The Pancreas

THE PANCREAS

The pancreas, located in the upper abdomen, has endocrine as well as exocrine functions (see Fig. 40-1). The secretion of pan-creatic enzymes into the gastrointestinal tract through the pan-creatic duct represents its exocrine function. The secretion of insulin, glucagon, and somatostatin directly into the bloodstream represents its endocrine function.

Exocrine Pancreas

The secretions of the exocrine portion of the pancreas are col-lected in the pancreatic duct, which joins the common bile duct and enters the duodenum at the ampulla of Vater. Surrounding the ampulla is the sphincter of Oddi, which partially controls the rate at which secretions from the pancreas and the gallbladder enter the duodenum.

The secretions of the exocrine pancreas are digestive enzymes high in protein content and an electrolyte-rich fluid. The secre-tions are very alkaline because of their high concentration of sodium bicarbonate and are capable of neutralizing the highly acid gastric juice that enters the duodenum. The enzyme secre-tions include amylase, which aids in the digestion of carbohy-drates; trypsin, which aids in the digestion of proteins; and lipase, which aids in the digestion of fats. Other enzymes thatpromote the breakdown of more complex foodstuffs are also secreted.

Hormones originating in the gastrointestinal tract stimulate the secretion of these exocrine pancreatic juices. Secretin is the major stimulus for increased bicarbonate secretion from the pan-creas, and the major stimulus for digestive enzyme secretion is the hormone CCK-PZ. The vagus nerve also influences exocrine pancreatic secretion.

Endocrine Pancreas

The islets of Langerhans, the endocrine part of the pancreas, are col-lections of cells embedded in the pancreatic tissue. They are com-posed of alpha, beta, and delta cells. The hormone produced by the beta cells is called insulin; the alpha cells secrete glucagon and the delta cells secrete somatostatin.

INSULIN

A major action of insulin is to lower blood glucose by permitting entry of the glucose into the cells of the liver, muscle, and other tissues, where it is either stored as glycogen or used for energy. In-sulin also promotes the storage of fat in adipose tissue and the synthesis of proteins in various body tissues. In the absence of in-sulin, glucose cannot enter the cells and is excreted in the urine. This condition, called diabetes mellitus, can be diagnosed by high levels of glucose in the blood. In diabetes mellitus, stored fats and protein are used for energy instead of glucose, with consequent loss of body mass. The level of glucose in the blood normally regulates the rate of insulin secretion from the pancreas.

GLUCAGON

The effect of glucagon (opposite to that of insulin) is chiefly to raise the blood glucose by converting glycogen to glucose in the liver. Glucagon is secreted by the pancreas in response to a de-crease in the level of blood glucose.

SOMATOSTATIN

Somatostatin exerts a hypoglycemic effect by interfering with re-lease of growth hormone from the pituitary and glucagon from the pancreas, both of which tend to raise blood glucose levels.

Endocrine Control of Carbohydrate Metabolism

Glucose for body energy needs is derived by metabolism of in-gested carbohydrates and also from proteins by the process of glu-coneogenesis. Glucose can be stored temporarily in the liver, muscles, and other tissues in the form of glycogen. The endocrine system controls the level of blood glucose by regulating the rate at which glucose is synthesized, stored, and moved to and from the bloodstream. Through the action of hormones, blood glucose is normally maintained at about 100 mg/dL (5.5 mmol/L). Insulin is the primary hormone that lowers the blood glucose level. Hor-mones that raise the blood glucose level are glucagon, epinephrine, adrenocorticosteroids, growth hormone, and thyroid hormone.

The endocrine and exocrine functions of the pancreas are in-terrelated. The major exocrine function is to facilitate digestion through secretion of enzymes into the proximal duodenum. Se-cretin and CCK-PZ are hormones from the gastrointestinal tract that aid in the digestion of food substances by controlling the se-cretions of the pancreas. Neural factors also influence pancreatic enzyme secretion. Considerable dysfunction of the pancreas must occur before enzyme secretion decreases and protein and fat di-gestion becomes impaired. Pancreatic enzyme secretion is nor-mally 1,500 to 2,500 mL/day.

Gerontologic Considerations

There is little change in the size of the pancreas with age. There is, however, an increase in fibrous material and some fatty depo-sition in the normal pancreas in patients older than 70 years of age. Some localized arteriosclerotic changes occur with age. There is also a decreased rate of pancreatic secretion (decreased lipase, amylase, and trypsin) and bicarbonate output in older patients. Some impairment of normal fat absorption occurs with increas-ing age, possibly because of delayed gastric emptying and pan-creatic insufficiency. Decreased calcium absorption may also occur. These changes require care in interpreting diagnostic tests in the normal elderly person and in providing dietary counseling.