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Transcript
SCIENCE CHEMICAL REGULATION Ch. 18 - CLASSWORK NAME________________________ DATE_________________________ PERIOD_______________________ CHEMICAL REGULATION - HORMONES In Chapter 17 we saw the many ways in which the nervous system controls the activities of the body. In this chapter we are going to discuss the endocrine system, which regulates many other body functions. The endocrine system consists of a set of glands, which are located at various sites in the body (see Figure 18-1). Each of these glands secretes one or more substances called hormones, which affect many metabolic processes. In general, the hormones are involved in the regulation of growth and development and maintenance of a constant internal environment. The secretions of the endocrine glands are released directly into the bloodstream. These secretions do not pass through ducts like the secretions of digestive glands. Thus the endocrine glands are sometimes referred to as the ductless glands. 1 The endocrine and nervous systems do not function independently of each other. Instead, the proper functioning of the body depends on the balanced and coordinated interaction of these two systems. The nervous system makes rapid adjustments in response to various types of stimuli. The endocrine system controls processes that require prolonged stimulation. Hormones are found in many different groups of organisms besides man and other vertebrates. They are found in plants and in many invertebrates. HORMONES. Hormones include a variety of types of chemical compounds. These substances are released from the endocrine glands directly into the bloodstream, which carries them to distant organs where they exert their effect. The tissue acted on by a given hormone is called its target tissue. In the nervous system, impulses from a given fiber can affect only a limited number of other cells. The hormones, on the other hand, are carried to many parts of the body in the blood. The action of some hormones is very specific, affecting only one type of tissue. Other hormones act on many different types of tissues. Hormones, like enzymes, regulate the rates of metabolic reactions. Only very small quantities of hormone are necessary to affect a reaction. However, these substances are rapidly inactivated in the body, so that the level of secretion requires constant adjustment. Exactly how hormones work is not known. They may act by controlling the enzyme systems involved in given reactions. Another possibility is that hormones may alter the permeability of cell membranes. Still a third possibility is that hormones may act on DNA, which in turn controls the synthesis of enzymes and other proteins. NEGATIVE FEEDBACK. Before we discuss the functioning of the endocrine system, there is a concept you should understand--this is negative feedback. It is a basic principle involved in control systems. An automatic thermostat is a common type of control system using negative feedback. You set the thermostat at a desired temperature, say 70 degrees. When the room temperature drops below 70 degrees, the thermostat turns on the furnace. When the temperature rises to 70 degrees, the thermostat automatically turns off the furnace. The condition regulated by the thermostat is the temperature, and information about the temperature is continually fed back to the thermostat. The system is called negative feedback because the final conditions and the response to them were opposite to the initial conditions and initial response. That is, the high temperature turned off the furnace, while low temperatures turned it on. Negative feedback is also involved in nervous control. We have already seen an example of this in Chapter 15 when we discussed the effects of increased carbon dioxide levels in the blood. In this case, increases in carbon dioxide concentrations stimulate the breathing center of the brain, which in turn increases respiratory rate and depth. This reduces carbon dioxide levels and increases the oxygen concentration. With the drop in blood carbon dioxide, the activity of the breathing center returns to normal and so does breathing. The secretion of hormones is generally controlled by negative feedback. As a simple example, we will use two hormones, A and B, secreted by organs A and B. 2 Release of hormone A into the bloodstream stimulates production and secretion of hormone B. When the level of hormone B in the blood reaches a certain point, it inhibits the further production of hormone A. Without hormone A, secretion of hormone B stops. When the blood level of hormone B drops below a certain point, production of hormone A resumes. In this simple example we showed negative feedback using two hormones. However, in the body endocrine activity is also affected by substances other than hormones. Through this cycle, the proper concentrations of the hormones are maintained. It also allows the body to manufacture these substances only when they are actually needed. ENDOCRINE GLANDS. We are only going to discuss the major endocrine organs of the body here. However, you should know that hormones are produced by the stomach and intestines. Also, the thymus gland in the chest and the pineal body in the head may also have endocrine functions, but relatively little is known about them. 1. PITUITARY GLAND. The pituitary gland, located under the brain, secretes many different hormones. It had often been called the "master gland" of the body because its hormones regulate such a wide variety of activities. Some pituitary hormones, including ACTH, gonadotropic hormones, and thyrotropic hormone, regulate the activity of other endocrine glands. Others, such as growth hormone, exert numerous effects on many different body tissues. If too much or too little of a hormone is secreted for any length of time, an abnormal condition will develop. For example, if too much growth hormone is present during childhood, the growth of bones and muscles is overstimulated. A person suffering from this condition may develop into a giant, reaching a height of 9 feet. If too little growth hormone is present, normal growth does not occur, and the person will be a dwarf. The secretion of thyrotropic, or thyroid-stimulating hormone (TSH), provides a good example of the regulation of endocrine activity by negative feedback. TSH secreted by the pituitary is carried in the blood to the thyroid gland, where it stimulates the secretion of the thyroid hormone, thyroxin. When the concentration of thyroxin in the blood reaches a certain level, it affects the pituitary in such a way that further secretion of TSH is inhibited. When the concentration of thyroid hormone in the blood decreases below a certain level, the pituitary once again secretes TSH. Among the other pituitary hormones, adrenocorticotropic hormone, ACTH, stimulates the production of hormones by the adrenal gland; the gonadotropic hormones are involved in reproduction; and antiduretic hormone, ADH, controls the reabsorption of water by the kidneys and affects blood pressure. 3 2. THYROID GLAND. The secretions of the thyroid gland control the rate of metabolism of the body. More specifically, thyroid hormone controls the rate of cellular respiration--the process by which the body metabolizes glucose for the release of energy. Because this hormone affects almost all the tissues in the body, any pronounced increase or decrease in its secretion has serious consequences. Both physical and mental health are affected. In children, whose tissues are rapidly growing and developing, too little thyroid hormone can result in cretinism. This condition is marked by mental retardation, a dwarflike appearance, a protruding abdomen, wrinkled skin, and deformed bones. If it is diagnosed before the damage becomes permanent, cretinism can be successfully treated by the administration of thyroid hormone. One of the basic ingredients in thyroid hormone is iodine. If iodine is lacking in the diet, the thyroid cannot produce adequate amounts of thyroid hormone. With continued stimulation, the thyroid becomes enlarged. The resulting condition, which is marked by a swelling of the front of the neck, is called a goiter. Although enlargement of the thyroid can also result from other conditions, simple goiter caused by an iodine deficiency is treated by increasing the amount of iodine in the diet. In the past there were certain parts of the world that were known as "goiter belts" because so many people in those regions suffered from goiter. It was found that soils in these areas, which were generally inland or in the mountains, had been depleted of iodine. When iodine was added to the diet, the goiters disappeared. Iodine is now routinely added to table salt, so that iodine deficiency has become relatively rare. Seafood is also an excellent source of iodine. 4 Embedded in the thyroid gland are two pairs of small endocrine glands called the parathyroids. They secrete the hormone parathormone, which is involved in calcium and phosphorus metabolism. 3. PANCREAS. Scattered throughout the pancreas are small masses of cells called the islands of Langerhans. These cells produce two hormones, insulin and glucagon. The effect of insulin is to lower the concentration of sugar in the blood, while the effect of glucagon is just the opposite. It raises the blood sugar level. Along with other hormones, insulin and glucagon regulate carbohydrate metabolism. Insulin stimulates the breakdown of glucose to carbon dioxide and water, the conversion of glucose to glycogen, and the conversion of glucose to fat. Thus, it works in three ways to keep the blood sugar level within a normal range. A deficiency of insulin results in a condition called diabetes. In diabetes the body cannot metabolize glucose normally. After a meal, the blood sugar level rises abnormally. The sugar level may be so high that some sugar is excreted in the urine. This does not occur under normal conditions, so that the presence of sugar in the urine is often a symptom 5 of diabetes. Without adequate insulin the body cannot obtain all the energy that it requires from the breakdown of glucose. Thus, body fat is broken down for energy. This disturbance of normal fat metabolism results in high levels of fats in the blood. The products of fat breakdown under these circumstances seriously upset the blood biochemistry, and are dangerous. Diabetes can be controlled by observing a strict diet and by the administration of insulin, if it is needed. The tendency to develop diabetes seems to be hereditary. It is also more common among overweight people that among those of normal weight. 4. ADRENAL GLANDS. There are two adrenal glands in the body, one on top of each kidney. Each gland is made up of two distinct layers--an outer layer called the cortex, and an inner layer called the medulla. Each layer secretes its own hormones. The two hormones of the adrenal medulla are epinephrine, or adrenalin, and norepinephrine, or noradrenalin. Although epinephrine has powerful effects when introduced into the body as a drug, its role in normal metabolism is not essential for life. Epinephrine increases the heart rate, raises blood pressure, increases the level of blood sugar, and produces gooseflesh. These effects often occur in times of sudden stress, such as fear or surprise. However, exactly the same effects can be caused by stimulation of part of the nervous system. Because of its stimulatory effects on the circulatory system, epinephrine is useful in treating shock, heart stoppage, and similar conditions. Unlike the adrenal medulla, the adrenal cortex is essential for life. It produces about thirty different hormones, which belong to a class of compounds called steroids. As a group, these hormones are known as the 6 adrenocortical steroids, and they have important metabolic effects in the body. Their production is stimulated by ACTH from the pituitary. A deficiency of these hormones affects the metabolism of carbohydrates, proteins, salts, and water. Skin, bone, cartilage, lymphatic structures, bone marrow, and many other tissues are affected. 5. GONADS. As part of their role in reproduction, the gonads---the testes of the male and the ovaries of the female---also function as endocrine organs. The testes secrete the hormone testosterone. Testosterone is necessary for normal reproductive functions, and it also produces and maintains the male secondary sex characteristics. These are the traits that we normally associate with maleness, such as the low voice, facial beard and body hair, and heavier muscular development. The ovaries produce two hormones---estrogen and progesterone. Both are involved in the regulation of the female reproductive cycle Estrogen regulates the female secondary sex characteristics---high voice, development of breasts, and so on. 7