CK-12 Biology I - Honors
Page 98
Thyroid and Parathyroid Glands
The thyroid is one of the largest endocrine glands in the body. This butterfly-shaped gland is found in the neck, wrapped around the trachea, as shown in Figure below. The hormones released by the thyroid control how quickly the body uses energy, makes proteins, and how sensitive the body should be to other hormones. The thyroid is controlled by the hypothalamus and pituitary. Thyroid hormone generally controls the pace of all of the processes in the body. This pace is related to your metabolism. If there is too much thyroid hormone, every function of the body tends to speed up. The thyroid gland regulates the body temperature by secreting two hormones that control how quickly the body burns calories. Hyperthyroidism (overactive thyroid) and hypothyroidism (under active thyroid) are the most common problems of the thyroid gland.
Figure 20.42
The position of the thyroid and parathyroid glands. A person can have more than four parathyroid glands.
The thyroid hormones thyroxine (T4) and triiodothyronine (T3) regulate the rate of metabolism and affect the growth and rate of function of many other systems in the body. As a result, problems with the under secretion or over secretion of thyroid hormones affect many body systems.
The element iodine is very important for making both T3 and T4. If a person’s diet does not have enough iodine, their thyroid cannot work properly and the person develops an iodine deficiency disease called goiter. Low amounts of T3 and T4 in the blood, due to lack of iodine to make them, causes the pituitary to secrete large amounts of thyroid stimulating hormone (TSH), which causes abnormal growth of the thyroid gland. The addition of small amounts of iodine to mass produced foods, such as table salt, has helped reduce the occurrence of iodine-deficiency in developed countries. The thyroid also produces the hormone calcitonin, which plays a role in calcium homeostasis. The hormones secreted by the thyroid are listed in Table below.
Hormones Secreted by the Thyroid and Parathyroid Glands Location Hormone Target Function
Thyroid Triiodothyroine (T3) Thyroxine (T4)
Calcitonin
Bone cells
Body Cells
Increase metabolic rate, stimulates mental and physical growth Increases calcium absorption by bones, lowers blood calcium level
Parathyroid Parathyroid hormone (PTH) Cells of the bone, kidney, and intestines Regulates blood calcium levels
Parathyroid Glands
The parathyroid glands are usually located behind the thyroid gland, but they are visible in Figure above. Parathyroid hormone (PTH), maintains blood calcium levels within a narrow range, so that the nervous and muscular systems can work properly. When blood calcium levels drop below a certain point, calcium-sensing receptors in the parathyroid gland release the hormone parathyroid hormone (PTH) into the blood. PTH has effects that are opposite to the action of calcitonin. It increases blood calcium levels by stimulating certain bone cells to break down bone and release calcium. It also increases gastrointestinal calcium absorption by activating vitamin D, and promotes calcium uptake by the kidneys. The hormones secreted by the parathyroid glands are listed in Table above.
Pineal Gland
The hormone melatonin is made in the pea-sized pineal gland, which is located at the base of the brain. Production of melatonin by the pineal gland is under the control of the hypothalamus which receives information from the retina about the daily pattern of light and darkness. Very little is currently known about the functions of melatonin, but scientists have found that it is involved in sleep cycles (circadian cycles), the onset of puberty, and immune function. Melatonin secretion also responds to seasonal changes in light, which could be a reason why getting out of bed on a dull, rainy morning can be so difficult, as the boy in Figure below probably knows.
Figure 20.43
Very little is currently known about the role of melatonin, but scientists do know that it is involved in sleep cycles. It is produced by the pineal gland, the retina and the intestines. Production of melatonin by the pineal gland is influenced of by the hypothalamus which receives information from the retina about the daily pattern of light and darkness.
Pancreas
The pancreas is both an exocrine gland as it secretes pancreatic juice containing digestive enzymes, and an endocrine gland as it produces several important hormones. It is located just below and behind the stomach, as shown in Figure below. The endocrine cells of the pancreas are grouped together in areas called islets of Langerhans, shown in Figure below. The islets produce the amino acid-based hormones insulin, glucagon, and somatostatin. Insulin and glucagon are both involved in controlling blood glucose levels. Insulin is produced by beta cells and causes excess blood glucose to be taken up by liver and muscle cells, where it is stored as glycogen, a polysaccharide. Glucagon is produced by alpha cells and stimulates liver cells to break down stores of glycogen into glucose which is then released into the blood. An alpha cell is another type of endocrine cell that is found within the islets of Langerhans. The hormones secreted by the pancreas are listed in Table below.
Figure 20.44
The location of the pancreas in relation to the stomach and gall bladder. The hormone-producing Islet cells a found in groups throughout the pancreas.
Figure 20.45
Micrograph of an islet of Langerhans isolated from a rat pancreas. Each islet in a human pancreas contains approximately 1000 cells and is 50 to 500 micrometers in diameter. Cell nuclei are stained blue, insulin-producing beta cells are green, and glucagon-producing alpha cells are red.
Hormones Secreted by the Pancreas Hormone Effects
Insulin Glucagon
Amylin
Somatostatin (inhibitory hormone)
Ghrelin
Reduces blood glucose concentration Raises blood glucose concentration
Suppresses glucagons secretion
Suppress the release of insulin, glucagon, and pancreatic enzymes
Stimulates appetite
Figure 20.46
The location of the adrenal glands, above the kidneys.
Adrenal Glands
An adrenal gland is located above each of the kidneys, as shown in Figure above. Each adrenal gland is separated into two structures, the adrenal medulla, which is the center of the gland, and the adrenal cortex, which is the outer layer. The medulla and the cortex work as two separate endocrine glands.
The adrenal medulla is the core of the adrenal gland, and is surrounded by the adrenal cortex. Secretion of hormones from the medulla is controlled by the sympathetic nervous system. The cells of the medulla are the body's main source of the hormones adrenaline (epinephrine) and noradrenaline (norepinephrine). These hormones are part of the fight-or-flight response initiated by the sympathetic nervous system. The hormone boosts the supply of oxygen and glucose to the brain and muscles, while suppressing other non-emergency bodily processes, such as digestion.
The adrenal cortex is the site of steroid hormone synthesis. Some cells make cortisol, while other cells make androgens such as testosterone. Other cells of the cortex regulate water and electrolyte concentrations by secreting aldosterone, which helps to regulate blood pressure. In contrast to the medulla that is controlled directly by the nervous system, the cortex is regulated by hormones secreted by the pituitary gland and hypothalamus.
Cortisol is an important steroid hormone that is often called the "stress hormone" as it is involved in the response to stress, and is involved in restoring homeostasis after a stressful event, such as the (good) stress caused by running around a soccer field [football pitch (for non-American-English speakers)], shown in Figure below. Cortisol increases blood pressure, blood sugar levels and has an immunosuppressive action. Long-term stress causes prolonged cortisol secretion, hyperglycemia, and weakening of the immune system. Excess levels of cortisol in the blood result in Cushing's syndrome, symptoms of which include rapid weight gain, a round face, excess sweating, and thinning of the skin and mucous membranes.
Figure 20.47
Regular activity through sport is a good way of allowing your body to respond naturally to its stress hormones, which prepare the body for quick movements or prolonged activity.
Hormones of the Adrenal Glands Location Hormone Function
Adrenal cortex Gonadotropins
Glucocortocoids (such as cortisol)
Mineralcorticoids (such as aldosterone)
Stimulates releases of sex hormones that develop sexual characteristics of males and females
Depress immune response, provide stress resistance, helps in fat, protein and carbohydrate metabolism
Regulate sodium reabsorption and potassium elimination in the kidneys
Adrenal medulla Norepinephrine (noradrenaline)
Epinephrine (adrenaline)
Increases alertness, physical effect similar to epinephrine
"Fight or flight" hormone, plays central role in short-term response to stress, increases heart rate and supply of blood and oxygen to the brain
Epinephrine, also called adrenaline, is a “fight or flight” hormone which is released from the adrenal medulla when stimulated by the sympathetic nervous system. Epinephrine plays a central role in the short-term stress reaction—the body’s response to threatening, exciting, or environmental stressors such as high noise levels or bright light. When secreted into the bloodstream, it binds to multiple receptors and has many effects throughout the body. Epinephrine increases heart rate, dilates the pupils, and constricts blood vessels in the skin and gut while dilating arterioles in leg muscles. It increases the blood sugar level, and at the same time begins the breakdown of lipids in fat cells. It also “turns down” non-emergency bodily processes such as digestion. Similar to other stress hormones, such as cortisol, epinephrine depresses the immune system.
Stress also releases norepinephrine in the brain. Norepinephrine has similar actions in the body as adrenaline, such as increasing blood pressure. Norepinephrine is also psychoactive because it affects alertness, which would be helpful for studying as shown in Figure below. The hormones secreted by the adrenal cortex and medulla are listed in Table above.
Figure 20.48
Thinking about an upcoming exam can cause your adrenal glands to produce adrenaline (epinephrine). Your bodys stress response can cause you to feel stressed out, but can also motivate you to study.
Gonads
The ovaries of females and the testes of males are the gamete producing organs, or gonads. Ovaries in females are homologous to testes in males. In addition to producing gametes, an exocrine action, the gonads are endocrine glands that produce steroid sex hormones. Sex hormones are responsible for the secondary sex characteristics that develop at puberty. Puberty is the process of physical changes during which the sex organs mature and a person become capable of reproducing. During puberty, among other changes, males begin producing sperm and females begin menstrual cycles.
Luteinizing hormone (LH) and follicle stimulating hormone (FSH), which are both secreted by the pituitary gland, are called gonadotropes because they are tropic hormones of the gonads. Recall that tropic hormones trigger the production of hormones in other endocrine glands. The secretion of LH and FSH are, in turn, controlled by gonadotropin-releasing hormone for the hypothalamus. Those pulses, in turn, are subject to the estrogen feedback from the gonads.
In males LH triggers the production of sex hormones called androgens in the testes. The main androgen produced by the testes is testosterone. Testosterone causes an increase in skeletal muscle mass and bone density and is also responsible for the secondary sex characteristics of males such as facial hair, shown in Figure below. The testes also produce small amounts of estrogen in the form of estradiol, which is believed to be important for sperm formation. On average, the human adult male body produces about eight to ten times more testosterone than an adult female body.
Hormones Produced by Gonads Organ Hormone Target Function
Ovaries Estrogen Bone cells, cells of sex organs Promotes growth and development of female sex organs
Maintains Uterine lining
Testes Progesterone
Testosterone
Bone cells, muscle cells, cells of sex organ Stimulates growth and development of male sex organs and sex drive
Figure 20.49
The male hormone testosterone stimulates the growth of facial hair. Many men develop facial hair in the later years of puberty, usually between the ages of 15 to 18 years. The amount of facial hair on a man's face varies between individuals, and also between ethnic groups. For example, men from many East Asian or West African backgrounds typically have much less facial hair than those of Western European, Middle Eastern, or South Asian descent.
In females a rise in LH concentration triggers the production of estrogen and progesterone by the ovaries. Estrogen causes the release of an egg from the ovaries and progesterone prepares the uterus for a possible implantation by a fertilized egg. The placenta is an endocrine gland of pregnancy because it secretes the hormones estrogen, human chorionic gonadatropin, and progesterone which are important for maintaining a pregnancy, shown in Figure below. The hormones secreted by the male and female gonads are listed in Table above.
Figure 20.50
Maintaining correct hormone levels (especially progesterone), throughout pregnancy is important for carrying a pregnancy to full term.
Other Hormone-Producing Tissues and Organs
Several organs that are generally nonendocrine in function, such as the stomach, the small intestine, the kidneys, and the heart have cells that secrete hormones. For example, the kidneys secrete erythropoietin (EPO), a hormone that regulates red blood cell production, and the heart secretes atriopeptin, a hormone that reduces water and sodium levels in the blood, which decreases blood pressure. Ghrelin is a hormone that stimulates appetite and is produced by certain cells that line the stomach. Certain cancer cells secrete hormones that can interfere with homeostasis.
Regulation: Feedback Mechanisms
Hormones regulate many cell activities and so are important to homeostatic regulation. The rate of hormone production and secretion is often controlled by homeostatic feedback control mechanisms, and the effect of hormones is also controlled by hormone antagonists. In these ways, the concentration of hormones and their products is kept within a narrow range so as to maintain homeostasis.
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. Most feedback mechanisms of the body are negative, only a few are positive. Hormone antagonists and hormone receptor antagonists are hormones or other molecules that block the action of hormones, and are also used by the body to control the action of hormones.
Negative Feedback
Negative feedback is a reaction in which the system responds in such a way as to reverse the direction of change. Since this tends to keep things constant, it allows for a process to return from a state of imbalance back to a homeostatic equilibrium.
A common, non-biological example of negative feedback happens in a home heating system. When you are home, you set your thermostat to 21˚C (about 70˚F), which is the set point. The thermometer in the thermostat monitors the room temperature and will sense when the temperature drops below the 21˚C set point (the stimulus). The thermometer will then send a message to the thermostat (control center), which in turn sends a message to the furnace to switch on and heat up the room. When the room temperature returns to the set temperature, the thermostat shuts the furnace off. In this home-heating example, the increase in air temperature is the negative feedback that results in the furnace being shut off. In this way a set room temperature of 21˚C (within a degree or two) is maintained.
An example of negative feedback in the body is the control of blood-glucose concentrations by insulin. A higher amount of glucose in the blood (the stimulus), signals the beta cells of the pancreas to release insulin into the blood. Hormone concentra
tion alone cannot trigger a negative feedback mechanism, negative feedback is instead triggered by an overproduction of the effect of the hormone, such as the lowering of blood glucose concentration (the effect), which causes a decrease in the secretion of insulin by the pancreas.
Negative Feedback: Regulation of Thyroid Hormones
The thyroid hormones thyroxine (T4) and triiodothyronine (T3) regulate the rate of metabolism. The production of T4 and T3 is regulated by a thyroid-stimulating hormone (TSH), which is released by the anterior pituitary. The thyroid and the TSH-producing cells of the anterior pituitary form a negative feedback loop, as shown in Figure below.
Thyroid-stimulating hormone production is decreased when the T4 levels are high, and when TSH levels are high, T4 production is decreased. The production and secretion of TSH is in turn controlled by thyrotropin-releasing hormone (TRH), which is produced by the hypothalamus. The rate of TRH secretion is increased in situations such as cold temperature because increasing the metabolic rate would generate more heat. Increased levels of T4 and T3 in the blood cause a reduction in TRH secretion. Among other things, TSH secretion is reduced by high levels of thyroid hormones, as well as the antagonistic hormone somatostatin. These feedback loops keep the concentration of thyroid hormones within a narrow range of concentrations.
Figure 20.51
Two negative feedback loops exist in the control of thyroid hormone secretion. (1) shows the loop between the TSH-producing cells of the anterior pituitary and the thyroid. Increased levels of T4 and T3 in the blood cause a reduction in TSH secretion. (2) shows that increased levels of T4 and T3 in the blood cause a reduction in TRH secretion.