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The Endocrine System / The Pancreas
In humans the pancreas weighs approximately 80 grams, has roughly the configuration of an inverted smoker's pipe, and is situated in the upper abdomen. The head of the pancreas (equivalent to the bowl of the pipe) is immediately adjacent to the duodenum, while its body and tail extend across the midline nearly to the spleen. The bulk of pancreatic tissue is devoted to its exocrine function, the elaboration of digestive enzymes that are secreted via the pancreatic ducts into the duodenum. Insulin, produced by the beta cells of the islets of Langerhans, is a moderate-sized protein composed of two chains, the alpha chain (with 21 amino acids) and the beta chain (with 30), linked by sulfur atoms. Insulin is derived from a larger prohormone molecule called proinsulin. Proinsulin is relatively inactive, and normally little of it is secreted. It contains a connecting peptide, or C-peptide, composed of 31 amino acids with an additional amino acid at either end linked to the alpha and beta chains, respectively. As is the case with other prohormones, the connecting peptide of proinsulin is cleaved off before insulin is released into the circulation. Insulin leaves the pancreas through veins, which empty into the portal vein perfusing the liver. Typically the pancreas of a normal adult contains approximately 200 units (eight milligrams) of insulin; the average daily secretion of insulin into the circulation ranges between 35 and 50 units. Although a number of physiological events influence insulin secretion, the most important is the concentration of glucose in the arterial (oxygenated) blood perfusing the pancreas. When the plasma glucose level rises, insulin release is stimulated; as plasma glucose falls, so does the rate of insulin secretion. Even during prolonged fasting, however, a baseline secretion of insulin continues. Insulin secretion also is influenced by neurotransmitters interacting with islet cell receptors, particularly those that bind norepinephrine. Insulin stimulates the transport of glucose and amino acids into muscle cells and prompts the conversion of amino acids into protein. Thus, insulin is required to replete the glycogen, a stored form of glucose, that is oxidized during exercise and to replenish protein needed for muscle growth and repair. Insulin is not required for the transport of glucose into liver cells, but the hormone profoundly affects intracellular metabolism in the liver. It promotes glycogen formation, stimulates the utilization of glucose, and suppresses those enzymes necessary for new glucose formation (gluconeogenesis) and glycogen breakdown (glycogenolysis). The overall effect of insulin is to increase glucose utilization and storage and decrease its release by the liver. Glucagon is produced by the alpha cells of the pancreas and also is secreted by cells scattered throughout the gastrointestinal tract. A number of forms of glucagon have been found; the biologically active one appears to contain 29 amino acids. Radioimmunoassays can distinguish between pancreatic glucagon and similar peptides from the gut. Circulating glucagon levels are high in the fasting state. Secretion is stimulated by amino acids and gastrointestinal peptide hormones. Normally, ingested glucose is a potent suppressor of glucagon release, an effect that is probably mediated by an increase in circulating insulin. Secretion of glucagon also is inhibited by free fatty acids and by somatostatin and appears to be modulated by the autonomic nervous system. Circulating glucagon binds to specific receptors on the surface of liver cells (hepatocytes), leading to the breakdown of liver glycogen into glucose, which is then released into the blood. Glucagon is estimated to be responsible for most of the hepatic glucose production after an overnight fast. Somatostatin, a peptide that was discovered initially in the hypothalamus (see Hypothalamus: Growth hormone-releasing hormone), contains 14 amino acids, is produced by the D cells of the islets, and has a number of effects on digestion. It inhibits gastrointestinal motility and blood flow, secretion of stomach acid, secretion of pancreatic exocrine, and the absorption of triglyceride from the bowel. In summary, it appears that the hormones insulin, glucagon, and somatostatin act in concert to control the flow of nutrients into and out of the circulation. The relative concentrations of these hormones regulate the rates of absorption and peripheral disposal of substances such as glucose, amino acids, and fatty acids. The anatomic proximity of the B, A, and D cells in the islets is significant. Somatostatin and glucagon appear to have a paracrine relationship whereby they influence the secretion of each other, and both affect the rate of insulin release. Pancreatic polypeptide, secreted by the F (or PP) cells, contains 36 amino acids amino acids. Circulating levels rise following ingestion of a meal. An increase in the level of free fatty acids in the blood suppresses its secretion. Pancreatic polypeptide can inhibit gallbladder contraction and pancreatic exocrine secretion, but its biologic role is uncertain. The following reflexes may be active when the pancreas is malfunctioning:
These reflexes should be tested and treated with the proper supplementation. Get Additional Help Free Health Tests & Professional Consultations Take this preliminary
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