Positive Feedback
Positive feedback is a reaction in which the system responds in such a way as to speed up the direction of change. Positive feedback mechanisms are not as common as negative feedback mechanisms because they cause an increase in the initial signal, which would tend to knock many systems out of balance. Take for example, the analogy of the home heating system. If this system were to work on a positive feedback loop, the furnace would not switch off when the temperature reached the set point of 21˚C. Instead, it would keep going and heat the room indefinitely.
An example of a positive feedback mechanism is milk production by a mother for her baby, as shown in Figure below. As the baby suckles, nerve messages from the mammary glands cause the hormone prolactin, to be secreted by the mother’s pituitary gland. The more the baby suckles, the more prolactin is released, which stimulates further milk production by the mother’s mammary glands. In this case, a negative feedback loop would be unhelpful because the more the baby nursed, the less milk would be produced. Another example of a positive feedback loop is the blood-clotting cascade that happens after a blood vessel is cut.
Figure 20.52
Production of breast milk is controlled by a positive feedback mechanism.
Hormone Antagonists
Many hormones work with hormone antagonists to control the concentrations of substances in the body. The hormones have opposite actions on the body and so are called antagonistic.
Insulin and glucagon make up an antagonistic hormone pair. The action of insulin is opposite that of glucagon. For example, your blood glucose concentration rises sharply after you eat food that contains simple carbohydrates, such as the blueberry muffins shown in Figure below. The increase in blood glucose level stimulates beta cells in the pancreas to release insulin into blood. In response to signals by insulin most body cells take up glucose, which removes it from the blood, and the blood glucose concentration returns to the set point. Later, you have missed eating lunch, you are hungry and feel a little light-headed. Your blood glucose concentration has dropped below the set point, which causes the release of glucagon from the pancreas. Glucagon causes the release of glucose from liver cells, which increases your blood-glucose concentration. If glucagon did not do its job correctly, your blood glucose concentration would continue to drop, and you would develop hypoglycemia (low blood sugar). This antagonistic relationship between the two hormones helps to maintain the narrow range of blood glucose concentration.
Figure 20.53
Insulin and glucagon work as an antagonistic pair to keep your blood glucose concentration within a narrow range even after you eat food containing carbohydrates, such as a muffin.
The actions of growth hormone releasing hormone (GHRH) are opposed by another hypothalamic hormone, somatostatin, also known as "growth-hormone-inhibiting hormone" (GHIH). Somatostatin and GHRH are secreted alternatively by the hypothalamus, which causes an increase and decrease in the secretion of growth hormone (GH) by the pituitary.
Many endocrine glands also work together as a group to control body processes. The major endocrine glands coordinate the control of various regulatory systems, such as metabolism, osmoregulation, and reproduction. Many individual glands are directly controlled by the nervous system, and all are in some way controlled by the pituitary and hypothalamus. Some of these glands and their hormone products are listed in Table below.
Coordination of the Endocrine Glands in the Control of Body Systems Function Organ or Glands Hormones Nervous System Control
Control of the Endocrine System Hypothalamus Pituitary Gland
Adrenocorticotropic hormone (ACTH)
TSH, FSH, LH, GH, prolactin (PRL)
ANS (sympathetic and parasympathetic nervous system)
Regulation of Metabolism Liver
Pineal Gland
Pancreas
Parathyroid Glands
Thyroid Gland
Melatonin
Glucagon
Insulin
Parathyroid hormone
T3 and T4
ANS (sympathetic and parasympathetic nervous system)
Response to Stress Adrenal Glands cortisol
norepinepherine,
Epinepherine,
ANS (sympathetic nervous system)
Reproduction Gonads (ovaries, testes) progestins (progesterone)
Androgens (testosterone), estrogens,
ANS (parasympathetic nervous system)
Osmoregulation Liver
Kidneys
Adrenal Glands
Angiotensin
ADH
Aldosterone
ANS (sympathetic and parasympathetic nervous system)
Homeostatic Imbalance: Endocrine System Disorders
Diseases of the endocrine system are common, and include diseases such as diabetes, thyroid disease, and obesity. An endocrine disease is usually characterized by hyposecretion or hypersecretion of hormones and an inappropriate response to hormone signaling by cells.
Cancer can occur in endocrine glands, such as the thyroid, and some hormones are involved in signaling distant cancer cells to multiply. For example, the estrogen receptor has been shown to be involved in certain types of breast cancers.
Hyposecretion
Hyposecretion is the production no hormone or too little of a hormone. It can be caused by the destruction of hormone-secreting cells, such as in Type 1 diabetes, or by a deficiency in a nutrient that is important for hormone synthesis. Hyposecretion can be treated with hormone-replacement therapies. Type 1 diabetes is an autoimmune disease that results in the destruction of the insulin-producing beta cells of the pancreas. A person with Type 1 diabetes needs insulin replacement therapy, usually by injection or insulin pump, in order to stay alive. An insulin pump is shown in Figure below.
Figure 20.54
In the treatment of Type 1 diabetes, an insulin pump is an alternative to multiple daily injections of insulin. The pump is usually used along with the monitoring of blood glucose concentration and carbohydrate intake.
Diabetes insipidus is characterized by excretion of large amounts of very dilute urine, even if liquid intake is reduced. It is caused by an inability of the kidney to concentrate urine due to a lack of antidiuretic hormone (ADH) also called vasopressin, or by an insensitivity of the kidneys to that hormone. Blood glucose levels are not affected in diabetes insipidus.
Growth hormone deficiency is caused by a lack of GH production by the pituitary. GH deficiency affects bone growth development, and people with growth hormone deficiency tend to have, among other things, low bone density and small stature, a condition called pituitary dwarfism. GH deficiency is treated by growth hormone replacement.
Hypothyroidism is the state in which not enough thyroid hormones are made. Thyroiditis is an autoimmune disease where the body's own antibodies attack the cells of the thyroid and destroy it. Thyroid hormones play an important role in brain development during fetal growth. Cells of the brain are a major target for the T3 and T4 hormones. As a result, hypothyroidism in children, either due to a thyroid problem from birth or a lack of iodine in the diet, is a major cause of physical and mental growth impairment in developing countries. In fact, iodine deficiency disorders are the single most common cause of preventable mental retardation and brain damage in the world.
Figure 20.55
There are many causes of goiter, but the most common is the world is iodine deficiency. Today, iodine deficiency remains mostly a problem in poorer countries which lack the means to add iodine to foods. Iodized salt has helped reduce the amount of iodine deficiency in the developed world. Governments of some countries add iodine to cattle feed to ensure that dairy foods will contain iodine.
Hypersecretion
Hypersecretion of a hormone happens when the body produces too much of a hormone. A hormone can be hypersecreted if the gland develops a tumor and grows out of control, or if the gland is signaled to produce too much of a hormone.
Hyperthyroidis
m is the result of excess thyroid hormone production, which causes an overactive metabolism and increased speed of all the body's processes. Hyperthyroidism is the most common cause of goiter in the developed world, shown in Figure above.
Hypersecretion of growth hormone causes acromegaly. A common cause of acromegaly is a benign tumor of the pituitary glands that releases too much GH. In some cases, acromegaly is also caused by overproduction of the hypothalamus hormone growth hormone release hormone (GHRH). Acromegaly most commonly affects middle-aged adults and can result in serious illness and premature death. Symptoms include enlarged hands and feet, protruding brow and chin, and enlarged internal organs. However, the disease is hard to diagnose in the early stages and is frequently missed for many years due to its slow progression. If the pituitary produced too much GH during childhood, the person will be taller than normal, a condition called pituitary gigantism. Pituitary gigantism is very rare, and some of the tallest people on record had this condition.
Hormone Insensitivity: Type 2 Diabetes
In some cases, the body makes enough hormones, but body cells do not respond. This can be due to missing or defective hormone receptors, or the body cells become resistant to normal concentration of the hormone, and do not respond to it.
Type 2 diabetes is characterized by hyperglycemia (high blood glucose concentrations), body cells that do not respond to normal amounts of insulin (insulin resistance), and the resulting inability of the pancreas to produce enough insulin. Insulin resistance in cells results in high amounts of free fatty acids and glucose in the blood. High plasma levels of insulin and glucose due to insulin resistance often lead to metabolic syndrome and Type 2 diabetes. Type 2 diabetes can be controlled by improving the diet, increasing levels of activity, and sometimes medication.
Gestational diabetes is a form of diabetes that affects pregnant women. There is no known single cause, but it's believed that the hormones produced during pregnancy reduce the ability of the cells in the pregnant woman's body to respond to insulin, which results in high blood glucose concentrations.
Hormones as Medicines
Many hormones and molecules like them are used as medicines. The most common type of therapy is called hormone-replacement therapy. The most commonly-prescribed hormones are estrogens and synthetic progesterone (as methods of hormonal contraception and as HRT therapy for post-menopausal women), thyroxine (as levothyroxine, for hypothyroidism) and corticosteroids (for autoimmune diseases and several respiratory disorders). Progestin, a synthetic progesterone, is also used to prolong pregnancy in women who have experienced a miscarriage due to a premature drop in progesterone levels. Hydrocortisone is a synthetic form of cortisol that is used to treat allergies and inflammation as well as cortisol production deficiencies. Hydrocortisone cream is a common over-the-counter medication for the topical treatment of rash. Insulin is used by many people with diabetes.
Epinephrine
Because of its anti inflammatory effect on the immune system, epinephrine is used to treat anaphylaxis. Anaphylaxis is a sudden and severe allergic reaction that involves the entire body. After an initial exposure to a substance like a certain food (such as peanuts), or bee sting, a person's immune system can becomes sensitized to that substance, which is called an allergen. Upon second exposure, an allergic reaction occurs.
Figure 20.56
An EpiPen epinephrine autoinjector. Auto injectors like this one can help save a persons life during an anaphylactic reaction.
Histamine and other substances that are released by body cells cause the blood vessels to dilate, which lowers blood pressure, and fluid to leak from the bloodstream into the tissues, which lowers the blood volume. The release of histamine causes the face and tongue to swell. Swelling of the lining of the throat can lead to breathing difficulties. The hormone epinephrine causes blood vessels to constrict which reduces swelling and causes blood pressure to increase. Epinephrine is used as a medicine in auto-injectors, shown in Figure above, which a person can use themselves should they have an anaphylactic reaction.
Anabolic Androgenic Steroids
Synthetic androgens, in the form of anabolic androgenic steroids (anabolic steroids), have many medical uses. It is used to stimulate bone growth and appetite, induce puberty in boys, and treat muscle-wasting conditions in patients that have diseases such as cancer and AIDS. In general, androgens, including testosterone, promote protein synthesis and the growth of muscle tissue and other tissues that have androgen receptors. Androgens also block the effects of the stress hormone cortisol on muscle tissue, so the breakdown of muscle is greatly reduced.
Figure 20.57
Athletes involved in sports that emphasize strength, weight, and shape may feel pressure to take anabolic steroids, however, the majority of school athletes do not take them.
Anabolic Steroid Abuse
As a result of their muscle-building action, anabolic steroids are used in sport and bodybuilding to increase muscle size and strength, to gain a competitive edge or to assist in recovery from injury. Steroids used to gain competitive advantage are forbidden by the rules of the governing bodies of many sports. Serious health risks can be produced by long-term use or excessive doses of anabolic steroids. Most of these side effects are dose dependent, the most common being an increase in low density lipoprotein (bad cholesterol), and a decrease in good high density lipoprotein (good cholesterol). Anabolic steroids also increase the risk of cardiovascular disease in men with high risk of bad cholesterol. Acne is fairly common among anabolic steroid users, mostly due to increases in testosterone which stimulates the sebaceous glands to produce more oil. High doses of anabolic steroids have been linked to liver damage.
Teenagers, particularly boys, who take anabolic steroids, are more likely to be involved in sports that emphasize weight and shape, (such as football or wrestling, which is shown in Figure above). Such teens also have higher rates of disordered eating, drug abuse, and generally have poorer attitudes towards health. Severe side effects can occur if a teenager uses anabolic steroids. For example, the steroids may prematurely stop the lengthening of bones, resulting in stunted growth. Other effects include, but are not limited to, accelerated bone maturation, increased acne outbreaks, and premature sexual development.
In addition to dangerous side effects of the steroids themselves, dangerous drug-taking habits that have been reported by abusers include: unsafe injection practices such as reusing needles, sharing needles, and sharing multidose vials. A common practice among anabolic steroid abusers is self–medicating with other hormones such as growth hormone and insulin, which in itself can lead to serious health consequences. Testosterone and other anabolic steroids are classified as a controlled substance in the United States (US), Canada, the United Kingdom (UK), Australia, Argentina, and Brazil.
Lesson Summary
The endocrine system is a system of organs that release hormones into the blood. Unlike the nervous system whose action helps the body react immediately to change, the endocrine system controls changes that happen to the body over a long period of time.
Hormones are made and secreted by cells in endocrine glands. The body produces many different hormones, but each hormone is very specific for its target cells.
The hypothalamus links the nervous system to the endocrine system by the pituitary gland. The pituitary gland secretes hormones that regulate homeostasis and also secretes hormones that stimulate other endocrine glands. Together the two glands serve as the major control centers of the endocrine system.
The ovaries of females and the testes of males are the gamete producing organs, or gonads. In addition to producing gametes, an exocrine action, the gonads are endocrine glands that produce steroid sex hormones.
The rate of hormone production and secretion is often regulated by homeostatic feedback control mechanisms, and the effect of hormones is controlled by hormone antagonists. In these ways, the concentration of hormones and their products is kept within a narrow range so as to maintain
life.
A feedback control mechanism, or a feedback loop, is a signaling system in which a product or effect of the system controls an earlier part of the system, either by shutting the process down or speeding it up.
Diseases of the endocrine system are common, and include diseases such as diabetes, thyroid disease, and obesity. Many hormones and hormone-like molecules are used as medicines. The most common type of therapy is called hormone-replacement therapy.
Review Questions
How does a hormone initially activate a target cell?
What is the main difference between the locations of the receptors for amino acid-based hormones and steroid-based hormones?
List five main endocrine glands, and identify their locations.
Name the two glands that control the nervous system.
Name three hormones that are involved in the stress response and identify their function.
Outline the role of the parathyroid glands in regulation of blood calcium levels.
What hormone is secreted by the pineal gland, and what is the function of the hormone?
How do feedback mechanisms help maintain homeostasis?
How does negative feedback differ from positive feedback?
CK-12 Biology I - Honors Page 99