Anatomy and Physiology: Pancreas and Diabetes

Pancreas and Diabetes

The pancreas is one of those dual citizenship organs, acting as both an endocrine organ, and as an accessory organ of the digestive system (see The Digestive System). The digestive enzymes produced by the exocrine pancreas (99 percent of the organ) are, by definition, picked up by ducts near the pancreatic acini, which ultimately empty into the small intestine. The remaining 1 percent is taken up by clusters of cells called pancreatic islets, which form the endocrine pancreas.

These pancreatic islets have four cells, each of which produces a different hormone: alpha cells produce glucagon, beta cells produce insulin, delta cells produce growth hormone- inhibiting hormone (GH-IH) or somatostatin, and F cells release the poorly named pancreatic polypeptide (discussed in The Digestive System). Pancreatic polypeptide regulates the release of pancreatic digestive enzymes, as well as inhibiting gall bladder contraction. GH-IH, in addition to living up to its name, also inhibits the secretion of both insulin and glucagon and slows absorption and enzyme secretions from the GI tract, usually in response to meals high in protein.

Now, on to blood glucose regulation (see Figure 18.6). Blood glucose usually lives somewhere between 70-120 mg/dl. The only way to maintain such a narrow range is through negative feedback. When blood sugar gets too high (such as after a meal), it is lowered with insulin; when blood sugar gets too low (from too long a break between meals), raise it with glucagon. In a nutshell, insulin attaches to insulin receptors on the capillary walls, and is thus used to remove glucose from blood plasma; insulin also helps with the storage of extra glucose through the formation of glycogen and lipids. Glucagon, on the other hand, targets the liver cells, causing the breakdown of glycogen, and thus releasing the glucose monomers and adding them to the bloodstream (raising blood glucose levels).

Imagine drinking a half gallon of milk in a half hour, and still feeling thirsty. Imagine getting up to pee over 20 times in a night. These are the signs of untreated diabetes that I developed just before my eighteenth birthday, at the end of my freshman year in college. Although there are different causes of diabetes, the signs are the same. In each case there isn't enough insulin to control the blood glucose levels. The two most common types are type I and type II.

Figure 18.6The opposing roles of insulin and glucagon in blood glucose regulation. (LifeART©1989-2001, Lippincott Williams & Wilkins)

In type I, juvenile onset, insulin-dependent diabetes, the beta cells of the pancreas have been destroyed in an autoimmune response; eventually, no insulin is produced and the patient must take insulin injections. Type II, adult onset diabetics, can often control their blood glucose with diet and certain pills (although insulin therapy might be necessary); these medication help the body to more efficiently use the insulin it already makes.

Testicles

In addition to churning out sperm, testicles are also little hormone factories. The testicles, or testes, produce two hormones: testosterone and inhibin. Even in the womb, testosterone production affects the development of the brain (explains a lot!); the hypothalamic nuclei, for example, will affect adult sexual behavior, some of which is influenced even before birth. Testosterone production, by the interstitial cells (Leydig cells), increases during puberty, ultimately affecting secondary sex characteristics, such as body hair, and the layout of body fat (a moment on the lips, a lifetime on your abdomen …). Testosterone has also been linked to male aggressive behavior.

FSH from the adenohypophysis stimulates the sustentacular cells (Sertoli cells) in the differentiation and maturation of spermatozoa (a process that takes about 70 days). FSH also causes these cells to release inhibin, which acts in a negative feedback loop, inhibiting the secretion of FSH. It is also thought that GnRH release is also inhibited this way.

Ovaries

The ovaries and the hormones of the menstrual cycle are given special treatment. The female gametes are produced only one at a time, and released during ovulation. In addition to the roles of FSH and LH discussed, there are other hormones called estrogens and progestins. Estrogens (estradiol is the main one) basically are involved in the final maturation of the secondary oocyte, and in the building of the uterine lining (endometrium).

The main progestin is the familiar progesterone. In addition to building up the endometrium, progesterone helps to speed up the movement of the secondary oocyte along the fallopian tube. If fertilization occurs, the division of the zygote to a morula, and then a blastula (a single cell, to a solid ball of cells, to a hollow ball of cells) is helped along by progesterone. Lastly, progesterone, with a little help from estrogen, prolactin, and even growth hormone, causes the breasts to enlarge in preparation for lactation and breast feeding.

The only hormone left would be almost funny if there weren't a powerful reason for it! This hormone, relaxin, which is also involved in breast development, is elevated during pregnancy. Its release is stimulated by LH in the beginning, but later it is a placental hormone called human chorionic gonadotropin (hCG) that stimulates the secretion of relaxin. Its most interesting function is to “relax” the cartilage in the pubic symphysis, making it easier for the baby to pass through the pelvic outlet (the bottom of the pelvis). As if it weren't hard enough to walk during the ninth month of pregnancy, a woman's pelvis has to feel like it's going to fall apart!

Excerpted from The Complete Idiot's Guide to Anatomy and Physiology © 2004 by Michael J. Vieira Lazaroff. All rights reserved including the right of reproduction in whole or in part in any form. Used by arrangement with Alpha Books, a member of Penguin Group (USA) Inc.

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