There are two regulatory systems in the body that control cellular functions: the NERVOUS SYSTEM and the ENDOCRINE SYSTEM. See Table 22.1. The nervous system controls muscular contractions and glandular secretions by conducting nerve impulses down nerve fibers. Nerve fibers tend to bring about their effects very rapidly, within a few milliseconds.
The endocrine system consists of endocrine glands. Endocrine glands secrete a chemical messenger called a hormone into the bloodstream. The blood carries the hormone to its target organ. The target organ's cells, called target cells, have specialized hormone receptors on the plasma membrane. Once a hormone reaches its target organ, it leaves the bloodstream and binds to its specific receptor on the target cell's plasma membrane. This causes changes in the physiological activities of the target cell. Read the Functions of Hormones on page 660.
Tissue or organ responses to hormones may occur within minutes, or it may take hours or even days for the target cells to respond. But once the response is initiated, the responses tend to be much more prolonged than those initiated by the nervous system. Nervous system effects, on the other hand, are generally brief.
The endocrine system and nervous system do not function in isolation of each other. The endocrine system interacts with the nervous system to coordinate and integrate the activity of body cells.
ENDOCRINE GLANDS
1. PITUITARY GLAND: See FIG. 18.1 (page 545); FIGS. 22.2 & 22.3; and TABLES 22.2 & 22.3.
The PITUITARY GLAND actually consists of two structurally and functionally distinct parts: the ANTERIOR PITUITARY (adenohypophysis) and the POSTERIOR PITUITARY (neurohypophysis). See FIG. 22.2. The pituitary gland is located in the SELLA TURCICA of the SPHENOID bone of the cranium (FIG. 6.8, p. 144). The pituitary gland is attached to a region of the brain called the HYPOTHALAMUS by a stalk called the INFUNDIBULUM.
The production of hormones by the ANTERIOR PITUITARY is controlled by regulatory hormones from the HYPOTHALAMUS. Release of anterior pituitary hormones (FIG. 22.2) is stimulated by releasing hormones from the HYPOTHALAMUS and suppressed by inhibitory hormones from the HYPOTHALAMUS.
See Table 22.2 for the anterior pituitary hormones and their functions.
The regulatory hormones from the HYPOTHALAMUS travel to the ANTERIOR PITUITARY by a portal vein system. Know the pathway of blood flow through the HYPOPHYSEAL PORTAL VEIN SYSTEM and what occurs along the way:
CEREBRAL ARTERIAL CIRCLE (CIRCLE OF WILLIS) to the SUPERIOR HYPOPHYSEAL ARTERY to the PRIMARY PLEXUS (a capillary bed in the hypothalamus that picks up regulatory hormones from the hypothalamus) to the HYPOPHYSEAL PORTAL VEINS to the SECONDARY PLEXUS (second capillary bed in the anterior pituitary that releases hypothalamus regulatory hormones & picks up anterior pituitary hormones) to the ANTERIOR HYPOPHYSEAL VEINS to the INTERNAL JUGULAR VEIN (carries anterior pituitary hormones into general circulation).
The POSTERIOR PITUITARY stores and releases hormones that are made by the HYPOTHALAMUS. Since the posterior pituitary does not make its own hormones, it is not a true endocrine gland. The HYPOTHALAMUS contains specialized nerve cells (neurons) called NEUROSECRETORY CELLS. (see FIG. 22.3) The cell bodies of the NEUROSECRETORY CELLS are located in the HYPOTHALAMUS and these neurosecretory cell bodies produce the hormones oxytocin (OT) and antidiuretic hormone (ADH). When OT and ADH are needed by the body (see * below & Table 22.3), they are synthesized by the neurosecretory cell bodies and passed down the nerve fibers, which run down through the INFUNDIBULUM to the POSTERIOR PITUITARY. Nerve impulses that travel down the nerve fiber trigger the release of OT and ADH from the nerve endings into the posterior pituitary. The released hormones then move into the capillary bed (plexus) of the posterior pituitary and are carried away by the posterior hypophyseal veins, which drain into the internal jugular vein (FIG. 22.2). Notice that there is not a portal vein system between the hypothalamus and the posterior pituitary.
* Antidiuretic hormone (ADH) is released when the body is dehydrated. ADH acts on the collecting ducts that receive filtrate from the nephrons of the kidneys. ADH stimulates the collecting ducts to remove more water from the filtrate and return the water to the bloodstream. This increases blood volume and makes the urine more concentrated (containing less water).
* Oxytocin stimulates labor contractions. Its pharmaceutical version is called "pitocin". Oxytocin also stimulates milk ejection from the breasts when a baby nurses ("milk letdown").
2. THYROID: see FIGURE 22.4 & TABLE 22.4
The THYROID GLAND is a bilobed gland in the neck. Notice its relative position to the thyroid cartilage of the larynx and the trachea. The thyroid produces
thyroid hormones that regulate metabolism and the basal metabolic rate (how much energy you burn up performing basic bodily functions). See
Table 22.4 for specifics.
3. PARATHYROID: see FIGURE 22.5 & TABLE 22.5
The 4 small PARATHYROID GLANDS are embedded on the posterior side of the thyroid gland. Parathyroid hormone increases blood calcium and magnesium levels by increasing calcium and magnesium absorption from the GI tract, by increasing the number and activity of osteoclasts (what do they do?), and by increasing calcium reabsorption by the
nephrons of the kidneys.
4. ADRENAL GLANDS: see FIGURE 22.6 and TABLE 22.6
The 2 ADRENAL GLANDS are located on the superior surface of the kidneys. The adrenals are actually 2 glands in one: the ADRENAL CORTEX and the ADRENAL MEDULLA.
The ADRENAL CORTEX produces several steroid hormones, called corticosteroids. Mineralocorticoids increase blood sodium levels and decrease blood potassium levels. Glucocorticoids affect glucose metabolism. The adrenal cortex also secretes androgens (male sex hormones) that may contribute to sex drive and sexual arousal in women.
The ADRENAL MEDULLA secretes epinephrine ("adrenaline") & norepinephrine ("noradrenaline"). These hormones stimulate the "fight-or-flight response", which is the body's response to stress, fright, emergencies, and exercise. Epinephrine and norepinephrine work with the SYMPATHETIC NERVOUS SYSTEM (which we will discuss later) to bring on the effects of the fight-or-flight response (increased heart rate, increased respiration, increased blood flow to the brain and skeletal muscles, dilation of the pupils of the eyeballs, etc.).
5. PANCREAS: FIG. 22.7 and TABLE 22.7
Review on your own the location of the pancreas. Remember that the pancreas is an
exocrine gland as well as an endocrine gland. In FIG. 22.7 (b), notice the
exocrine cells surrounding the ducts into which they secrete the pancreatic juice used
for digestion into the duodenum of the small intestine.
Then endocrine cells are found in clusters called the ISLETS OF LANGERHANS (pancreatic islets). The alpha cells of the ISLETS OF LANGERHANS produce glucagon, a hormone which raises blood glucose levels when blood sugar levels drop below normal. The beta cells of the ISLETS OF LANGERHANS produce insulin, which lowers blood glucose levels when they are higher than normal. One of the target organs of insulin and glucagon is the liver (see the functions of the liver in your digestive system notes).
6. OVARIES: See FIGS. 26.15 (p. 799), 26.18 (p. 801), & 26.26 (p. 811), and
TABLE 22.8.
Recall that the OVARIAN FOLLICLES produce estrogens and the CORPUS LUTEUM produces
estrogens & progesterone. Estrogens and progesterone develop and maintain female sexual characteristics, regulate the menstrual cycle, prepare the endometrium of the uterus for embryo implantation, and maintain pregnancy. See
Table 22.8 for other functions.
7. TESTES: See FIGS 26.3 (p. 784), 26.5 (p. 787), & 26.6 (p. 788), and TABLE
22.8.
The INTERSTITIAL ENDOCRINOCYTES of the testes produce testosterone, which develops and maintains male sexual characteristics, and regulates sperm production.
8. PINEAL GLAND: See FIG. 18.1 (p. 545), FIG. 22.1, and page 674.
The PINEAL GLAND is located in the brain, posterior to the hypothalamus. The pineal gland produces a hormone called
melatonin. The release of melatonin is controlled by daily dark and light cycles. When more light enters the eyes, melatonin production drops. Increased levels of melatonin are produced during darkness, so melatonin is thought to control the body's biological clock (day-night cycles) and to promote sleepiness. Increased production of melatonin during the longer dark hours in winter may contribute to SAD (Seasonal Affective Disorder), a form of wintertime depression.
9. THYMUS GLAND: see FIG. 1.7 (p. 13) & FIG. 15.5 (p. 480)
The THYMUS GLAND produces hormones that promote the proliferation (cell division) and maturation of
T cell lymphocytes. These hormones may also slow down the aging process.
10. Hormones are produced by cells found in other organs in the body. See Table 22.9.
For example, the hormone GASTRIN is secreted by G cells located in the STOMACH MUCOSA (FIG. 24.11, p. 730). Gastrin stimulates the secretion of gastric juice by the stomach and increases peristaltic smooth muscle contractions of the GI tract. The enteroendocrine cells of the SMALL INTESTINE (FIG. 24.17, p. 741) also produces hormones that stimulate the secretion of pancreatic juice and bile, as well as the release of bile by the gallbladder.
The KIDNEYS produce a hormone called erythropoietin, that stimulates red blood cell production by red bone marrow.
The PLACENTA in the uterus of a pregnant woman is an organ that has endocrine functions. Early in pregnancy, the placenta produces
human chorionic gonadotropin (hCG), which maintains the CORPUS LUTEUM so that the corpus luteum will produce
estrogen and progesterone during pregnancy. (Where is the corpus luteum located?) Later in pregnancy, the PLACENTA takes over the production of
estrogen & progesterone. Estrogens & progesterone maintain the endometrium of the uterus during pregnancy and prepare the mammary glands to produce milk.