; beta cells form the hormone insulin.
Hormones of The Pancreas
|Glucagon||Blood glucose level||Increases blood glucose by stimulating the liver to convert glycogen and other nutrients into glucose|
|Insulin||Blood glucose level||Decreases blood glucose by aiding movement of glucose into cells and promoting the conversion of glucose into glycogen||Hyposecretion causes type I diabetes mellitus.
Hypersecretion may cause hypoglycemia.
Glucagon (glu’-kah-gon) increases the concentration of glucose in the blood. It does this by activating the liver to convert glycogen and certain noncarbohydrates, such as amino acids, into glucose. Glucagon helps to maintain the blood level of glucose within normal limits even when carbohydrates are depleted due to long intervals between meals. Epinephrine stimulates a similar action, but glucagon is more effective. Glucagon secretion is controlled by the blood level of glucose via a negative-feedback mechanism. A low level of blood glucose stimulates glucagon secretion, and a high level of blood glucose inhibits glucagon secretion.
The effect of insulin on the level of blood glucose is opposite that of glucagon. Insulin decreases blood glucose by aiding the movement of glucose into body cells, where it can be used as a source of energy. Without insulin, glucose is not readily available to most cells for cellular respiration. Insulin also stimulates the liver to convert glucose into glycogen for storage. The antagonistic functions of glucagon and insulin maintain the concentration of glucose in the blood within normal limits. Like glucagon, the level of blood glucose regulates the secretion of insulin. High blood glucose levels stimulate insulin secretion; low levels inhibit insulin secretion.
Disorders Diabetes mellitus (d–-ah-be i-tez mel-li i-tus) is caused by the hyposecretion of insulin or the inability of target cells to recognize it due to a loss of insulin receptors. Type I or insulin-dependent diabetes is an autoimmune metabolic disorder that usually appears in persons less than 20 years of age. For this reason, it is sometimes called juvenile diabetes, although the condition persists for life. Type I diabetes results when the immune response destroys the beta cells in pancreatic islets. Because the metabolism of carbohydrates, fats, and proteins is affected, persons with type I diabetes must follow a restrictive diet. They must also check their blood glucose level several times a day and inject themselves with insulin, or receive insulin from an implanted insulin pump, to keep their blood glucose concentration within normal limits.
The vast majority of diabetics have type II or insulin- independent diabetes, which is caused by a reduction of the insulin receptors on target cells. This form of diabetes, also called adult-onset diabetes, usually appears after 40 years of age in persons who are overweight. The symptoms are less severe than in type I diabetes and can be controlled by a careful diet and oral medications that help regulate blood levels of glucose. The current increase in obesity among children and young adults is of concern because it may lead to an increase in type II diabetes. In either case, the result is hyperglycemia, excessively high levels of glucose in the blood. With insufficient insulin or a reduction in target insulin receptors, glucose cannot get into cells easily, and cells must rely more heavily on triglycerides as an energy source for cellular respiration. The products of this reaction tend to decrease blood pH (acidosis), which can inactivate vital enzymes and may lead to death.
An excessive production of insulin, or overdose of insulin, may lead to hypoglycemia, a condition characterized by excessively low blood glucose levels. Symptoms include acute fatigue, weakness, increased irritability, and restlessness. In extreme conditions, it may lead to an insulin- triggered coma.