Endochrine System Flashcards

Question

Answer 1

metabolism of fatty acids conversion of amino acids to glucose

Answer 2

Peptide Hormones v. Steroid Hormones

Answer 3

Mineralocorticoids (Fig. 16.15). (see Section 16.2) Figure 16.15 Bottom: When the blood sodium level is low, a low blood pressure causes the kidneys to secrete renin. Renin leads to the secretion of aldosterone from the adrenal cortex. Aldosterone causes the kidneys to reabsorb sodium ions +(Na+), and water follows, so that blood volume and pressure return to normal. Top: When a high blood sodium level accompanies a high blood volume, the heart secretes atrial natriuretic hormone (ANH). ANH causes the kidneys to excrete sodium ions +(Na+), and water follows. The blood volume and pressure return to normal.

Answer 4

Gonads found in males  Produce androgens (e.g., testosterone)  Stimulates growth of the penis and testes  Responsible for male sex characteristics such as facial, underarm, and pubic hair  Prompts the larynx and vocal cords to enlarge, resulting in a lower voice  Promotes muscular strength Gonads found in males  Produce androgens (e.g., testosterone)  Stimulates growth of the penis and testes  Responsible for male sex characteristics such as facial, underarm, and pubic hair  Prompts the larynx and vocal cords to enlarge, resulting in a lower voice  Promotes muscular strength Figure 16.20 Gonads found in males  Produce androgens (e.g., testosterone)  Stimulates growth of the penis and testes  Responsible for male sex characteristics such as facial, underarm, and pubic hair  Prompts the larynx and vocal cords to enlarge, resulting in a lower voice  Promotes muscular strength 11. Pineal gland  Located in the brain  Secretes melatonin that regulates the sleep/wake cycle (circadian rhythm)  May also regulate sexual development Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display. 6 P.M. 6 A.M. b. Winter a. Experimental © The McGraw-Hill Companies, Inc./Evelyn Jo Johnson, photographer c. Summer Figure 16.21 Melatonin production changes by season. 16.6 Other Endocrine Glands 22  Erythropoietin is secreted by the kidneys to increase red blood cell production.  Leptin is produced by fat cells, and acts on the hypothalamus to give a feeling of being satiated. Hormones from other tissues 16.6 Other Endocrine Glands Hormones from other tissues  Prostaglandins  Groups of potent chemicals that are not carried in the bloodstream, but work locally on neighboring cells  Some cause smooth muscle contraction  Major impact on reproductive organs  Many other roles in the body  Aspirin and ibuprofen block the synthesis of these  Homeostasis The nervous and endocrine systems are important in maintaining homeostasis.  The hypothalamus bridges regulatory functions of both systems.  The nervous system is able to respond to changes in the external environment. H

Answer 5

16.12 * Type 1  Usually early-onset  * Autoimmune disorder that tends to run in families  * Pancreatic cells are attacked and cannot produce insulin  * Need insulin injections  * * Type 2  Usually adult-onset and most common type  * Tends to occur in obese, sedentary people  * Cells do not respond to insulin  * Usually diet and exercise are important for controlling this and may even prevent this

Answer 6

Testes and testosterone

Answer 7

* The nervous system and endocrine system both use chemical signals when they respond to changes that might affect homeostasis. * However, they have different means of **delivering these signals** (Fig. 16.2). * As discussed in Section 14.1, the nervous system is composed of neurons. * In this system, **sensory receptors** detect changes in the internal and external environments. * The **central nervous system (CNS) then integrates the information and responds by stimulating muscles and glands**. * Communication depends on nerve signals, conducted in axons, and neurotransmitters, which cross synapses. Axon conduction occurs rapidly, as does the diffusion of a neurotransmitter across the short distance of a synapse. * In other words, the nervous system is organized to respond rapidly to stimuli. This is particularly useful if the stimulus is an external event that endangers our safety—we can move quickly to avoid being hurt.

Answer 8

1. Page 330 Exochrine vs Endochrine glands

Answer 9

16.3 Thyroid and Parathyroid Glands LEARNING OUTCOMES Upon completion of this section, you should be able to List the hormones produced by the thyroid and parathyroid glands and provide a function for each. Describe the negative feedback mechanism involved in the maintenance of blood calcium homeostasis. Summarize the diseases and conditions associated with the thyroid and parathyroid glands. The thyroid gland is a large gland located in the neck, where it is attached to the trachea just below the larynx (see Fig. 16.1). The parathyroid glands are embedded in the posterior surface of the thyroid gland. Thyroid Gland The thyroid gland regulates the metabolic rate of the body, and it has a role in calcium homeostasis. The thyroid gland is composed of a large number of follicles, each containing thyroid cells filled with triiodothyronine (T3), which contains three iodine atoms, and thyroxine (T4), which contains four.

Answer 10

Figure 16.20 Gonads found in males  Produce androgens (e.g., testosterone)  Stimulates growth of the penis and testes  Responsible for male sex characteristics such as facial, underarm, and pubic hair  Prompts the larynx and vocal cords to enlarge, resulting in a lower voice  Promotes muscular strength

Answer 11

Not all hormones act between body parts. As we will see, prostaglandins are a good example of local hormones. After prostaglandins are produced, they are not carried elsewhere in the bloodstream. Instead, they affect neighboring cells, sometimes promoting pain and inflammation. Also, growth factors are local hormones that promote cell division and mitosis.

Answer 12

Comparing Nervous and Endochrine Systems (see Fig. 16.2).

Answer 13

1. blood glucose level rise causes 2. the pancreas to release insulin. 3. This, in turn, promotes glucose uptake by the liver, muscles, and other cells of the body 4. When the blood glucose level falls, the pancreas no longer secretes insulin.

Answer 14

Pancreas Fish-shaped organ behind the stomach  Composed of 2 tissues  Exocrine function: produces and secretes digestive juice Endocrine function (by islets of Langerhans): produces and secretes hormones 1. Insulin – secreted when blood glucose is high; stimulates the uptake of glucose by cells (muscle and liver) 2. Glucagon – secreted when blood glucose is low; stimulates the breakdown of glycogen in the liver Figure 16.18  It is the inability to control blood glucose levels.  There are two types: type 1 and type 2.  25.8 million people in the US have diabetes. What is diabetes What is diabetes?  General symptoms include  frequent urination.  unusual hunger and/or thirst.  unexplained change in weight.  blurred vision.  sores that heal slowly or not at all.  excessive fatigue.  Long-term effects are blindness, loss of limbs, nerve deterioration, kidney and cardiovascular disease.

Answer 15

16.3 Thyroid and Parathyroid Glands LEARNING OUTCOMES Upon completion of this section, you should be able to List the hormones produced by the thyroid and parathyroid glands and provide a function for each. Describe the negative feedback mechanism involved in the maintenance of blood calcium homeostasis. Summarize the diseases and conditions associated with the thyroid and parathyroid glands. The thyroid gland is a large gland located in the neck, where it is attached to the trachea just below the larynx (see Fig. 16.1). The parathyroid glands are embedded in the posterior surface of the thyroid gland. Thyroid Gland The thyroid gland regulates the metabolic rate of the body, and it has a role in calcium homeostasis. The thyroid gland is composed of a large number of follicles, each containing thyroid cells filled with triiodothyronine (T3), which contains three iodine atoms, and thyroxine (T4), which contains four. Effects of Thyroid Hormones To produce triiodothyronine (T3) and thyroxine (T4), the thyroid gland actively requires iodine. The concentration of iodine in the thyroid gland can increase to as much as 25 times that in the blood. If iodine is lacking in the diet, the thyroid gland is unable to produce the thyroid hormones. In response to constant stimulation by TSH from the anterior pituitary, the thyroid enlarges, resulting in a condition called endemic goiter (Fig. 16.11a). In the 1920s, it was discovered that the use of iodized salt allows the thyroid to produce the thyroid hormones and, therefore, helps prevent goiter. However, iodine deficiencies are still common in many parts of the world, with an estimated 2 billion people still experiencing some degree of deficiency. Figure 16.11 Endemic goiter, hypothyroidism, and hyperthyroidism. a. An enlarged thyroid gland is often caused by a lack of iodine in the diet. Without iodine, the thyroid is unable to produce its hormones, and continued anterior pituitary stimulation causes the gland to enlarge. b. Individuals who develop hypothyroidism during infancy or childhood do not grow and develop as others do. Unless medical treatment is begun, the body is short and stocky; intellectual disabilities are also likely. c. In exophthalmic goiter, a goiter is due to an overactive thyroid and the eyes protrude because of edema in eye socket tissue. (a): ©Bruce Coleman, Inc./Alamy; (b): ©Medical-on-Line/Alamy; (c): ©Dr. P. Marazzi/Science Source While thyroid hormones increase the metabolic rate, they do not have a target organ. Instead, they stimulate all cells of the body to metabolize at a faster rate. More glucose is broken down, and more energy is used. Mechanism of Thyroxine Action If the thyroid fails to develop properly, a condition called congenital hypothyroidism results (Fig. 16.11b). Individuals with this condition are short and stocky and have had extreme hypothyroidism (undersecretion of thyroid hormone) since infancy or childhood. Thyroid hormone therapy can initiate growth, but unless treatment is begun within the first 2 months of life, intellectual disability results. The occurrence of hypothyroidism in adults produces the condition known as myxedema. Lethargy, weight gain, loss of hair, slower pulse rate, lowered body temperature, and Page 339thickness and puffiness of the skin are characteristics of myxedema. The administration of adequate doses of thyroid hormones restores normal function and appearance. In the case of hyperthyroidism (oversecretion of thyroid hormone), the thyroid gland is overactive and enlarges, forming a goiter. This type of goiter is called exophthalmic goiter (Fig. 16.11c). The eyes protrude because of edema in eye socket tissues and swelling of the muscles that move the eyes. The patient usually becomes hyperactive, nervous, and irritable and suffers from insomnia. Surgical removal or destruction of a portion of the thyroid by means of radioactive iodine is sometimes effective in curing the condition. Hyperthyroidism can also be caused by a thyroid tumor, usually detected as a lump during physical examination. Again, the treatment is surgery in combination with administration of radioactive iodine. The prognosis for most patients is excellent. Calcitonin Calcium ions (Ca2+) play a significant role in both nervous conduction and muscle contraction. They are also necessary for blood clotting. The blood calcium level is regulated in part by calcitonin, a hormone secreted by the thyroid gland when the blood calcium level rises (Fig. 16.12). The primary effect of calcitonin is to bring about the deposit of calcium ions in the bones. It also temporarily reduces the activity and number of osteoclasts. When the blood calcium level lowers to normal, the thyroid’s release of calcitonin is inhibited. Figure 16.12 Blood calcium homeostasis. Top: When the blood calcium level is high, the thyroid gland secretes calcitonin. Calcitonin promotes the uptake of calcium ions +(Ca2+) by the bones; therefore, the blood calcium level returns to normal. Bottom: When the blood calcium level is low, the parathyroid glands release parathyroid hormone (PTH). PTH causes the bones to release calcium ions +(Ca2+). It also causes the kidneys to reabsorb +Ca2+ and activate vitamin D; thereafter, the intestines absorb +Ca2+. Therefore, the blood calcium level returns to normal. Parathyroid Glands Parathyroid hormone (PTH), produced by the parathyroid glands, causes the blood calcium level to increase. A low blood calcium level stimulates the release of PTH, which promotes the activity of osteoclasts and the release of calcium from the bones. PTH also activates vitamin D in the kidneys. Activated vitamin D, a hormone sometimes called calcitriol, then promotes calcium reabsorption by the kidneys. The absorption of calcium ions from the intestine is also stimulated by calcitriol. These effects bring the blood calcium level back to the normal range, and PTH secretion stops. Many years ago, the four parathyroid glands were sometimes mistakenly removed during thyroid surgery because of their size and location. Gland removal caused insufficient PTH production, which resulted in hypoparathyroidism. Hypoparathyroidism causes a dramatic drop in blood calcium, followed by excessive nerve excitability. Nerve signals happen spontaneously and without rest, causing a phenomenon called tetany. In tetany, the body shakes from continuous muscle contraction. Without treatment, severe hypoparathyroidism causes seizures, heart failure, and death. Untreated hyperparathyroidism (oversecretion of PTH) can result in osteoporosis because of continuous calcium release from the bones. Hyperparathyroidism may also cause formation of calcium kidney stones. When a bone is broken, homeostasis is disrupted. For the fracture to heal, osteoclasts will have to destroy old bone, and osteoblasts will have to lay down new bone. Many factors influence the formation of new bone, including parathyroid hormone, calcitonin, and vitamin D. The calcium needed to repair the fracture is made readily available as new blood capillaries penetrate the fractured area. CHECK YOUR PROGRESS 16.3 Explain how the hormones of the thyroid gland influence the metabolic rate. Answer T3 and T4 increase the metabolic rate of all cells of the body stimulating them to break down glucose and to use more energy. Describe how calcitonin and parathyroid hormones interact to regulate blood calcium levels. Answer When blood calcium is high, the thyroid gland secretes calcitonin, promoting calcium uptake by the bones and lowering blood calcium. When blood calcium is low, the parathyroid glands secrete parathyroid hormone, causing bones to release calcium, and the kidneys to reabsorb calcium and activate vitamin D, so that the intestines can absorb more calcium. These effects continue until the blood calcium levels return to normal. Distinguish between hyperthyroidism and hyperparathyroidism with regard to the effects on the body. Answer Hyperthyroidism is usually an oversecretion of T3 and T4; overactivity and irritability may result, along with an exophthalmic goiter in some cases. Hyperparathyroidism results in osteoporosis and kidney stones due to the oversecretion of PTH, which causes calcium release from the bones. Page 340 CONNECTING THE CONCEPTS For more information on the importance of calcium, refer to the following discussions: Section 12.5 explains the role of the bones in maintaining calcium homeostasis. Section 13.2 examines how calcium ions are involved in muscle contraction. Section 14.1 explores how calcium ions are involved in the activity of a neural synapse.

Answer 16

1.  A large gland located below the larynx  2. Iodine is needed in the diet to allow the thyroid gland to produce thyroid hormone 3. Thyroid hormone works like a steroid hormone and can directly enter the target cell even though it is a non-steroidal hormone  4. It produces:  * Thyroid hormone (TH): regulates metabolism  * Triiodothyronine (T3 ), thyroxine (T4 )  * Calcitonin: helps lower blood Ca2+ levels by stimulating the deposition of calcium in the bones 4

Answer 17

Ca+ high: Calcitonin Ca+ low: PTSH

Answer 18

 Inner portion of the adrenal glands  Hypothalamus initiates stimulation of hormone secretion in the adrenal medulla  Produces hormones that allow a short-term response to stress (“fight or flight” response)  Epinephrine (adrenaline)  Norepinephrine (noradrenaline) A

Answer 19

Posterior Pituitary Gland

Answer 20

**pancreas** B cells and lowering blood glucose level Figure 16.19 Blood glucose homeostasis. Top: When the blood glucose level is high, the pancreas secretes insulin. Insulin promotes the storage of glucose as glycogen and the synthesis of proteins and fats. Therefore, insulin lowers the blood glucose level. Bottom: When the blood glucose level is low, the pancreas secretes glucagon. Glucagon acts opposite to insulin; therefore, glucagon raises the blood glucose level to normal.

Answer 21

1. the nervous and endocrine systems work together to govern internal systems: The sympathetic system immediately acts to **_increase heart rate and constrict the blood vessels, so your blood pressure rises_**. Sweating may have upset the water-salt balance of your blood. If so, the hormone **aldosterone from the adrenal cortex will act** on the kidney tubules to conserve sodium ions (Na+), and water reabsorption will follow. The hypothalamus can also help by sending antidiuretic hormone (ADH) to the posterior pituitary gland, which releases it into the blood. ADH actively promotes water reabsorption by the kidney tubules. * This usually occurs below the level of consciousness. * Subconscious control often depends on _reflex actions_ that involve the * hypothalamus * medulla oblongata. * Let’s take **blood pressure** as an example. 1. After a 3-mile run, you decide to sit down under a tree to rest a bit. When you stand up to push off again, you feel faint. 2. The feeling quickly passes, because the _medulla oblongata_ responds to input from the **baroreceptor**s in the **aortic arch and carotid arteries.** 3. Recall from Section 14.5 that certain drugs, such as alcohol, can affect **ADH secretion.** When you consume alcohol, it is quickly absorbed across the stomach lining into the bloodstream, where it travels to the **hypothalamus and inhibits ADH secretion.** 4. When **ADH levels fall, the kidney tubules absorb less water**. 5. The result is increased production of dilute urine. 6. Excessive water loss, or dehydration, is a disturbance of homeostasis. 7. This is why drinking alcohol when you are exercising or perspiring heavily on a hot day is not a good idea. Instead of keeping you hydrated, an alcoholic beverage, such as beer, has the opposite effect.

Answer 22

The endocrine system is largely composed of glands (see Fig. 16.1). These glands secrete hormones, which are carried by the bloodstream to target cells throughout the body. It takes time to deliver hormones, and it takes time for cells to respond. The effect initiated by the endocrine system is longer lasting. In other words, the endocrine system is organized for a slow but prolonged response. Both the nervous system and the endocrine system make use of negative feedback mechanisms (see Section 4.8). If the blood pressure falls, sensory receptors signal a control center in the brain. This center sends out nerve signals to the arterial walls, so that they constrict, and blood pressure rises. Now the sensory receptors are no longer stimulated, and the feedback mechanism is inactivated. Similarly, a rise in blood glucose level causes the pancreas to release insulin. This, in turn, promotes glucose uptake by the liver, muscles, and other cells of the body (see Fig. 16.2). When the blood glucose level falls, the pancreas no longer secretes insulin.

Answer 23

growth hormone thyroid-stimulating hormone adrenocorticotropic hormone

Answer 24

Erythropoietin

Answer 25

Figure 16.1 The endocrine system. This diagram indicates the major endocrine glands of the body. Other organs also produce hormones, such as the kidneys, the gastrointestinal tract, and the heart, but this is not the primary function of these organs.

Answer 26

Gonads are considered endochrine glands

Answer 27

16.3 Thyroid and Parathyroid Glands LEARNING OUTCOMES Upon completion of this section, you should be able to List the hormones produced by the thyroid and parathyroid glands and provide a function for each. Describe the negative feedback mechanism involved in the maintenance of blood calcium homeostasis. Summarize the diseases and conditions associated with the thyroid and parathyroid glands. The thyroid gland is a large gland located in the neck, where it is attached to the trachea just below the larynx (see Fig. 16.1). The parathyroid glands are embedded in the posterior surface of the thyroid gland. Thyroid Gland The thyroid gland regulates the metabolic rate of the body, and it has a role in calcium homeostasis. The thyroid gland is composed of a large number of follicles, each containing thyroid cells filled with triiodothyronine (T3), which contains three iodine atoms, and thyroxine (T4), which contains four. Effects of Thyroid Hormones To produce triiodothyronine (T3) and thyroxine (T4), the thyroid gland actively requires iodine. The concentration of iodine in the thyroid gland can increase to as much as 25 times that in the blood. If iodine is lacking in the diet, the thyroid gland is unable to produce the thyroid hormones. In response to constant stimulation by TSH from the anterior pituitary, the thyroid enlarges, resulting in a condition called endemic goiter (Fig. 16.11a). In the 1920s, it was discovered that the use of iodized salt allows the thyroid to produce the thyroid hormones and, therefore, helps prevent goiter. However, iodine deficiencies are still common in many parts of the world, with an estimated 2 billion people still experiencing some degree of deficiency. Figure 16.11 Endemic goiter, hypothyroidism, and hyperthyroidism. a. An enlarged thyroid gland is often caused by a lack of iodine in the diet. Without iodine, the thyroid is unable to produce its hormones, and continued anterior pituitary stimulation causes the gland to enlarge. b. Individuals who develop hypothyroidism during infancy or childhood do not grow and develop as others do. Unless medical treatment is begun, the body is short and stocky; intellectual disabilities are also likely. c. In exophthalmic goiter, a goiter is due to an overactive thyroid and the eyes protrude because of edema in eye socket tissue. (a): ©Bruce Coleman, Inc./Alamy; (b): ©Medical-on-Line/Alamy; (c): ©Dr. P. Marazzi/Science Source While thyroid hormones increase the metabolic rate, they do not have a target organ. Instead, they stimulate all cells of the body to metabolize at a faster rate. More glucose is broken down, and more energy is used. Mechanism of Thyroxine Action If the thyroid fails to develop properly, a condition called congenital hypothyroidism results (Fig. 16.11b). Individuals with this condition are short and stocky and have had extreme hypothyroidism (undersecretion of thyroid hormone) since infancy or childhood. Thyroid hormone therapy can initiate growth, but unless treatment is begun within the first 2 months of life, intellectual disability results. The occurrence of hypothyroidism in adults produces the condition known as myxedema. Lethargy, weight gain, loss of hair, slower pulse rate, lowered body temperature, and Page 339thickness and puffiness of the skin are characteristics of myxedema. The administration of adequate doses of thyroid hormones restores normal function and appearance. In the case of hyperthyroidism (oversecretion of thyroid hormone), the thyroid gland is overactive and enlarges, forming a goiter. This type of goiter is called exophthalmic goiter (Fig. 16.11c). The eyes protrude because of edema in eye socket tissues and swelling of the muscles that move the eyes. The patient usually becomes hyperactive, nervous, and irritable and suffers from insomnia. Surgical removal or destruction of a portion of the thyroid by means of radioactive iodine is sometimes effective in curing the condition. Hyperthyroidism can also be caused by a thyroid tumor, usually detected as a lump during physical examination. Again, the treatment is surgery in combination with administration of radioactive iodine. The prognosis for most patients is excellent. Calcitonin Calcium ions (Ca2+) play a significant role in both nervous conduction and muscle contraction. They are also necessary for blood clotting. The blood calcium level is regulated in part by calcitonin, a hormone secreted by the thyroid gland when the blood calcium level rises (Fig. 16.12). The primary effect of calcitonin is to bring about the deposit of calcium ions in the bones. It also temporarily reduces the activity and number of osteoclasts. When the blood calcium level lowers to normal, the thyroid’s release of calcitonin is inhibited. Figure 16.12 Blood calcium homeostasis. Top: When the blood calcium level is high, the thyroid gland secretes calcitonin. Calcitonin promotes the uptake of calcium ions +(Ca2+) by the bones; therefore, the blood calcium level returns to normal. Bottom: When the blood calcium level is low, the parathyroid glands release parathyroid hormone (PTH). PTH causes the bones to release calcium ions +(Ca2+). It also causes the kidneys to reabsorb +Ca2+ and activate vitamin D; thereafter, the intestines absorb +Ca2+. Therefore, the blood calcium level returns to normal. Parathyroid Glands Parathyroid hormone (PTH), produced by the parathyroid glands, causes the blood calcium level to increase. A low blood calcium level stimulates the release of PTH, which promotes the activity of osteoclasts and the release of calcium from the bones. PTH also activates vitamin D in the kidneys. Activated vitamin D, a hormone sometimes called calcitriol, then promotes calcium reabsorption by the kidneys. The absorption of calcium ions from the intestine is also stimulated by calcitriol. These effects bring the blood calcium level back to the normal range, and PTH secretion stops. Many years ago, the four parathyroid glands were sometimes mistakenly removed during thyroid surgery because of their size and location. Gland removal caused insufficient PTH production, which resulted in hypoparathyroidism. Hypoparathyroidism causes a dramatic drop in blood calcium, followed by excessive nerve excitability. Nerve signals happen spontaneously and without rest, causing a phenomenon called tetany. In tetany, the body shakes from continuous muscle contraction. Without treatment, severe hypoparathyroidism causes seizures, heart failure, and death. Untreated hyperparathyroidism (oversecretion of PTH) can result in osteoporosis because of continuous calcium release from the bones. Hyperparathyroidism may also cause formation of calcium kidney stones. When a bone is broken, homeostasis is disrupted. For the fracture to heal, osteoclasts will have to destroy old bone, and osteoblasts will have to lay down new bone. Many factors influence the formation of new bone, including parathyroid hormone, calcitonin, and vitamin D. The calcium needed to repair the fracture is made readily available as new blood capillaries penetrate the fractured area. CHECK YOUR PROGRESS 16.3 Explain how the hormones of the thyroid gland influence the metabolic rate. Answer T3 and T4 increase the metabolic rate of all cells of the body stimulating them to break down glucose and to use more energy. Describe how calcitonin and parathyroid hormones interact to regulate blood calcium levels. Answer When blood calcium is high, the thyroid gland secretes calcitonin, promoting calcium uptake by the bones and lowering blood calcium. When blood calcium is low, the parathyroid glands secrete parathyroid hormone, causing bones to release calcium, and the kidneys to reabsorb calcium and activate vitamin D, so that the intestines can absorb more calcium. These effects continue until the blood calcium levels return to normal. Distinguish between hyperthyroidism and hyperparathyroidism with regard to the effects on the body. Answer Hyperthyroidism is usually an oversecretion of T3 and T4; overactivity and irritability may result, along with an exophthalmic goiter in some cases. Hyperparathyroidism results in osteoporosis and kidney stones due to the oversecretion of PTH, which causes calcium release from the bones. Page 340 CONNECTING THE CONCEPTS For more information on the importance of calcium, refer to the following discussions: Section 12.5 explains the role of the bones in maintaining calcium homeostasis. Section 13.2 examines how calcium ions are involved in muscle contraction. Section 14.1 explores how calcium ions are involved in the activity of a neural synapse.

Answer 28

6.1 Endocrine Glands LEARNING OUTCOMES Upon completion of this section, you should be able to Distinguish between the mode of action of a neurotransmitter and that of a hormone. Distinguish between endocrine and exocrine glands. Identify the organs and glands of the endocrine system. Compare the actions of peptide and steroid hormones.

Answer 29

16.7 Hormones and Homeostasis LEARNING OUTCOMES Upon completion of this section, you should be able to Summarize how the endocrine and nervous systems respond to external changes in the body. Summarize how the endocrine and nervous systems respond to internal changes in the body. The nervous and endocrine systems exert control over the other systems and thereby maintain homeostasis (Fig. 16.23). Figure 16.23 The nervous system and endocrine system interact to control homeostasis. The nervous and endocrine systems work together to regulate and control the other systems. Responding to External Changes The nervous system is particularly able to respond to changes in the external environment. Some responses are automatic, as you can verify by trying this: Hold a piece of clear plastic in front of your face. Get someone to gently toss a soft object, such as a wadded-up piece of paper, at the plastic. Can you prevent yourself from blinking? This reflex protects your eyes. The eyes and other organs that have sensory receptors provide us with valuable information about the external environment. The central nervous system, on the receiving end of millions of bits of information, integrates information, compares it with previously stored memories, and “decides” on the proper course of action. The nervous system often responds to changes in the external environment through body movement. It gives us the ability to stay in as moderate an environment as possible. Otherwise, we test the ability of the nervous system to maintain homeostasis despite extreme conditions. Responding to Internal Changes The governance of internal organs usually requires that the nervous and endocrine systems work together. This usually occurs below the level of consciousness. Subconscious control often depends on reflex actions that involve the hypothalamus and the medulla oblongata. Let’s take blood pressure as an example. After a 3-mile run, you decide to sit down under a tree to rest a bit. When you stand up to push off again, you feel faint. The feeling quickly passes, because the medulla oblongata responds to input from the baroreceptors in the aortic arch and carotid arteries. The sympathetic system immediately acts to increase heart rate and constrict the blood vessels, so your blood pressure rises. Sweating may have upset the water-salt balance of your blood. If so, the hormone aldosterone from the adrenal cortex will act on the kidney tubules to conserve sodium ions (Na+), and water reabsorption will follow. The hypothalamus can also help by sending antidiuretic hormone (ADH) to the posterior pituitary gland, which releases it into the blood. ADH actively promotes water reabsorption by the kidney tubules. Recall from Section 14.5 that certain drugs, such as alcohol, can affect ADH secretion. When you consume alcohol, it is quickly absorbed across the stomach lining into the bloodstream, where it travels to the hypothalamus and inhibits ADH secretion. When ADH levels fall, the kidney tubules absorb less water. The result is increased production of dilute urine. Excessive water loss, or dehydration, is a disturbance of homeostasis. This is why drinking alcohol when you are exercising or perspiring heavily on a hot day is not a good idea. Instead of keeping you hydrated, an alcoholic beverage, such as beer, has the opposite effect. Controlling the Reproductive System Few systems intrigue us more than the reproductive system, which couldn’t function without nervous and endocrine control. The hypothalamus controls the anterior pituitary, which controls the release of hormones from the testes and the ovaries and the production of their gametes. The nervous system directly controls the muscular contractions of the ducts that propel the sperm. Page 350Contractions of the uterine tubes, which move a developing embryo to the uterus, where development continues, are stimulated by the nervous system, too. Without the positive feedback cycle involving oxytocin produced by the hypothalamus, birth might not occur. The Neuroendocrine System The nervous and endocrine systems work so closely together that they form what is sometimes called the neuroendocrine system. As we have seen, the hypothalamus certainly bridges the regulatory activities of the nervous and endocrine systems. In addition to producing the hormones released by the posterior pituitary, the hypothalamus produces hormones that control the anterior pituitary. The nerves of the autonomic system, which control other organs, are acted upon directly by the hypothalamus. The hypothalamus truly belongs to both the nervous and endocrine systems. Indeed, it is often and appropriately referred to as a neuroendocrine organ.Page 351 CHECK YOUR PROGRESS 16.7 Summarize the role of the endocrine system in maintaining homeostasis. Answer The nervous system can detect inputs from sensory receptors both internally and externally. The endocrine system, which works with the nervous system, is a mechanism for responding to these stimuli to maintain homeostasis. Explain how the body restores its water-salt balance after it has lost water and salt through sweating. Answer Aldosterone from the adrenal cortex acts on kidney tubules to conserve sodium, and water reabsorption will follow. ADH from the anterior pituitary also increases water reabsorption by the kidneys. Explain why the nervous and endocrine systems are integrated with one another. Answer The nervous and endocrine systems are integrated with each other because of the role of the hypothalamus in each system. CONNECTING THE CONCEPTS For more information on the organ systems presented in this section, refer to the following discussions: Section 5.3 examines the factors that regulate heart rate. Section 11.4 explains the role of aldosterone and ADH on the function of the kidney. Section 14.2 explores the roles of the hypothalamus and medulla oblongata in the CNS. CONCLUSION For diabetics, the prospects of controlling their blood glucose levels and living a healthy life are better today than in the past. Prior to the development of recombinant DNA technology, which now allows human insulin to be produced in large quantities, insulin was derived from the pancreases of pigs or cows. This required laborious purification, and because the animal insulins were not identical to the human form, sometimes immunological reactions occurred. Increasingly, insulin pumps are being used to treat diabetes. An insulin pump is a device a little bigger than a cell phone, which can deliver precise amounts of insulin under the skin using a small plastic catheter. The insulin pump more accurately mimics the pancreas’s natural release of the correct amount of insulin needed by the body. Studies have shown that insulin pumps are more effective than traditional injections of insulin in controlling blood sugar levels. In the near future, it may be possible to implant a device—sometimes called an “artificial pancreas”—into patients with diabetes. Such a device would not only monitor the blood sugar level but also provide appropriate doses of insulin.

Answer 30

16.7 Hormones and Homeostasis LEARNING OUTCOMES Upon completion of this section, you should be able to Summarize how the endocrine and nervous systems respond to external changes in the body. Summarize how the endocrine and nervous systems respond to internal changes in the body. The nervous and endocrine systems exert control over the other systems and thereby maintain homeostasis (Fig. 16.23). Figure 16.23 The nervous system and endocrine system interact to control homeostasis. The nervous and endocrine systems work together to regulate and control the other systems. Responding to External Changes The nervous system is particularly able to respond to changes in the external environment. Some responses are automatic, as you can verify by trying this: Hold a piece of clear plastic in front of your face. Get someone to gently toss a soft object, such as a wadded-up piece of paper, at the plastic. Can you prevent yourself from blinking? This reflex protects your eyes. The eyes and other organs that have sensory receptors provide us with valuable information about the external environment. The central nervous system, on the receiving end of millions of bits of information, integrates information, compares it with previously stored memories, and “decides” on the proper course of action. The nervous system often responds to changes in the external environment through body movement. It gives us the ability to stay in as moderate an environment as possible. Otherwise, we test the ability of the nervous system to maintain homeostasis despite extreme conditions. Responding to Internal Changes The governance of internal organs usually requires that the nervous and endocrine systems work together. This usually occurs below the level of consciousness. Subconscious control often depends on reflex actions that involve the hypothalamus and the medulla oblongata. Let’s take blood pressure as an example. After a 3-mile run, you decide to sit down under a tree to rest a bit. When you stand up to push off again, you feel faint. The feeling quickly passes, because the medulla oblongata responds to input from the baroreceptors in the aortic arch and carotid arteries. The sympathetic system immediately acts to increase heart rate and constrict the blood vessels, so your blood pressure rises. Sweating may have upset the water-salt balance of your blood. If so, the hormone aldosterone from the adrenal cortex will act on the kidney tubules to conserve sodium ions (Na+), and water reabsorption will follow. The hypothalamus can also help by sending antidiuretic hormone (ADH) to the posterior pituitary gland, which releases it into the blood. ADH actively promotes water reabsorption by the kidney tubules. Recall from Section 14.5 that certain drugs, such as alcohol, can affect ADH secretion. When you consume alcohol, it is quickly absorbed across the stomach lining into the bloodstream, where it travels to the hypothalamus and inhibits ADH secretion. When ADH levels fall, the kidney tubules absorb less water. The result is increased production of dilute urine. Excessive water loss, or dehydration, is a disturbance of homeostasis. This is why drinking alcohol when you are exercising or perspiring heavily on a hot day is not a good idea. Instead of keeping you hydrated, an alcoholic beverage, such as beer, has the opposite effect. Controlling the Reproductive System Few systems intrigue us more than the reproductive system, which couldn’t function without nervous and endocrine control. The hypothalamus controls the anterior pituitary, which controls the release of hormones from the testes and the ovaries and the production of their gametes. The nervous system directly controls the muscular contractions of the ducts that propel the sperm. Page 350Contractions of the uterine tubes, which move a developing embryo to the uterus, where development continues, are stimulated by the nervous system, too. Without the positive feedback cycle involving oxytocin produced by the hypothalamus, birth might not occur. The Neuroendocrine System The nervous and endocrine systems work so closely together that they form what is sometimes called the neuroendocrine system. As we have seen, the hypothalamus certainly bridges the regulatory activities of the nervous and endocrine systems. In addition to producing the hormones released by the posterior pituitary, the hypothalamus produces hormones that control the anterior pituitary. The nerves of the autonomic system, which control other organs, are acted upon directly by the hypothalamus. The hypothalamus truly belongs to both the nervous and endocrine systems. Indeed, it is often and appropriately referred to as a neuroendocrine organ.Page 351 CHECK YOUR PROGRESS 16.7 Summarize the role of the endocrine system in maintaining homeostasis. Answer The nervous system can detect inputs from sensory receptors both internally and externally. The endocrine system, which works with the nervous system, is a mechanism for responding to these stimuli to maintain homeostasis. Explain how the body restores its water-salt balance after it has lost water and salt through sweating. Answer Aldosterone from the adrenal cortex acts on kidney tubules to conserve sodium, and water reabsorption will follow. ADH from the anterior pituitary also increases water reabsorption by the kidneys. Explain why the nervous and endocrine systems are integrated with one another. Answer The nervous and endocrine systems are integrated with each other because of the role of the hypothalamus in each system. CONNECTING THE CONCEPTS For more information on the organ systems presented in this section, refer to the following discussions: Section 5.3 examines the factors that regulate heart rate. Section 11.4 explains the role of aldosterone and ADH on the function of the kidney. Section 14.2 explores the roles of the hypothalamus and medulla oblongata in the CNS. CONCLUSION For diabetics, the prospects of controlling their blood glucose levels and living a healthy life are better today than in the past. Prior to the development of recombinant DNA technology, which now allows human insulin to be produced in large quantities, insulin was derived from the pancreases of pigs or cows. This required laborious purification, and because the animal insulins were not identical to the human form, sometimes immunological reactions occurred. Increasingly, insulin pumps are being used to treat diabetes. An insulin pump is a device a little bigger than a cell phone, which can deliver precise amounts of insulin under the skin using a small plastic catheter. The insulin pump more accurately mimics the pancreas’s natural release of the correct amount of insulin needed by the body. Studies have shown that insulin pumps are more effective than traditional injections of insulin in controlling blood sugar levels. In the near future, it may be possible to implant a device—sometimes called an “artificial pancreas”—into patients with diabetes. Such a device would not only monitor the blood sugar level but also provide appropriate doses of insulin.

Answer 31

Figure 16.6 Anterior Pituitary Gland

Answer 32

* Figure 16.16 Addison disease. Addison disease is characterized by a peculiar bronzing of the skin in all areas of the body, not just those exposed to sunlight. Note the color of the hands on the left and center compared with the hand of an individual without the disease on the right. * ©BSIP/Science Source *

Answer 33

Glucocorticoids

Answer 34

What happens when the body produces too little or too much GH?

Answer 35

Figure 16.18 The endocrine and exocrine tissues of the pancreas. This light micrograph shows that the pancreas has two types of cells. The exocrine tissue produces a digestive juice, and the endocrine tissue produces the hormones insulin and glucagon.

Answer 36

* The **hypothalamus** initiates nerve signals that travel by way of the brain stem, spinal cord, and preganglionic sympathetic nerve fibers to the **adrenal medulla**. These signals stimulate the adrenal medulla to **secrete its hormones**. The cells of the adrenal medulla are thought to be **modified postganglionic neurons**. **_hormones produced by the adrenal medulla:_** These hormones rapidly bring about all the body changes that occur when an individual reacts to an emergency situation in a fight-or-flight manner. These hormones provide a short-term response to stress (Fig. 16.14a). 1. Epinephrine (adrenaline) and 2. norepinephrine (noradrenaline) are the 3. Adrenal Cortex

Answer 37

16. 6 Other Endocrine Glands 22  16. 6 Other Endocrine Glands Hormones from other tissues 

Answer 38

1. Some of these effects induce a target cell to **increase its uptake of particular substances (such as glucose) or ions (such as calcium).** 2. Other effects bring about an **alteration of the target cell’s structure** in some way. 3. A few hormones simply influence **cell metabolism**. 1. ***_Growth hormone_*** is a **peptide** that influences cell metabolism leading to a change in the structure of bone. 4. The term **peptide hormone** is used to include hormones that are: 1. peptides, 2. proteins, 3. glycoproteins, and 4. modified amino acids. 1. Growth hormone is a protein produced and secreted by the **anterior pituitary.** 5. All **steroid hormones** have a similar structure of **four carbon rings** because they are all derived from cholesterol (see Fig. 2.20).

Answer 39

(Fig. 16.1) Figure 16.1 The endocrine system. This diagram indicates the major endocrine glands of the body. Other organs also produce hormones, such as the kidneys, the gastrointestinal tract, and the heart, but this is not the primary function of these organs.

Answer 40

What is the endochrine system? Endocrine system 16.1 Endocrine Glands 5 Copyright

Answer 41

Pineal gland Check out Endochrine slides See Figure 16.21

Answer 42

The importance of these receptors can be demonstrated by examining a condition called **androgen insensitivity syndrome**. Individuals with this syndrome have both X and Y sex chromosomes. Because they possess a Y chromosome, they produce the sex hormone testosterone (see Section 16.6), even though the testes usually remain in the abdominal cavity. However, the body cells lack receptors for testosterone, and therefore do not respond to the hormone. Therefore, the individuals appear to be normal females, although genetically they are males. **_Endocrine Gland | Hormone | Released Target Tissues/Organs | Chief Functions_** Hypothalamus | Hypothalamic-releasing Anterior pituitary | Regulates anterior pituitary hormones Pituitary gland Posterior pituitary| Antidiuretic (ADH)| Kidneys | Stimulates water reabsorption by kidneys * Oxytocin | Uterus, mammary glands| Stimulates uterine muscle contraction, release of milk by mammary glands **_Anterior pituitary_**| Thyroid-stimulating (TSH)| ThyroidStimulates thyroid * Adrenocorticotropic (ACTH)| Adrenal cortex| Stimulates adrenal cortex * Gonadotropic (FSH, LH)| Gonads Egg and sperm production, sex hormone production * Prolactin (PRL)| Mammary glands | Milk production * Growth (GH) | Soft tissues | bonesCell division, protein synthesis, bone growth * Melanocyte-stimulating (MSH) | Melanocytes in skin | Unknown function in humans; regulates skin color in lower vertebrates * Thyroid| Thyroxine (T4) and triiodothyronine (T3)| All tissues| Increase metabolic rate, regulate growth and development * CalcitoninBones,| kidneys,| intestineLowers blood calcium level Parathyroids| Parathyroid (PTH)| Bones, kidneys, intestine| Raises blood calcium level Adrenal gland Adrenal cortex | Glucocorticoids (cortisol)| All tissues| Raise blood glucose level, stimulate breakdown of protein * Mineralocorticoids| (aldosterone)| KidneysReabsorb sodium and excrete potassium * Sex hormones| Gonads, skin, muscles, bones| Stimulate reproductive organs and bring about sex characteristics Adrenal medulla| Epinephrine and norepinephrine| Cardiac and other muscles| Are released in emergency situations, raise blood glucose level Pancreas| Insulin| Liver, muscles, adipose tissue| Lowers blood glucose level, promotes glycogen formation * Glucagon| Liver, muscles, adipose tissue| Raises blood glucose level Gonads Testes| Androgens (testosterone)| Gonads, skin, muscles, bones| Stimulate male sex characteristics Ovaries| Estrogens, progesterone, | small amounts of testosterone| Gonads, skin, muscles, bonesStimulate female sex characteristics Thymus| Thymosins| T lymphocytes| Stimulate production and maturation of T lymphocytes Pineal gland| Melatonin| BrainControls circadian rhythms, possibly involved in maturation of sexual organs

Answer 43

Difference from Nervous System (see Fig. 16.1)

Answer 44

Figure 16.12 Blood calcium homeostasis. Top: When the **blood calcium level** is high, the **thyroid** gland secretes calcitonin. **Calcitonin** promotes the uptake of calcium ions +(Ca2+) by the bones; therefore, the blood calcium level returns to normal. Bottom: When the **blood calcium level is low,** the parathyroid glands release **parathyroid hormone (PTH). PTH causes the bones to release calcium ions +(Ca2+)**. It also causes the kidneys to reabsorb +Ca2+ and **activate vitamin D;** thereafter, the intestines absorb +Ca2+. Therefore, the blood calcium level returns to normal.

Answer 45

Figure 16.11 Endemic goiter, hypothyroidism, and hyperthyroidism. a. An enlarged thyroid gland is often caused by a lack of iodine in the diet. Without iodine, the thyroid is unable to produce its hormones, and continued anterior pituitary stimulation causes the gland to enlarge. b. Individuals who develop hypothyroidism during infancy or childhood do not grow and develop as others do. Unless medical treatment is begun, the body is short and stocky; intellectual disabilities are also likely. c. In exophthalmic goiter, a goiter is due to an overactive thyroid and the eyes protrude because of edema in eye socket tissue.

Answer 46

Insulin, See Figure 16.2

Answer 47

16.3 Thyroid and Parathyroid Glands LEARNING OUTCOMES Upon completion of this section, you should be able to List the hormones produced by the thyroid and parathyroid glands and provide a function for each. Describe the negative feedback mechanism involved in the maintenance of blood calcium homeostasis. Summarize the diseases and conditions associated with the thyroid and parathyroid glands. The thyroid gland is a large gland located in the neck, where it is attached to the trachea just below the larynx (see Fig. 16.1). The parathyroid glands are embedded in the posterior surface of the thyroid gland. Thyroid Gland The thyroid gland regulates the metabolic rate of the body, and it has a role in calcium homeostasis. The thyroid gland is composed of a large number of follicles, each containing thyroid cells filled with triiodothyronine (T3), which contains three iodine atoms, and thyroxine (T4), which contains four. Effects of Thyroid Hormones To produce triiodothyronine (T3) and thyroxine (T4), the thyroid gland actively requires iodine. The concentration of iodine in the thyroid gland can increase to as much as 25 times that in the blood. If iodine is lacking in the diet, the thyroid gland is unable to produce the thyroid hormones. In response to constant stimulation by TSH from the anterior pituitary, the thyroid enlarges, resulting in a condition called endemic goiter (Fig. 16.11a). In the 1920s, it was discovered that the use of iodized salt allows the thyroid to produce the thyroid hormones and, therefore, helps prevent goiter. However, iodine deficiencies are still common in many parts of the world, with an estimated 2 billion people still experiencing some degree of deficiency. Figure 16.11 Endemic goiter, hypothyroidism, and hyperthyroidism. a. An enlarged thyroid gland is often caused by a lack of iodine in the diet. Without iodine, the thyroid is unable to produce its hormones, and continued anterior pituitary stimulation causes the gland to enlarge. b. Individuals who develop hypothyroidism during infancy or childhood do not grow and develop as others do. Unless medical treatment is begun, the body is short and stocky; intellectual disabilities are also likely. c. In exophthalmic goiter, a goiter is due to an overactive thyroid and the eyes protrude because of edema in eye socket tissue. (a): ©Bruce Coleman, Inc./Alamy; (b): ©Medical-on-Line/Alamy; (c): ©Dr. P. Marazzi/Science Source While thyroid hormones increase the metabolic rate, they do not have a target organ. Instead, they stimulate all cells of the body to metabolize at a faster rate. More glucose is broken down, and more energy is used. Mechanism of Thyroxine Action If the thyroid fails to develop properly, a condition called congenital hypothyroidism results (Fig. 16.11b). Individuals with this condition are short and stocky and have had extreme hypothyroidism (undersecretion of thyroid hormone) since infancy or childhood. Thyroid hormone therapy can initiate growth, but unless treatment is begun within the first 2 months of life, intellectual disability results. The occurrence of hypothyroidism in adults produces the condition known as myxedema. Lethargy, weight gain, loss of hair, slower pulse rate, lowered body temperature, and Page 339thickness and puffiness of the skin are characteristics of myxedema. The administration of adequate doses of thyroid hormones restores normal function and appearance. In the case of hyperthyroidism (oversecretion of thyroid hormone), the thyroid gland is overactive and enlarges, forming a goiter. This type of goiter is called exophthalmic goiter (Fig. 16.11c). The eyes protrude because of edema in eye socket tissues and swelling of the muscles that move the eyes. The patient usually becomes hyperactive, nervous, and irritable and suffers from insomnia. Surgical removal or destruction of a portion of the thyroid by means of radioactive iodine is sometimes effective in curing the condition. Hyperthyroidism can also be caused by a thyroid tumor, usually detected as a lump during physical examination. Again, the treatment is surgery in combination with administration of radioactive iodine. The prognosis for most patients is excellent. Calcitonin Calcium ions (Ca2+) play a significant role in both nervous conduction and muscle contraction. They are also necessary for blood clotting. The blood calcium level is regulated in part by calcitonin, a hormone secreted by the thyroid gland when the blood calcium level rises (Fig. 16.12). The primary effect of calcitonin is to bring about the deposit of calcium ions in the bones. It also temporarily reduces the activity and number of osteoclasts. When the blood calcium level lowers to normal, the thyroid’s release of calcitonin is inhibited. Figure 16.12 Blood calcium homeostasis. Top: When the blood calcium level is high, the thyroid gland secretes calcitonin. Calcitonin promotes the uptake of calcium ions +(Ca2+) by the bones; therefore, the blood calcium level returns to normal. Bottom: When the blood calcium level is low, the parathyroid glands release parathyroid hormone (PTH). PTH causes the bones to release calcium ions +(Ca2+). It also causes the kidneys to reabsorb +Ca2+ and activate vitamin D; thereafter, the intestines absorb +Ca2+. Therefore, the blood calcium level returns to normal. Parathyroid Glands Parathyroid hormone (PTH), produced by the parathyroid glands, causes the blood calcium level to increase. A low blood calcium level stimulates the release of PTH, which promotes the activity of osteoclasts and the release of calcium from the bones. PTH also activates vitamin D in the kidneys. Activated vitamin D, a hormone sometimes called calcitriol, then promotes calcium reabsorption by the kidneys. The absorption of calcium ions from the intestine is also stimulated by calcitriol. These effects bring the blood calcium level back to the normal range, and PTH secretion stops. Many years ago, the four parathyroid glands were sometimes mistakenly removed during thyroid surgery because of their size and location. Gland removal caused insufficient PTH production, which resulted in hypoparathyroidism. Hypoparathyroidism causes a dramatic drop in blood calcium, followed by excessive nerve excitability. Nerve signals happen spontaneously and without rest, causing a phenomenon called tetany. In tetany, the body shakes from continuous muscle contraction. Without treatment, severe hypoparathyroidism causes seizures, heart failure, and death. Untreated hyperparathyroidism (oversecretion of PTH) can result in osteoporosis because of continuous calcium release from the bones. Hyperparathyroidism may also cause formation of calcium kidney stones. When a bone is broken, homeostasis is disrupted. For the fracture to heal, osteoclasts will have to destroy old bone, and osteoblasts will have to lay down new bone. Many factors influence the formation of new bone, including parathyroid hormone, calcitonin, and vitamin D. The calcium needed to repair the fracture is made readily available as new blood capillaries penetrate the fractured area. CHECK YOUR PROGRESS 16.3 Explain how the hormones of the thyroid gland influence the metabolic rate. Answer T3 and T4 increase the metabolic rate of all cells of the body stimulating them to break down glucose and to use more energy. Describe how calcitonin and parathyroid hormones interact to regulate blood calcium levels. Answer When blood calcium is high, the thyroid gland secretes calcitonin, promoting calcium uptake by the bones and lowering blood calcium. When blood calcium is low, the parathyroid glands secrete parathyroid hormone, causing bones to release calcium, and the kidneys to reabsorb calcium and activate vitamin D, so that the intestines can absorb more calcium. These effects continue until the blood calcium levels return to normal. Distinguish between hyperthyroidism and hyperparathyroidism with regard to the effects on the body. Answer Hyperthyroidism is usually an oversecretion of T3 and T4; overactivity and irritability may result, along with an exophthalmic goiter in some cases. Hyperparathyroidism results in osteoporosis and kidney stones due to the oversecretion of PTH, which causes calcium release from the bones. Page 340 CONNECTING THE CONCEPTS For more information on the importance of calcium, refer to the following discussions: Section 12.5 explains the role of the bones in maintaining calcium homeostasis. Section 13.2 examines how calcium ions are involved in muscle contraction. Section 14.1 explores how calcium ions are involved in the activity of a neural synapse.

Answer 48

Distinguish between hyperthyroidism and hyperparathyroidism with regard to the effects on the body. Page 340 CONNECTING THE CONCEPTS For more information on the importance of calcium, refer to the following discussions: Section 12.5 explains the role of the bones in maintaining calcium homeostasis. Section 13.2 examines how calcium ions are involved in muscle contraction. Section 14.1 explores how calcium ions are involved in the activity of a neural synapse.

Answer 49

6. 6 Other Endocrine Glands 22  16. 6 Other Endocrine Glands

Answer 50

**_General functions of hormones _**

Answer 51

Exocrine glands Endocrine glands

Answer 52

16.5 Pancreas LEARNING OUTCOMES Upon completion of this section, you should be able to Explain how the pancreas functions as both an endocrine and an exocrine gland. Describe how the pancreatic hormones help maintain blood glucose homeostasis. Distinguish between type 1 and type 2 diabetes mellitus. The pancreas is a fish-shaped organ that stretches across the abdomen behind the stomach and near the duodenum of the small intestine. It is composed of both exocrine and endocrine tissue. The exocrine tissue produces and secretes enzymes and other compounds involved in digestion. These are delivered to the small intestine by the pancreatic duct (see Section 9.4). The endocrine tissue is called the pancreatic islets (islets of Langerhans). As Figure 16.18 illustrates, each pancreatic islet is surrounded by exocrine tissue. Within each islet are a variety of cell types, several of which play an important role in the endocrine functions of this organ. A cells are responsible for the secretion of the hormone glucagon, whereas B cells (not to be confused with the B cells of the immune system) secrete insulin. A third cell type, D cells, releases somatostatin, a hormone that is released at the same time as insulin to regulate the digestive processes. Figure 16.18 The endocrine and exocrine tissues of the pancreas. This light micrograph shows that the pancreas has two types of cells. The exocrine tissue produces a digestive juice, and the endocrine tissue produces the hormones insulin and glucagon. ©Victor P. Eroschenko Unlike most other endocrine organs, the pancreas is not under pituitary control, but instead responds directly to changes in blood glucose levels. Insulin is secreted by the B cells when the blood glucose level is high, which usually occurs just after eating. Insulin stimulates the uptake of glucose by cells, especially liver cells, muscle cells, and adipose tissue cells. In liver and muscle cells, glucose is then stored as glycogen. In muscle cells, the glucose supplies energy for muscle contraction. Glucose enters the metabolic pool in fat cells and thereby supplies glycerol for the formation of fat. In these various ways, insulin lowers the blood glucose level (Fig. 16.19, top). Figure 16.19 Blood glucose homeostasis. Top: When the blood glucose level is high, the pancreas secretes insulin. Insulin promotes the storage of glucose as glycogen and the synthesis of proteins and fats. Therefore, insulin lowers the blood glucose level. Bottom: When the blood glucose level is low, the pancreas secretes glucagon. Glucagon acts opposite to insulin; therefore, glucagon raises the blood glucose level to normal. (both photos) (islet of Langerhans): ©Science Photo Library/Getty Images Glucagon is secreted by the A cells of the pancreas, usually between eating, when the blood glucose level is low. The major target tissues of glucagon are the liver and adipose tissue. Glucagon stimulates the liver to break down glycogen to glucose. It also promotes the use of fat and protein in preference to glucose as energy sources. Adipose tissue cells break down fat to glycerol and fatty acids. The liver takes these up and uses them as substrates for glucose formation. In these ways, glucagon raises the blood glucose level (Fig. 16.19, bottom). Diabetes Mellitus An estimated 30.3 million Americans (9.4% of the population) have diabetes mellitus, often referred to simply as diabetes. Of these, an estimated 7.2 million are undiagnosed. Diabetes is characterized by the body’s inability to maintain blood glucose homeostasis (Fig. 16.19), resulting in an excess of glucose in the blood. This creates a number of problems with homeostasis. As the blood glucose level rises, glucose, along with water, is excreted in the urine. The term mellitus, from Greek, refers to “honey” or “sweetness.” As a result, diabetics urinate frequently and are always thirsty. Other symptoms of diabetes include fatigue, constant hunger, and weight loss. The high blood glucose levels often cause an increase in blood pressure due to osmosis and as a result can damage the small capillaries of the kidneys, eyes, and other areas of the circulatory system. Diabetics often experience vision problems due to diabetic retinopathy and swelling in the lens of the eye. If untreated, diabetics often develop serious and even fatal complications. Sores that don’t heal develop into severe infections. Blood vessel damage causes kidney failure, nerve destruction, heart attack, or stroke. Blood Sugar Regulation in Diabetics Types of Diabetes There are two types of diabetes. Type 1 diabetes is sometimes called juvenile diabetes or insulin-dependent diabetes mellitus (IDDM). Type 2 diabetes is also known as adult-onset diabetes, or non-insulin-dependent diabetes mellitus (NIDDM). Although the causes of these forms of diabetes are different, they can occur in children or adults. Type 1 Diabetes In type 1 diabetes, the pancreas is not producing enough insulin. This condition is believed to be brought on by exposure to an environmental agent, most likely a virus, whose presence causes cytotoxic T cells to destroy the pancreatic islets as Page 345part of an autoimmune response (see Section 7.5). The body turns to the metabolism of fat, which leads to the buildup of ketones in the blood, called ketoacidosis, which increases the acidity of the blood and can lead to coma and death. Individuals with type 1 diabetes must have daily insulin injections. These injections control the diabetic symptoms but still can cause inconveniences, because the blood sugar level may swing between hypoglycemia (low blood glucose) and hyperglycemia (high blood glucose). Without testing the blood glucose level, it is difficult to be certain which of these is present, because the symptoms can be similar. These symptoms include perspiration, pale skin, shallow breathing, and anxiety. Whenever these symptoms appear, immediate attention is required to bring the blood glucose back within the normal range. If the problem is hypoglycemia, the treatment is one or two glucose tablets, hard candy, or orange juice. If the problem is hyperglycemia, the treatment is insulin. Better control of blood glucose levels can often be achieved with an insulin pump, a small device worn outside the body that is connected to a plastic catheter inserted under the skin. Because diabetes is such a common problem, many researchers are working to develop more effective methods for treating diabetes. Recently, progress has been made in the development of an artificial pancreas, which would act as an automated system to provide insulin based on real-time changes in blood sugar levels. It is also possible to transplant a working pancreas, or even fetal pancreatic islet cells, into patients with type 1 diabetes. Another possibility is xenotransplantation, in which insulin-producing islet cells of another species, such as pigs, are placed inside a capsule that allows insulin to exit but prevents the immune system from attacking the foreign cells. Finally, researchers are close to testing a vaccine that could block the immune system’s attack on the islet cells, perhaps by inducing the T cells capable of suppressing these responses. Type 2 Diabetes Most adult diabetics have type 2 diabetes. Often, the patient is overweight or obese, and adipose tissue produces a substance that impairs insulin receptor function. However, complex genetic factors can be involved, as shown by the tendency for type 2 diabetes to occur more often in certain families or even ethnic groups. For example, the condition is 77% more common in African Americans than in non-Hispanic whites. Normally, the binding of insulin to its plasma membrane receptor causes the number of protein carriers for glucose to increase, causing more glucose to enter the cell. In the type 2 diabetic, insulin still binds to its receptor, but the number of glucose carriers does not increase. Therefore, the cell is said to be insulin resistant. It is possible to prevent or at least control type 2 diabetes by adhering to a low-fat, low-sugar diet and exercising regularly. If this fails, oral drugs are available that stimulate the pancreas to secrete more insulin and enhance the metabolism of glucose in the liver and muscle cells. Millions of Americans may have type 2 diabetes without being aware of it, yet the effects of untreated type 2 diabetes are as serious as those of type 1 diabetes. BIOLOGY TODAY Science Identifying Insulin as a Chemical Messenger In 1920, physician Frederick Banting decided to try to isolate insulin in order to identify it as a chemical messenger. Previous investigators had been unable to do this, because the enzymes in the digestive juices destroyed insulin (a protein) during the isolation procedure. Banting hit upon the idea of ligating (tying off) the pancreatic duct, which he knew from previous research would lead to the degeneration only of the cells that produce digestive juices and not of the pancreatic islets (islets of Langerhans), where insulin is made. His professor, J. J. Macleod, made a laboratory available to him at the University of Toronto and assigned a graduate student, Charles Best, to assist him. Banting and Best had limited funds and spent that summer working, sleeping, and eating in the lab. By the end of the summer, they had obtained pancreatic extracts that did lower the blood glucose level in diabetic dogs. Macleod then brought in biochemists, who purified the extract. Insulin therapy for the first human patient began in 1922, and large-scale production of purified insulin from pigs and cattle followed. The basic experimental system used by Banting and Best is shown in Table 16A. Table 16AExperimental System Used by Banting and Best Table Summary: ProcedureResults 1. Identify the source of the chemical.Pancreatic islets are source. 2. Identify the effect to be studied.Presence of pancreatic secretions in body lowers blood glucose. 3. Isolate the chemical.Insulin isolated from pancreatic secretions. 4. Show that the chemical has the desired effect.Insulin lowers blood glucose. For their discovery, Banting and Macleod received the Nobel Prize in Physiology or Medicine in 1923. The amino acid sequence of insulin was determined in 1953. Insulin is now synthesized using recombinant DNA technology, using bacteria, such as Escherichia coli, to produce the hormone. The availability of recombinant insulin (Fig. 16A), sometimes called synthetic insulin, has played a major role in improving the health of diabetics around the world. Figure 16A Synthetic insulin. Insulin is now a product of recombinant engineering and biotechnology. ©McGraw-Hill Education/Jill Braaten, photographer Questions to Consider What are some advantages, and potential disadvantages, of producing a medicine destined to be injected into humans (such as insulin) in a bacterium such as E. coli? Some people oppose the use of animals for medical research. Do you think insulin would have eventually been discovered without animal experimentation? Why or why not? SCIENCE IN YOUR LIFE What is gestational diabetes, and what causes it? Women who were not diabetic prior to pregnancy but have high blood glucose during pregnancy have gestational diabetes. Gestational diabetes affects a small percentage of pregnant women. This form of diabetes is caused by insulin resistance—body insulin concentration is normal, but cells fail to respond normally. Gestational diabetes and insulin resistance generally develop later in the pregnancy. Carefully planned meals and exercise often control this form of diabetes, but insulin injections may be necessary. If the woman is not treated, additional glucose crosses the placenta, causing high blood glucose in the fetus. The extra energy in the fetus is stored as fat, resulting in macrosomia, or a “fat” baby. Delivery of a very large baby can be dangerous for both the infant and the mother; thus, cesarean section is often necessary. Complications after birth are common for these babies. Further, there is a greater risk that the child will become obese and develop type 2 diabetes mellitus later in life. Gestational diabetes usually goes away after the birth of the child. However, once a woman has experienced gestational diabetes, she has a greater chance of developing it again during future pregnancies. These women also tend to develop type 2 diabetes later in life. Page 346 Testing for Diabetes The oral glucose tolerance test assists in the diagnosis of diabetes mellitus. After the patient is given 100 g of glucose, the blood glucose concentration is measured at intervals. In a diabetic, the blood glucose level rises greatly and remains elevated for several hours (Fig. 16.20), and glucose appears in the urine. In a nondiabetic person, the blood glucose level rises somewhat and then returns to normal after about 2 hours. Figure 16.20 The results of a glucose tolerance test for diabetes. Following the administration of 100 g of glucose, the blood glucose level rises dramatically in the diabetic, and glucose appears in the urine. Also, the blood glucose level at 2 hours is equal to more than 200 mg/dl. SCIENCE IN YOUR LIFE Are there alternatives to injections for insulin? Until recently, diabetics had to rely mostly on insulin pumps or injections to receive insulin. However, in 2014 the FDA approved an insulin inhaler as a method of delivering insulin. The inhaler provides a dry powder to the lungs, where it is absorbed into the bloodstream. The inhaler is not designed to replace injections but, instead, to provide a small dose of insulin around mealtimes. CHECK YOUR PROGRESS 16.5 Distinguish between the exocrine and endocrine functions of the pancreas. Answer Exocrine—produces and secretes enzymes and digestive juices, which are delivered to the small intestine via the pancreatic duct. Endocrine—produces and secretes insulin, glucagon, and somatostatin into the blood. Describe how the pancreatic hormones interact to regulate blood glucose levels. Answer When blood glucose levels are high, insulin is released, which aids in storing excess glucose. When blood glucose levels are low, glucagon is released. This breaks down glycogen to glucose, which is delivered to the bloodstream. Explain the difference in the relationship of the pancreas to type 1 and type 2 diabetes. Answer In type 1 diabetes, the pancreas does not produce insulin. In type 2 diabetes, cells become insulin resistant, and thus the pancreas may secrete more insulin than normal. CONNECTING THE CONCEPTS For additional information on the various forms of diabetes, refer to the following discussions: Section 4.8 examines how feedback mechanisms are involved in maintaining blood glucose homeostasis. Section 9.6 explains the body mass index (BMI), an indicator that is used to determine obesity and the subsequent risk of diabetes. Section 11.3 examines the influence of diabetes on the urinary system.

Answer 53

Figure 16.3 Hormones target specific cells. Most hormones are distributed by the bloodstream to target cells. Target cells have receptors for the hormones, and a hormone combines with a receptor like a key fits a lock.Page 331

Answer 54

Figure 16.21 The hormones produced by the testes and the ovaries. The testes and ovaries secrete the sex hormones. The testes secrete testosterone, and the ovaries secrete estrogens and progesterone. In each sex, secretion of GnRH from the hypothalamus and secretion of FSH and LH from the pituitary are controlled by their respective hormones.

Answer 55

* **_Peptide Hormones_** (Fig. 16.4). 1. Figure 16.4 Action of a peptide hormone. A peptide hormone (first messenger) binds to a receptor in the plasma membrane. Thereafter, cyclic AMP (second messenger) forms and activates an enzyme cascade. 2. Tutorial: Action of a Peptide Hormone

Answer 56

Controlling the Reproductive System NeuroEndochrine Collaboration Page 350

Answer 57

Mechanism of Thyroxine Action If the thyroid fails to develop properly, a condition called **congenital hypothyroidism r**esults (Fig. 16.11b). Individuals with this condition are short and stocky and have had extreme hypothyroidism (undersecretion of thyroid hormone) since infancy or childhood. Thyroid hormone therapy can initiate growth, but unless treatment is begun within the first 2 months of life, intellectual disability results. The occurrence of hypothyroidism in adults produces the condition known as **myxedema.** Lethargy, weight gain, loss of hair, slower pulse rate, lowered body temperature, and Page 339thickness and puffiness of the skin are characteristics of myxedema. The administration of adequate doses of thyroid hormones restores normal function and appearance. In the case of hyperthyroidism (oversecretion of thyroid hormone), the thyroid gland is overactive and enlarges, forming a goiter. This type of goiter is called exophthalmic goiter (Fig. 16.11c). The eyes protrude because of edema in eye socket tissues and swelling of the muscles that move the eyes. The patient usually becomes hyperactive, nervous, and irritable and suffers from insomnia. Surgical removal or destruction of a portion of the thyroid by means of radioactive iodine is sometimes effective in curing the condition. Hyperthyroidism can also be caused by a thyroid tumor, usually detected as a lump during physical examination. Again, the treatment is surgery in combination with administration of radioactive iodine. The prognosis for most patients is excellent.

Answer 58

Pineal Gland Figure 16.22 Melatonin production changes by season. Melatonin production is greatest at night when we are sleeping. a. Light suppresses melatonin production. Melatonin is secreted for a longer time in the (b) winter than in the (c) summer.

Answer 59

adrenal glands (see Fig. 16.1). (Fig. 16.13).

Answer 60

Mechanisms of Hormone Action

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* secretion is controlled by **ACTH,** * regulate carbohydrate, protein, and fat metabolism. * produced in adrenal cortex. * Cortisol is one of these that is active in the stress response and the repair of damaged tissues in the body. * raise the blood glucose level in at least two ways. * (1) They **promote the breakdown of muscle proteins to amino acids,** taken up by the liver from the bloodstream. * The liver then converts these excess amino acids to glucose, which enters the blood. * (2) They **promote the metabolism of fatty acids rather than carbohydrates,** and this spares glucose.

Answer 62

CHECK YOUR PROGRESS 16.2 Explain how the endocrine system and nervous system communicate with one another. Answer Through neurotransmitters and hormones—for example, the nervous system sends input to the adrenal medullae, so that a fight-or-flight response can be triggered when needed. Meanwhile, several hormones secreted by the endocrine system regulate the hypothalamus and/or anterior pituitary. List the hormones produced by the posterior pituitary and provide a function for each. Answer Posterior pituitary does not produce any hormones, but it stores and releases ADH and oxytocin produced in the hypothalamus. ADH conserves water, and oxytocin stimulates uterine contractions and milk letdown. List the hormones produced by the anterior pituitary and provide a function for each. Answer TSH stimulates the thyroid to produce T3 and T4; ACTH stimulates the adrenal cortex to produce glucocorticoids; gonadotropic hormones FSH and LH stimulate the gonads to produce gametes and sex hormones; PRL causes breast development and milk production; MSH causes skin color changes; GH promotes skeletal and muscular growth. CONNECTING THE CONCEPTS For more information on the hormones presented in this section, refer to the following discussions: Section 12.2 examines the influence of growth hormone on bone growth. Section 17.2 describes the role of pituitary hormones in the production of sperm cells in males. Section 17.4 describes the role of pituitary hormones in the female ovarian cycle.

Answer 63

Gonads found in males

Answer 64

What do prolactin and Growth Hormone do?

Answer 65

**Parathyroid Glands** * **_Parathyroid hormone (PTH)_**, produced by the parathyroid glands, causes the **blood calcium level to increase.** 1. A low blood calcium level stimulates the release of PTH, which promotes the activity of *osteoclasts and the release of calcium from the bones.* * PTH also **activates vitamin D i**n the kidneys. * Activated vitamin D, a hormone sometimes called **calcitriol, t**hen promotes **calcium reabsorption by the kidneys.** * The absorption of calcium ions from the intestine is also stimulated by **calcitriol.** These effects bring the blood calcium level back to the normal range, and PTH secretion stops. Many years ago, the four parathyroid glands were sometimes mistakenly removed during thyroid surgery because of their size and location. Gland removal caused insufficient PTH production, which resulted in hypoparathyroidism. Hypoparathyroidism causes a dramatic drop in blood calcium, followed by excessive nerve excitability. Nerve signals happen spontaneously and without rest, causing a phenomenon called tetany. In tetany, the body shakes from continuous muscle contraction. Without treatment, severe hypoparathyroidism causes seizures, heart failure, and death. Untreated hyperparathyroidism (oversecretion of PTH) can result in osteoporosis because of continuous calcium release from the bones. Hyperparathyroidism may also cause formation of calcium kidney stones. When a bone is broken, homeostasis is disrupted. For the fracture to heal, osteoclasts will have to destroy old bone, and osteoblasts will have to lay down new bone. Many factors influence the formation of new bone, including parathyroid hormone, calcitonin, and vitamin D. The calcium needed to repair the fracture is made readily available as new blood capillaries penetrate the fractured area.

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Parathyroid glands 16.10

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(Fig. 16.1)

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(Fig. 16.8). **_Anterior Pituitary Hormones_**

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Hormones from Other Organs or Tissues- Non Endochrine secreters

Answer 70

* Stores antidiuretic hormone (ADH) and oxytocin that are produced by the hypothalamus  * ADH regulates water balance by reabsorbing water into the bloodstream. *  Oxytocin causes uterine contractions during childbirth and allows milk to be released during nursing

Answer 71

**_Anterior Pituitary- controlled by Hypothlamus indirectly_** _Four of the seven hormones produced by the anterior pituitary have an effect on other glands._ 1. Thyroid-stimulating hormone (TSH) stimulates the thyroid to produce the thyroid hormones. 2. Adrenocorticotropic hormone (ACTH) stimulates the adrenal cortex to produce cortisol. 3. The gonadotropic hormones—follicle-stimulating hormone (FSH) and luteinizing hormone (LH)—stimulate the gonads (the testes in males and the ovaries in females) to produce gametes and sex hormones. 4. In each instance, the blood level of the last hormone in the sequence exerts negative feedback control over the secretion of the first two hormones (Fig. 16.8). 5. The other three hormones produced by the anterior pituitary do not affect other endocrine glands. 6. Prolactin is produced in quantity only after childbirth. It causes the mammary glands in the breasts to develop and produce milk. It also plays a role in carbohydrate and fat metabolism. 7. Melanocyte-stimulating hormone causes skin-color changes in many fishes, amphibians, and reptiles having melanophores, skin cells that produce color variations. The concentration of this hormone in humans is very low. 8. Growth hormone (GH), or somatotropic hormone, promotes skeletal and muscular growth. It stimulates the rate at which amino acids enter cells and protein synthesis occurs. It also promotes fat metabolism as opposed to glucose metabolism. The production of insulin-like growth factor 1 (IGF-1) by the liver is stimulated by growth hormone as well. IGF-1 is often measured as a means of determining GH level. Growth and development are also stimulated by IGF-1, and it may well be the means by which GH influences growth and development. Figure 16.7 Hormones produced by the anterior pituitary. The hypothalamus controls the secretions of the anterior pituitary, and the anterior pituitary controls the secretions of the thyroid, adrenal cortex, and gonads, which are also endocrine glands.Figure 16.8 Negative feedback mechanisms in the endocrine system. Feedback mechanisms (red arrows) provide means of controlling the amount of hormones produced (blue arrows) by the hypothalamus and pituitary glands.

Answer 72

Thyroid Gland

Answer 73

Hormone Behavior Types Steroid hormone action • Diffuse through the plasma membrane of target cells • Enter the nucleus • Bind to a specific receptor within the nucleus • The hormone-receptor complex binds to specific sites on the cell’s DNA • Activate genes that result in synthesis of new proteins = direct gene activation

Answer 74

The Action of Peptide Hormones Most endocrine glands secrete peptide hormones. The actions of peptide hormones can vary depending on the type of target cell. Peptide hormones do not have the ability to cross the plasma membrane, and therefore must interact with a receptor on the surface of the membrane. As an example, we will explore what happens when the hormone epinephrine binds to a plasma membrane receptor of a muscle cell (Fig. 16.4). In muscle cells, the reception of epinephrine leads to the breakdown of glycogen to glucose, which provides energy for ATP production. The immediate result of binding is the formation of cyclic adenosine monophosphate (cAMP). Cyclic AMP contains one phosphate group attached to adenosine at two locations, producing a circular, or cyclic, molecule. Cyclic AMP activates a protein kinase enzyme in the cell. This enzyme, in turn, activates another enzyme, and so forth. The series of enzymatic reactions that follows cAMP formation is called an enzyme cascade. Each enzyme can be used over and over at every step of the cascade, so more enzymes are involved. Finally, many molecules of glycogen are broken down to glucose, which enters the bloodstream. Typical of a peptide hormone, epinephrine never enters the cell. Therefore, the hormone is called the first messenger; cAMP, which sets the metabolic machinery in motion, is called the second messenger. To explain this terminology, let’s imagine that the adrenal medulla, which produces epinephrine, is like a company’s home office that sends out a courier (the hormone epinephrine is the first messenger) to a factory (the cell). The courier doesn’t have a pass to enter the factory, so when he arrives at the factory, he tells a supervisor through the intercom that the home office wants the factory to produce a particular product. The supervisor (cAMP, the second messenger) enters a command in the computer that instructs the machinery (the enzymatic pathway) to make the product.

Answer 75

**Gonads** **See Figure 16.12**

Answer 76

Posterior Pituitary * Neurons in the hypothalamus called neurosecretory cells produce the **_hormones antidiuretic hormone (ADH) and oxytocin_** (Fig. 16.6). These hormones pass through axons into the **posterior pituitary**, where they are stored in **axon endings.** *

Answer 77

1. There is a difference in function between an endocrine gland and an exocrine gland. Exocrine glands have ducts and secrete their products into these ducts. The glands’ products are carried to the interior of other organs or outside the body. The accessory glands of the digestive system (see Section 9.4) are good examples Page 330of the exocrine glands. For example, the salivary glands send saliva into the mouth by way of the salivary ducts. In contrast, endocrine glands secrete their products into the bloodstream, which delivers them throughout the body. Only certain cells, called target cells, can respond to a specific hormone. A target cell for a particular hormone has a receptor protein for that hormone. The hormone and the receptor protein bind together like a key that fits a lock. The target cell then responds to that hormone. .Comparison of the Endocrine and Nervous Systems The nervous system and endocrine system both use chemical signals when they respond to changes that might affect homeostasis. However, they have different means of delivering these signals (Fig. 16.2). As discussed in Section 14.1, the nervous system is composed of neurons. In this system, sensory receptors detect changes in the internal and external environments. The central nervous system (CNS) then integrates the information and responds by stimulating muscles and glands. Communication depends on nerve signals, conducted in axons, and neurotransmitters, which cross synapses. Axon conduction occurs rapidly, as does the diffusion of a neurotransmitter across the short distance of a synapse. In other words, the nervous system is organized to respond rapidly to stimuli. This is particularly useful if the stimulus is an external event that endangers our safety—we can move quickly to avoid being hurt.

Answer 78

Thyroid abnormalities  Simple goiter – thyroid enlarges due to lack of iodine in the diet  Hypothyroidism – low blood levels of thyroid hormones A. Congenital hypothyroidism: thyroid does not develop properly and is characterized in a short, stocky person that may be developmentally disabled B. Myxedema: hypothyroidism in adults characterized by lethargy, weight gain, loss of hair, cold intolerant, and thick, puffy skin  Hyperthyroidism – excess thyroid hormones in the blood A. Exophthalmic goiter: such as seen in Graves' disease and is characterized by enlargement of the thyroid gland, protrusion of the eyes, hyperactive, and suffers from insomnia B. Thyroid tumor: can also cause hyperthyroidism

Answer 79

In addition to glucocorticoids, the **adrenal cortex** secrete small amounts of sex hormones called These include male sex hormones (androgens) and female sex hormones (estrogen). The primary androgen hormone is **dehydroepiandrosterone (DHEA),** which is a precursor for testosterone, the primary male sex hormone. While primarily active in males following puberty, androgens do play a role in the sexual development of both males and females. In addition, these regions produce small amounts of **estradiol,** a form of estrogen. Although most estrogen in females is produced by the ovaries, the adrenal estradiol does play an important role in **regulating growth of the skeleton in puberty and maintaining bone mass.**

Answer 80

1. The nervous and endocrine systems work so closely together that they form what is sometimes called the neuroendocrine system. 2. As we have seen, the **hypothalamus** certainly bridges the regulatory activities of the nervous and endocrine systems. * In addition to producing the hormones released by the **posterior pituitary,** the hypothalamus produces hormones that control the **anterior pituitary**. 3. The nerves of the **autonomic system**, which control other organs, are acted upon directly by the **hypothalamus**. 4. The hypothalamus truly belongs to both the nervous and endocrine systems. Indeed, it is often and appropriately referred to as a neuroendocrine organ.Page 351

Answer 81

Homeostasis

Answer 82

**_Posterior Pituitary_** Figure 16.6 Hormones produced by the hypothalamus and posterior pituitary. The hypothalamus produces two hormones, ADH and oxytocin, stored and secreted by the posterior pituitary.

Answer 83

Control of Hormone Release • Hormone levels in the blood are maintained by negative feedback: • Stimulus triggers release of hormone that inhibits further hormone release

Answer 84

hypothalamus

Answer 85

Effects of Growth Hormone Growth hormone is produced by the anterior pituitary. The quantity is greatest during childhood and adolescence, when most body growth is occurring. If too little GH is produced during childhood, the individual has pituitary dwarfism, characterized by perfect proportions but small stature. The Bioethics feature “Growth Hormones and Pituitary Dwarfism” in this section discusses how a synthetic growth hormone Page 336can be used to treat some forms of dwarfism. If too much GH is secreted, gigantism may result (Fig. 16.9). Individuals with gigantism often have additional health problems, primarily because GH has a secondary effect on the blood sugar level, promoting an illness called diabetes mellitus (see Section 16.5). Figure 16.9 Growth hormone influences height. Irregularities in growth hormone can lead to gigantism. ©Xinhua News/Associated Press BIOLOGY TODAY Bioethics Growth Hormones and Pituitary Dwarfism Without treatment, children with a deficiency of growth hormone (GH) experience pituitary dwarfism: slow growth, short stature, and in some cases failure to begin puberty. Prior to the advent of biotechnology in the 1980s, treating these children was incredibly difficult and expensive. The GH needed to treat deficiencies had to be obtained from cadaver pituitaries. Although the treatment was generally very successful, the use of cadaveric GH caused Creutzfeldt–Jakob disease (a neurological disease similar to “mad cow” disease) in a small number of treated individuals. Thanks to biotechnology, technologists are now able to synthesize human GH (HGH) using bacteria. These bacteria have had the gene for HGH inserted into their genetic information. The altered bacteria are then grown in laboratories and make unlimited amounts of GH. Children with insufficient GH can be treated more safely and inexpensively with this GH. Recombinant HGH can also be used to treat other disorders, such as the chromosomal deficiency known as Turner syndrome (discussed in Section 19.6). It may even be possible to slow or reverse the aging process with HGH treatments. There is some controversy surrounding treating short children without HGH deficiency for essentially cosmetic reasons. Unfortunately, Americans are obsessed with height. Shorter children are often bullied and teased by their peers. Some data suggest that shorter individuals are discriminated against at their jobs. Their salaries are often lower than those of their taller counterparts with equivalent education and experience. Many people of short stature report having greater self-esteem problems than individuals of average to above-average height. Treatment with HGH could be the solution to these problems. Although the supply of HGH is seemingly unlimited, the cost of treatments is still quite high (though much cheaper than cadaveric GH), with annual treatments costing up to $25,000. In most cases, insurance companies will not cover these costs. Of greater concern, however, are the potential side effects of supplemental HGH therapy, which are not well understood. Moreover, it is not clear whether HGH treatment will result in a significant increase in the final height of short children. Questions to Consider Now that HGH is easier to obtain, what potential abuses would you predict? Do you think insurance companies should be expected to pay for HGH treatment if a child shows no hormone deficiency and is simply short? On occasion, GH is overproduced in the adult and a condition called acromegaly results. Long bone growth is no longer possible in adults, so only the feet, hands, and face (particularly the chin, nose, and eyebrow ridges) can respond, and these portions of the body become overly large (Fig. 16.10).Page 337 Figure 16.10 Overproduction of growth hormone in adults leads to acromegaly. Acromegaly is caused by overproduction of GH in the adult. It is characterized by enlargement of the bones in the face, fingers, and toes as a person ages. (both hands): ©Bart's Medical Library/Medical Images; (man): ©Yasser Al-Zayyat/AFP/Getty Images CHECK YOUR PROGRESS 16.2 Explain how the endocrine system and nervous system communicate with one another. Answer Through neurotransmitters and hormones—for example, the nervous system sends input to the adrenal medullae, so that a fight-or-flight response can be triggered when needed. Meanwhile, several hormones secreted by the endocrine system regulate the hypothalamus and/or anterior pituitary. List the hormones produced by the posterior pituitary and provide a function for each. Answer Posterior pituitary does not produce any hormones, but it stores and releases ADH and oxytocin produced in the hypothalamus. ADH conserves water, and oxytocin stimulates uterine contractions and milk letdown. List the hormones produced by the anterior pituitary and provide a function for each. Answer TSH stimulates the thyroid to produce T3 and T4; ACTH stimulates the adrenal cortex to produce glucocorticoids; gonadotropic hormones FSH and LH stimulate the gonads to produce gametes and sex hormones; PRL causes breast development and milk production; MSH causes skin color changes; GH promotes skeletal and muscular growth. CONNECTING THE CONCEPTS For more information on the hormones presented in this section, refer to the following discussions: Section 12.2 examines the influence of growth hormone on bone growth. Section 17.2 describes the role of pituitary hormones in the production of sperm cells in males. Section 17.4 describes the role of pituitary hormones in the female ovarian cycle.

Answer 86

SCIENCE IN YOUR LIFE How is labor induced if a woman’s pregnancy extends past her due date? After medication to prepare the birth canal for delivery, oxytocin (Pitocin) is used to induce labor. Pitocin is a synthetic version of the oxytocin released by the posterior pituitary. During labor, oxytocin may also be given to increase the strength of contractions. Stronger contractions speed the labor process if necessary (e.g., if the woman’s uterus is contracting poorly or if the health of the mother or child is at risk during delivery). Oxytocin is routinely used following delivery to minimize postpartum bleeding by ensuring that strong uterine contractions continue. Use of oxytocin must be monitored carefully, because it may cause excessive uterine contractions. Should this occur, the uterus could tear itself. Further, reduced blood supply to the fetus caused by very strong contractions may be fatal to the baby. Though it reduces the duration of labor, inducing labor with oxytocin can be very painful for the mother. Whenever possible, physicians prefer gentler and more natural methods to induce labor and/or strengthen contractions.

Answer 87

Prostaglandins

Answer 88

Figure 16.7 Hormones produced by the anterior pituitary. The hypothalamus controls the secretions of the anterior pituitary, and the anterior pituitary controls the secretions of the thyroid, adrenal cortex, and gonads, which are also endocrine glands.

Answer 89

Type 1 Diabetes

Answer 90

1. **counteract the inflammatory response** that leads to **pain and swelling.** * Very high levels of glucocorticoids in the blood can suppress the body’s defense system, including the inflammatory response that occurs at infection sites. * Cortisone and other glucocorticoids can relieve swelling and pain from inflammation. * However, by suppressing pain and immunity, they can also make a person highly susceptible to injury and infection. 2.