Bisc 102 - Chapter 25: The Endocrine System
Bisc 102 - Chapter 25: The Endocrine System Bisc 102
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25.1 The Endocrine System Uses Hormones to Communicate - An animal’s body has two main communication systems. The n ervous system, described in chapter 24, is a network of cells that specialize in sending speedy signals that vanish as quickly as they arrive. The e ndocrine system is the other main communication system. As this chapter explains, the endocrine system does not act with the speed of neural impulses, but its chemical message have something else: staying power. - The endocrine system has two main components: glands and hormones. An e ndocrine gland consists of cells that produce and secrete hormones into the bloodstream, which carries the secretions throughout the body. A h ormone is a biochemical that travels in the bloodstream and alters the metabolism of one or more cells. - Instead, a limited selection of arget cells respond to each hormone. Inside or on the surface of each target cell is a receptor protein, which binds to the hormone and initiates the cell’s response. 25.2 Hormones Stimulate Responses in Target Cells - The term t arget cells is a little misleading, because it implies that hormones somehow travel straight from their source to a limited act of cells. In reality, the blood circulating throughout the body contains many hormones at once. - The receptors may occur on the target cell’s surface or inside the cytoplasm. In general, receptors for water-soluble hormones are on the surface of the target cell. In contrast, lipid-soluble hormones typically interact with receptors inside cells. Water-Soluble Hormones Trigger Second Messenger Systems - Most water-soluble hormones are either proteins or short chains of amino acids (“peptide hormones”). Because these hormones are water-soluble, they cannot pass readily through the cell membrane. Instead, they bind to receptors on the surface of target cells. - The hormone receptor interaction triggers a cascade of chemical reactions within the cell. The product of this chain reaction is a s econd messenger, which is the molecule that actually provokes the cell’s response. The second messenger typically activates the enzymes that produce the hormone’s effects. For example, one hormone may stimulate the target cell to transport stored proteins to its outer membrane, another might activate the enzymes needed to cleave stored glycogen into its glucose subunits. - Whatever the outcome, the entire cascade of reactions converts the external “message” - the arrival of the hormone at the outer membrane - into a signal inside the cell. Lipid-Soluble Hormones Directly Alter Gene Expression - Some hormones are lipid-soluble. The most familiar are the s teroid hormones, such as testosterone and estrogen. The body synthesizes these and other steroid hormones from cholesterol, which is one reason humans need at least some cholesterol in their diets. Two other lipid-soluble hormones, the thyroid hormones, are derived from a single amino acid. - Unlike protein or peptide hormones, the lipid-soluble hormones easily cross the cell membrane; no second messenger is involved. Once inside the cell, the hormone may enter the nucleus and bind to a receptor associated with DNA, triggering the production of proteins that carry out the target cell’s response. Alternatively, the hormone may bind to a receptor in the cytoplasm, and the two molecules may travel together to the nucleus. Either way, response time is much slower than for water-soluble hormones, because the cell must produce new proteins before the hormone takes effect. 25.3 The Hypothalamus and Pituitary Gland Oversee Endocrine Control - The almond-sized hypothalamus is a part of the brain, and the p ituitary gland is a pea-sized structure attached to a stalk extending from the hypothalamus. The pituitary is really two glands in one: the larger a nterior pituitary (toward the front) and the smaller posterior pituitary (toward the back). Anatomically, the posterior pituitary is a continuation of the hypothalamus, whereas the anterior pituitary consists of endocrine cells. - Note that the posterior pituitary does not synthesize hormones of its own, but it does store and release two of the hormones produced in the hypothalamus. The anterior pituitary does produce hormones, but it takes orders from the hypothalamus. That is, hormones from the hypothalamus enter a specialized system of blood vessels leading directly to the anterior pituitary; these hormones, in turn, regulate the release of the anterior pituitary hormones. The Posterior Pituitary Stores and Releases Two Hormones - One of the two hormones produced by the hypothalamus and released by the posterior pituitary is antidiuretic hormone (ADH), called vasopressin. If cells in the hypothalamus detect that the body’s fluids are too concentrated, the posterior pituitary releases more ADH. This hormone stimulates kidney cells to return water to the blood (rather than eliminating the water in urine). The body’s fluids become more dilute. Once balance is restored, ADH production slows. - Oxytocin is the other posterior pituitary hormone. When a baby suckles, sensory neurons in the mother’s nipple relay the information to the brain, which stimulates the release of oxytocin. The hormone causes cells in the breast to contract, squeezing the milk through ducts leading to the nipple. Oxytocin also triggers muscle contraction in the uterus, which pushes a baby out during labor. Physicians use synthetic oxytocin to induce labor or accelerate contractions in a woman who is giving birth. The Anterior Pituitary Produces and Secretes Six Hormones - One of the six hormones that the anterior pituitary gland produces is growth hormone (GH). This hormone promotes growth and development in all tissues by increasing protein synthesis and cell division rates. Levels of GH pea in the preteen years and help spark adolescent growth spurts. A severe deficiency of GH during childhood leads to pituitary dwarfism, which is associated with extremely short stature; at the other extreme, a child with too much GH becomes a pituitary giant. In an adult, GH does not affect height because the long bones of the body are no longer growing. However, excess GH can cause acromegaly, a thickening of the bones in the hands and face. - Prolactin is an anterior pituitary hormone that stimulates milk production in a woman’s breasts after she gives birth. In males and in women who are not breast feeding, a hormone from the hypothalamus suppresses prolactin synthesis. In nursing mothers, however, a sucking infant triggers nerve impulses that overcome this inhibition. - Four other anterior pituitary hormones all influence hormone secretion by other endocrine glands. Thyroid-stimulating hormone (TSH) prompts the thyroid gland to release hormones, whereas adrenocorticotropic hormone (ACTH) s timulates hormone release from parts of the adrenal glands. The remaining two anterior pituitary hormones stimulate hormone release from the ovaries and testes; f ollicle-stimulating hormone (FSH) and luteinizing hormone (LH). - The anterior pituitary also produces e ndorphins, which are natural painkillers that bind to receptors on target cells in the brain. 25.4 Hormones from Many Glands Regulate Metabolism - The thyroid gland, parathyroid glands, adrenal glands, and pancreas secrete hormones that influence metabolism. Hormones from the anterior pituitary control many, but not all, of the activities of these glands. The Thyroid Gland Sets the Metabolic Pace - The thyroid gland is a two-lobed structure in the neck. The lobes secrete two thyroid hormone, thyroxine and t riiodothyronine, that increase the rate of metabolism in target cells. (e.g. lungs exchange gases faster, small intestine absorbs nutrients more readily, fat levels in cells and in blood plasma decline) - The thyroid hormones illustrate how the hypothalamus and pituitary interact in negative feedback loops. When blood levels of thyroid hormones are low, the hypothalamus secretes thyrotropin-releasing hormone (TRH), which stimulates the anterior pituitary to increase production of thyroid-stimulating hormone (TSH). In response, cells in the thyroid secrete thyroxine and triiodothyronine. In the opposite situation, TRH secretion slows, so the thyroid glands reduce their production of hormones. - One disorder that affects the thyroid gland is hypothyroidism, a condition in which the thyroid does not release enough hormones. The metabolic rate slows, and weight increases. Synthetic hormones can treat many cases of hypothyroidism. In the past, the most common cause of hypothyroidism was iodine deficiency. Both thyroid hormones contain iodine; a deficiency of this essential element causes a goiter, or swollen thyroid gland. Today iodine-deficient goiter is rare in nations where iodine is added to table salt. - An overactive thyroid causes hyperthyroidism. This disorder is associated with hyperactivity, an elevated heart rate a high metabolic rate, and rapid weight loss. - Scattered cells throughout the thyroid gland produce a third hormone, c alcitonin, which decreases blood calcium level by increasing the deposition of calcium in bone. The Parathyroid Glands Control Calcium Level - The parathyroid glands are four small groups of cells embedded in the back of the thyroid gland. When calcium-sensitive cells in the glands detect low blood calcium, the glands secrete parathyroid hormone (PTH). This hormone increases calcium levels in blood and tissue fluid by releasing calcium from bones and by enhancing calcium absorption at the digestive tract and kidneys. PTH action therefore opposes that of calcitonin. - Calcium is vital to muscle contraction, blood clotting, bone formation, and the activities of many enzymes. Underactivity of the parathyroids can therefore be fatal. Excess PTH can also be harmful if calcium leaves bones faster than it accumulates. This condition, called osteoporosis, is most common in women who have reached menopause (cessation of menstrual periods). The estrogen decrease that accompanies menopause makes bone-forming cells more sensitive to PTH, which depletes bone mass. The Adrenal Glands Coordinate the Body’s Responses - The paired, walnut-sized a drenal glands sit on top of the kidneys (ad- means near r means kidney). Each adrenal gland is divided into two regions, which are controlled in different ways and secrete different hormones. The adrenal medulla, releases its hormones when stimulated by the sympathetic nervous system (which mediates the body’s overall “fight or flight” response). The a drenal cortex is the outer portion, and it is under endocrine control. - The adrenal medulla’s hormones, e pinephrine (adrenaline) and n (noradrenaline), help the body respond to exercise, trauma, fear, excitement, and other short-term stresses. These water-soluble hormones cause heart rate and blood pressure to climb. In addition, the airway increases in diameter, making breathing easier. The metabolic rate increases, while digestion and other “nonessential” processes slow. - Unlike the adrenal medulla, the adrenal cortex secretes steroid hormones, including mineralocorticoids maintain blood volume and salt balance. One example, aldosterone, stimulates the kidneys to return sodium ions and water to the blood while excreting potassium ions. This action conserves water and increases blood pressure, which is especially important in compensating for fluid loss from severe bleeding. - Glucocorticoids are essential in the body’s response to prolonged stress. Cortisol is the most important glucocorticoid. This hormone mobilizes energy reserves by stimulating the production of glucose from amino acids. Glucocorticoids also indirectly constrict blood vessels, which slows blood loss and prevents inflammation after an injury. These same effects, however, also account for the unhealthy effects of chronic stress. Narrowed blood vessels can lead to heart attacks, and the suppressed immune system leaves a person vulnerable to illness. The Pancreas Regulates Blood Glucose - The pancreas is an elongated gland, about the size of a hand. Located beneath the stomach and attached to the small intestine. Clusters of cells in the pancreas secrete insulin and glucagon, two hormones that regulate the use of glucose. - Insulin and glucagon oppose each other in regulating blood glucose levels. After a meal rich in carbohydrates, glucose enters the circulation at the small intestine. The resulting rise in blood sugar triggers specialized cells in the pancreas to secrete i stimulates cells throughout the body to absorb glucose from the bloodstream. The cells may then consume the glucose in cellular respiration to generate energy or use it as a reactant in other metabolic reactions. Liver cells also absorb glucose and store it as glycogen. As cells take up sugar, the blood glucose concentration declines, and insulin secretion slows. If blood sugar dips too low, however, other cells in the pancreas secrete glucagon, which stimulates target cells in the liver to break down stored glycogen and release glucose into the bloodstream. Too Much Glucose in Blood: Diabetes - Failure to regulate blood sugar can be deadly. In d iabetes glucose accumulates too dangerously high in levels in the bloodstream. - Symptoms of diabetes include frequent urination, excessive thirst, extreme hunger, blurred vision, weakness, fatigue, irritability, nausea, and weight loss. - The accumulation of blood sugar can occur for multiple reasons, but two forms of diabetes are most common. In type 1 diabetes, the pancreas fails to produce insulin, so the body’s cells never receive the signal to “open the door” and admit glucose. In type 2 diabetes, the body’s cells fail to absorb glucose even when insulin is present; this condition is called insulin resistance. In type 2 diabetes, the insulin “rings the doorbell”, but the cell never opens the door. Either way, the body’s cells starve for lack of glucose. - Fifteen percent of affected individuals have type 1 diabetes, which usually begins in childhood or early adulthood. Typically, the underlying cause is an autoimmune attack on cells of the pancreas, which therefore cannot produce insulin. Type 1 diabetes is sometimes also called insulin-dependent diabetes because insulin injections can replace the missing hormone. - Type 2 diabetes is much more common. Although it usually begins in adulthood, the incidence of type 2 diabetes among both adults and children is rising in developed countries (including the United States). This disease is strongly associated with obesity; nearly all type 2 diabetes patients are overweight. The exact cause-effect relationship between obesity and type 2 diabetes, however, is unclear. Not Enough Glucose in Blood: Hypoglycemia - The opposite of diabetes is hypoglycemia, in which excess insulin production or insufficient carbohydrate intake causes low blood sugar. A person with this condition feels weak, sweaty, anxious, and shaky; in severe cases, hypoglycemia can cause seizures or loss of consciousness. Frequent, small meals low in sugar and high in protein and complex carbohydrates can prevent insulin surges and help relieve symptoms of hypoglycemia. The Pineal Gland Secretes Melatonin - The pineal gland, a small brain structure near the hypothalamus, produces the hormone melatonin. Darkness stimulates melatonin synthesis in the pineal gland, whereas exposing the eye to light inhibits melatonin production. The amount of melatonin in blood therefore “tells” the other cells of the body how much light the eyes are receiving. This interaction, in turn, sets the stage for the regulation of sleep - wake cycles and other circadian rhythms. 25.5 Hormones from the Ovaries and Testes Control Reproduction - The reproductive organs include the o varies in females and the t estes in males. These egg- and sperm-producing organs secrete the steroid hormones that enable these gametes to mature. In addition, hormones from the ovaries and testes promote the development of secondary sex characteristics, which are features that differentiate the sexes but do not participate directly in reproduction. - In a woman of reproductive age, the levels of several sex hormones cycle approximately every 28 days. The hypothalamus produces a hormone that stimulates the anterior pituitary to release FSH and LH into the bloodstream. At target cells in the ovary, these two hormones trigger the events that lead to the release of an egg cell. Meanwhile, the cells surrounding the egg produce the sex hormones e strogen and progesterone, which exert negative feedback control on both the hypothalamus and the pituitary. Estrogen also promotes development of the female secondary sex characteristics, such as breasts and wider hips, whereas progesterone helps prepare the uterus for pregnancy. - Sperm production is completed under the influence of LH, which also prompts cells in the testes to release the sex hormone t estosterone. This hormone stimulates the formation of male structures in the embryo and promotes later development of male secondary sex characteristics, including facial hair, deepening of the voice, and increased muscle growth.