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Chapter 12
The Endocrine System
Glands - A group of cells that secrete a substance that perform certain function in the body. There are two types of glands in the body— exocrine glands and endocrine glands a. Exocrine glands: These glands secrete their products into ducts (called ductile glands) that empty onto a surface or into a cavity. For example, sweat glands, salivary glands, ceruminous or wax glands. b. Endocrine glands are called ductless glands. They produced chemicals known as hormone which act as chemical messengers. These hormones are directly released into the blood and are carried to the target sites. Each hormone molecule may then bind to a cell that has specific receptors for that hor- mone, triggering a reaction in the cell. Such a cell is called a target organ cell. The glands of the endocrine system are located in widely separated parts of the body—in the neck; in the cranial, thoracic, abdominal, and pelvic cavities; and also outside of the body cavities The names, locations, and functions and dysfunctions of the well-known endocrine glands are shown in Figure 12-1 and Table 12- The two major classes of hormones—nonsteroid hormones and steroid hormones —differ in the mechanisms by which they influence target organ cells o Nonsteroid hormones are protein in nature serve as first messengers, providing communication between endocrine glands and target organs. Another molecule, such as cyclic AMP, then acts as the second messenger, delivering information within a hormone’s target cells. Example- Thyroid stimulating hormone. o Steroid hormones are lipid in nature and the primary actions of these hormones such as estrogen do not occur by the second-messenger mechanism. Because they are lipid soluble, steroid
hormones can pass intact directly through the cell membrane of the target cell enter the nucleus, where they bind with a receptor to form a hormone-receptor complex. This complex acts on DNA, which ultimately causes the formation of a new protein in the cytoplasm that then produces specific effects in the target cell.
Regulation of Hormone Secretion
Regulation of hormone levels in the blood depends on a homeostatic mechanism called negative feedback Negative feedback —mechanisms that reverse the direction of an abnormal body process to normal. The principle of negative feedback can be illustrated by using the hormone insulin as an example. The elevated blood sugar after meal stimulates the release of insulin from the pancreas. Insulin then assists in the transfer of sugar from the blood into cells, causing blood sugar levels to drop. Low blood sugar levels then cause endocrine cells in the pancreas to cease the production and release of insulin. Positive feedback —(uncommon) mechanisms that amplify physiological changes rather than reverse them. For example, during labor, the muscle contractions that push the baby through the birth canal become stronger and stronger by means of a positive feedback mechanism that regulates secretion of the hormone oxytocin (OT). Two examples of positive feedback in the body-- blood clotting and uterine contractions during labor.
Mechanisms of Endocrine Disease
Tumors or other abnormalities frequently cause the glands to secrete too much or too little of their hormones o Hypersecretion —secretion of an excess of hormone o Hyposecretion — secretion of insufficient hormone o Polyendocrine disorders —hyper or hyposecretion of more than one hormone o Target cell insensitivity produces results similar to hyposecretion. If the usual target cells of a particular hormone have damaged receptors, too few receptors, or some other abnormality, they will not respond to that hormone properly. Diabetes mellitus (DM), for example, can result from insu- lin hyposecretion or from the target cells' insensitivity to insulin
Prostaglandins
o Prostaglandins (PGs) or tissue hormones (modified version of fatty acids) are powerful substances found in a wide variety of body tissues o PGs are often produced in a tissue and diffuse only a short distance to act on cells in that tissue. Typical hormones influence and control activities of widely separated organs; typical prostaglandins influence activities of neighboring cells. o The prostaglandins in the body can be divided into several groups. Three classes of prostaglandins —prostaglandin A (PGA), prostaglandin E (PGE), and prostaglandin F (PGF)—are among the best known. o PGs influence many body functions, including respiration, blood pressure, gastrointestinal secretions, inflammation and reproduction. o Most PGs regulate cells by influencing the production of cyclic AMP like the non steroid hormones. Endocrine Glands of the Body
Pituitary Gland
The pituitary gland lies buried deep in the cranial cavity, in the small depression of the sphenoid bone that is shaped like a saddle and called the sella turcica (Turkish saddle). A stem like structure, the pituitary
Tropic hormones are hormones that have other endocrine glands as their target. Most tropic hormones are produced and secreted by the anterior pituitary. Tropic hormones from the anterior pituitary include: TSH, ACTH, GH, LH, FSH. Non-tropic hormones are hormones that directly stimulate target cells to induce effects. Most endocrine glands, such as the gonads, pancreas, and adrenal glands, produce non-tropic hormones also ADH and oxytocin released from the posterior pituitary gland are example of non tropic hormone. B. Posterior pituitary gland (neurohypophysis) The posterior pituitary gland stores and releases two hormones—antidiuretic hormone and oxytocin. Both hormones are actually produced in the hypothalamus and then passed down to be stored in the posterior pituitary gland.. Antidiuretic hormone (ADH ) - accelerates water reabsorption from urine in the kidney tubules into the blood, thereby decreasing urine secretion. Hyposecretion causes diabetes insipidus, characterized by excessive volume of urine Oxytocin (OT) – o Oxytocin stimulates contraction of the smooth muscle of the pregnant uterus and is believed to initiate and maintain labor. o Causes glandular cells of the breast to release milk into ducts (“milk let down”). o Oxytocin is also thought to enhance social bonding—a function helpful in supporting the mother- infant bond Hypothalamus
thus helps maintain homeostasis of blood calcium. It prevents hypercalcemia (excess calcium in blood). Hyperthyroidism or oversecretion of thyroid hormones dramatically increases the metabolic rate. Food material is burned by the cells at an excessive rate, and individuals who suffer from this condition lose weight, have an increased appetite, and show signs of nervous irritability. They appear restless, jumpy, and excessively active. Many patients with hyperthyroidism also have very prominent, almost protruding eyes—a condition called exophthalmos Graves disease is an inherited form of hyperthyroidism Hypothyroidism or under secrerion of thyroid hormones may result from different conditions o Goiter —painless enlargement of thyroid caused by inadequate production of thyroxin by thyroid gland due to dietary deficiency of iodine. The use of iodized salt has dramatically reduced the incidence of goiters caused by low iodine intake. To produce a goiter, the gland enlarges in an attempt to compensate for the lack of iodine in the diet necessary for the synthesis of thyroid hormones. o Cretinism : Hyposecretion of thyroid hormones during the growing years leads to a condition called cretinsm. It is characterized by a low metabolic rate, retarded growth and sexual devel- opment, and often, mental retardation. o Myxedema : Later in life, deficient thyroid hormone secretion produces the disease called myxedema. The low metabolic rate that characterizes myxedema leads to lessened mental and physical vigor, weight gain, loss of hair, and an accumulation of mucous fluid in the subcutaneous tissue that is often most noticeable around the eyes Fig. Exophthalmos
Fig: Goiter Parathyroid Glands The parathyroid glands are small glands. There are usually four of them, and they are found on the back of the thyroid gland. The parathyroid glands secrete parathyroid hormone (PTH). Function of hormone —PTH increases the concentration of calcium in the blood thus maintains the calcium balance in body—the opposite effect of the thyroid glands calcitonin. PTH stimulates bone- resorbing cells or osteoclasts to increase their breakdown of bone's hard matrix, a process that frees the calcium stored in the matrix. The released calcium then moves out of bone into blood and this in turn increases the blood's calcium concentration. Disorders o Hypersecretion causes possible hypercalcemia o Hyporsecretion causes possible hypocalcemia. Hypo function of parathyroid gland cause a condition called tetany resulting in low levels of calcium that affects nerves leading to twitching and spasm of respiratory muscles. Adrenal Glands The two adrenal glands are located on top of each kidney. Each gland has two parts: the cortex and the medulla. The adrenal cortex is the outer part of the adrenal gland and is made up of glandular epithelium. The adrenal medulla is the inner part, and it is made up of secretory nervous tissue—much like the secretory nervous tissue of the posterior pituitary. Adrenocorticotropic hormone (ACTH) from the pituitary glands stimulates the activity of the cortex of the adrenal A. Adrenal cortex - Three different zones or layers of cells make up the adrenal cortex Three cell layers (zones) and the hormones they produced a. Outer layer— Mineralocorticoids (MCs)—chiefly aldosterone b. Middle layer— Glucocorticoids (GCs)—chiefly cortisol (hydrocortisone) c. Inner layer—secretes sex hormones (small amounts of male hormones (androgens) resemble testosterone secreted by adrenal cortex of both sexes)
conducted by sympathetic nerve fibers stimulate the adrenal medulla. When stimulated, it literally squirts epinephrine and norepinephrine into the blood. As with glucocorticoids, these hormones may help the body resist or avoid stress. In other words, these hormones produce the body’s “fight-or-flight” response to danger (stress) as they prepare the body for an emergency situation. Adrenaline also causes bronchial relaxation, dilation of iris, increase basal metabolic rate, and increase blood pressure. (In stress conditions, the hypothalamus acts on the anterior pituitary gland to cause the release of ACTH, which stimulates the adrenal cortex to secrete glucocorticoids. In addition, the sympathetic subdivision of the autonomic nervous system is stimulated with the adrenal medulla, so the release of epinephrine and norepinephrine occurs to assist) Adrenal abnormalities o Hypersecretion of glucocorticoids causes Cushing syndrome : characterized by moon face, hump on back ( buffalo hump) , elevated blood sugar levels, frequent infections. o Hypersecretion of adrenal androgens may result from a virilizing tumor and cause masculinization of affected in women (beard growth, development of body hair, and increased muscle mass) o Hyposecretion of cortical hormones may result in Addison disease : characterized by muscle weak ness, reduced blood sugar, nausea, loss of appetite, and weight loss Pancreas (Pancreatic Islets) The pancreas is posterior to the stomach. The pancreatic islets, or islets of Langerhans of pancreas produce insulin and glucagon. Names of hormones
- Glucagon—secreted by alpha cells of islets of Langerhans
- Insulin—secreted by beta cells islets of Langerhans. Functions of hormones o Glucagon increases the blood glucose level by accelerating glycogenolysis in liver (conversion of
glycogen to glucose) o Insulin decreases the blood glucose by accelerating the movement of glucose out of the blood into cells, which increases glucose metabolism by cells. Insulin is the only hormone that can decrease blood glucose concentration. Other hormones, however, increase its concentration, including glucocorticoids, growth hormone, and glucagon. o Glucagon and insulin are antagonists of each other. Disorders Diabetes mellitus o Type 1DM results from hyposecretion of insulin o Type 2 DM results from target cell insensitivity to insulin preventing the normal effects of insulin on its target cells and thus also raising blood glucose levels In diabetes mellitus, excess glucose is filtered out of the blood by the kidneys and lost in the urine, producing the condition glycosuria (glucose in the urine). Other symptoms include polyuria and polydipsia (increase thirst). Gonads or Sex Glands
Female Sex Glands (Ovaries)
The ovaries contain two types of glandular structures that secrete sex hormones— the ovarian follicles and the corpus luteum o Ovarian follicles/Graafian follicle are little pockets in which egg cells, or ova, develop. It also secrete estrogen (sex hormone), the "feminizing hormone." o The corpus luteum chiefly secretes progesterone but also some estrogen. Effects of estrogen (feminizing hormone) o Development and maturation of breasts and external genitals (reproductive organs) o Development of adult female body contours and secondary sex characteristics. Effects of progesterone o Builds the lining of uterus for the fertilized egg and then maintain the pregnancy. o Initiation of menstrual cycle
Male Sex Glands (Testes)
The interstitial cells of testes secrete the male sex hormone testosterone. Effects of testosterone (masculinizing hormone) a. Maturation of external genitals b. Beard growth c. Voice changes at puberty d. Development of musculature and body contours typical of the male Thymus The thymus is located in the mediastinum. Like the adrenal gland, the thymus has a cortex and medulla. Both portions produced T lymphocytes (white blood cells). Name of hormone —thymosin Function of hormone —plays an important role in the development and function of the body's immune system Placenta The placenta functions as a temporary endocrine gland. During pregnancy, it produces human chorionic
