Other Glands, Organs, or Tissues

Other Glands, Organs, or Tissues

Other organs in the body produce hormones, but their main functions are not to produce these hormones. These structures include the thymus, reproductive organs, digestive glands, pancreas, heart, kidneys, adipose tissue, skeleton, and skin.

Thymus

The thymus, located deep inside the mediastinum posterior to the sternum between the lungs, is larger in ­children than in adults. It shrinks with age and is import-ant in early immunity. The lobulated thymus secretes hormones called thymosins, affecting production and differentiation of lymphocytes as well as thymulin and thymopoietins. Although considered hormones, thesesubstances primarily act locally as paracrines. By the time of old age, the thymus has changed to a structure made of adipose and fibrous connective tissues.

Reproductive Organs

The reproductive organs important for hormone secretion include the ovaries, which produce estrogens­ and progesterone; the testes, which produce testosterone­ in their interstitial cells; and the placenta, which pro-duces estrogens, progesterone, and gonadotropins. The placenta is a temporary endocrine organ that sustains the fetus during pregnancy and secretes steroid and protein hormones that regulate pregnancy. Gonado-tropins regulate the release of gonadal hormones. Estro-gens help the reproductive organs to mature and cause the appearance of the secondary sex characteristics of females at the time of puberty. Along with progester-one, estrogens promote the menstrual cycle and breast development. In males at puberty, testosterone initiates maturation of reproductive organs and appearance of secondary sex characteristics as well as sex drive. It is also required for normal production of sperm and to maintain reproductive organs in adult males.

Digestive Glands

The digestive glands that secrete hormones are found in the linings of the stomach and small intestine. For example, the stomach secretes gastrin and ghrelin. The duodenum releases secretin, cholecystokinin, and incretins.

Pancreas

The pancreas functions as two things: an exocrine gland secreting digestive juice and an endocrine gland releasing hormones. It is an elongated, slightly flattened organ posterior to the stomach, behind the parietal peritoneum. It is joined to the duodenum of the small intestine, transporting digestive juice into the intestine (FIGURE 16-18).

The endocrine part of the pancreas consists of groups of cells called pancreatic islets or islets ofLangerhans. Of these cells,alpha cellssecrete the hor-mone glucagon and beta cells secrete the hormone insulin. Delta cells produce a peptide hormone that is identical to GH-inhibiting hormone. It suppresses release of glucagon and insulin and slows food absorp-tion and enzyme secretion in the digestive tract. F cells produce pancreatic polypeptide, a hormone that inhib-its gallbladder contractions while regulating pancre-atic enzyme production. Glucagon, a 29-­amino-acid polypeptide, stimulates the liver to break down glyco-gen in the process known as ­glycogenolysis and to con-vert certain noncarbohydrates, including amino acids, into glucose in the process called ­gluconeogenesis. This raises the blood sugar concentration much more effectively than epinephrine is able to do. Gluca-gon secretion is regulated by negative feedback and ­prevents hypoglycemia from occurring when glu-cose concentration is relatively low. Glucagon is so powerful­ that just one molecule can trigger the release of 100 million glucose molecules into the bloodstream. Humoral stimuli cause the alpha cells to secrete gluca-gon, although stimulation from the sympathetic ner-vous system and rising amino acid levels also play a role. The release of glucagon is suppressed by insulin, somatostatin, and rising blood glucose levels.

Insulin, a 51-amino-acid protein, works in amanner opposite of glucagon by stimulating the liver to form glycogen from glucose and inhibiting conver-sion of noncarbohydrates into glucose. It consists of two amino acid chains that are linked by disulfide or -S-S- bonds and is synthesized as part ofproinsulin, alarger polypeptide chain. Insulin decreases blood glu-cose concentration, promotes amino acid transport into cells, increases protein synthesis, and stimulates adipose cells to make and store fat. Insulin secretion is also controlled by negative feedback and insulin pre-vents high blood glucose concentrations by promot-ing glycogen formation. Insulin secretion decreases as glucose concentrations fall. Just after we eat, insulin lowers blood glucose levels and also influences pro-tein and fat metabolism. Insulin enhances membrane transport of glucose into primarily fat and muscle cells, inhibits glycogen breakdown into glucose, and inhibits conversion of fats or amino acids to glucose.

Because the brain, kidneys, and liver have easy access to blood glucose, no matter what the insulin level currently is, insulin is not required for glucose entry into these organs. In the brain it plays roles in feeding behaviors, learning, memory, and neuronal development. Insulin and glucagon function together to maintain stable blood glucose concentration, even though the amount of carbohydrates ingested by a person may vary widely. Nerve cells are partially sen-sitive to blood glucose concentration changes. Such changes can alter brain functions.

Prior to the development of type 2 diabetes, most patients develop prediabetes, which is blood glu-cose levels that are higher than normal but not high enough to be diagnosed as diabetes. Prediabetes may be referred to as impaired glucose tolerance or impairedfasting glucose. This condition increases the risk fordeveloping type 2 diabetes and cardiovascular disease. Prediabetes exists when the average blood glucose via the A1C test is revealed to be between 5.7% and 6.4%. Prediabetes can also be diagnosed via a fasting plasmaglucose test, oral glucose tolerance test, or a random plasma glucose test. There may be no clear symptomsof prediabetes. Early treatment is able to return blood glucose levels to normal. It is possible to lower the risk for type 2 diabetes by losing 7% of body weight and by exercising moderately such as brisk walking for 30 minutes a day, five days per week.

Heart

The atria of the heart secrete atrial natriuretic peptide­ (ANP), which stimulates urinary sodium excretion. This peptide decreases sodium in the extracellular fluid, reducing blood pressure and ­volume. It also reduces the sensation of thirst, and suppresses ADH secretion.

Kidneys

The kidneys secrete erythropoietin, which is a red blood cell GH. Erythropoietin is a glycoprotein hor-mone that causes the bone marrow to increase pro-duction of red blood cells. They also release renin, initiating the renin–angiotensin–aldosterone mech-anism of aldosterone release. Therefore, the enzyme renin is responsible for the activation of angiotensin.

Adipose Tissue

The adipose cells release leptin, which is a peptide hormone. Leptin functions to inform the body how much stored fat is present that may be used for energy. Blood levels of leptin are higher when there is more stored fat. It helps to control appetite and stimulate increased expenditure of energy. Two other adipose cell hormones play different roles. Resistin is an insu-lin antagonist, whereas adiponectin enhances the sensitivity to insulin.

Skeleton

The bones via their osteoblasts secrete osteocalcin, which causes pancreatic beta cells to divide, secreting more insulin. Osteocalcin restricts fat storage by the adipocytes and triggers adiponectin release, improv-ing handling of glucose and reducing body fat. Insulin encourages the conversion of inactive osteocalcin to active osteocalcin in the bones. This forms a two-way mode of communication between the bones and the pancreas. In type 2 diabetes, osteocalcin levels are low. Increasing the level of osteocalcin may be an effective form of treatment for type 2 diabetes.

Skin

In the skin, cholecalciferol, which is an inactive form of vitamin D₃, is produced because of ultraviolet radi-ation exposure. Eventually, cholecalciferol becomes fully activated by the kidneys, forming calcitriol, which is required to regulate how intestinal cells absorb calcium from the diet. Without calcitriol, the bones soften and weaken. Osteocalcin is also involved in regulating the mineralization of the bones and teeth. Vitamin D is involved with immune functions. It also decreases inflammation and can reduce risks for cancer. TABLE 16-7 illustrates the hormones pro-duced by other organs.