Hormones. Background Recall that homeostasis is the consistent internal conditions that your body maintains. KEY IDEAS READING TOOLBOX

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1 CHAPTER 39 1 SECTION KEY IDEAS Endocrine System Hormones As you read this section, keep these questions in mind: What are the major functions of the endocrine system? Which structures produce and release hormones? Which cells can be affected by hormones? What are two ways that hormones cause changes inside a cell? How are hormone levels in the blood regulated? What Is the Endocrine System? Like the nervous system, the endocrine system helps to regulate and control other systems in your body. Remember that signals from the nervous system travel along neurons. They move very quickly. In contrast, signals from the endocrine system travel more slowly. In general, endocrine signals travel in the form of chemicals in the blood. They control changes that happen in the body over relatively long periods of time. The endocrine system has four main functions: regulate metabolism maintain salt, water, and nutrient balance in the blood control the body s responses to stress regulate growth, development, and reproduction What Carries Endocrine Signals? Hormones carry most signals from the endocrine system. Hormones are substances produced in one part of the body that cause changes in another part of the body. These changes generally help the body maintain homeostasis. READING TOOLBOX Summarize As you read this section, underline the main ideas. When you finish reading, write an outline of the section using the ideas you underlined. 1. Compare Give two differences between endocrine signals and signals from the nervous system. Background Recall that homeostasis is the consistent internal conditions that your body maintains. As these bicyclists sweat, they lose water and salt. In response, their endocrine systems produce hormones that help their bodies retain water and salt. This helps them maintain homeostasis. 2. Explain How does the endocrine system help maintain homeostasis when a person sweats? Interactive Reader 439 Endocrine System

2 SECTION 1 Hormones continued 3. Identify What are the main functions of endocrine glands? Where Are Hormones Made? Not all cells can make hormones. Only cells that are part of endocrine glands or endocrine tissues can produce and secrete, or give off, hormones. A gland is a structure that secretes hormones or other substances. There are two main kinds of glands: endocrine glands and exocrine glands. Endocrine glands release hormones directly into the blood or the fluid around cells. The main functions of endocrine glands are to produce and secrete hormones. The figure below shows the main endocrine glands. 4. List Name four endocrine glands that are found in both males and females. There are many endocrine glands in the human body. The hormones that endocrine glands secrete have many different functions. 5. Compare Give two differences between endocrine glands and exocrine glands. Exocrine glands do not produce or secrete hormones. They secrete nonhormonal substances, such as enzymes, through tubelike structures called ducts. Unlike exocrine glands, endocrine glands do not have ducts. Some organs are both endocrine glands and exocrine glands. For example, the pancreas acts as an exocrine gland when it secretes digestive enzymes to the small intestine. However, the pancreas acts as an endocrine gland when it secretes hormones into the bloodstream. Some organs, such as the brain, stomach, kidneys, and liver, contain cells that release hormones. These organs are not considered endocrine glands because their main function is not to secrete hormones. The cells in these organs that secrete hormones are called endocrine tissues. Interactive Reader 440 Endocrine System

3 SECTION 1 Hormones continued How Do Hormones Affect Cells? Recall that hormones travel through the bloodstream. As a result, they come into contact with most cells of the body. However, a hormone affects only certain cells. The cells that are affected by a hormone are called target cells. Target cells have specific receptor proteins that recognize and bind to a specific hormone. These receptor proteins can be on the cell membrane or inside the cell. There are two main types of hormones: amino-based hormones and cholesterol-based hormones. Aminobased hormones cannot pass through cell membranes. Cholesterol-based hormones can. Because of this difference, the two types of hormones affect cells in different ways. Type of hormone Amino-based hormones Cholesterol-based hormones How it works Amino-based hormones bind to receptor proteins on the surfaces of target cells. The proteins then cause enzymes inside the cell to produce molecules called second messengers. Second messengers activate other enzymes within the cell. Cholesterol-based hormones bind to receptor proteins inside target cells. When the hormone is bound to the receptor protein, it can directly activate or inactivate genes. Amino-based hormones and cholesterol-based hormones can produce the same effect through different processes. For example, glucagon is an amino-based hormone. Cortisol is a cholesterol-based hormone. Both hormones can increase the amount of glucose in the bloodstream. However, they produce this effect in different ways. Glucagon, like most amino-based hormones, binds to proteins on the outsides of target cells. The protein activates an enzyme, which produces a second messenger called camp. The camp molecules activate other enzymes, which break a compound called glycogen down into glucose. The glucose is then released into the blood. In contrast, cortisol passes through cell membranes and binds to receptor proteins inside target cells. The proteins then activate genes. The genes produce proteins that break down fats and proteins into glucose. The glucose is then released into the blood. 6. Compare How is the location of receptor proteins that bind to amino-based hormones different from that of receptor proteins that bind to cholesterol-based hormones? Summarize After you read this page, make a diagram showing the similarities and differences between glucagon and cortisol. Share your diagram with a small group. Explain how your diagram describes the two hormones. Interactive Reader 441 Endocrine System

4 SECTION 1 Hormones continued 7. Identify What are two ways the body controls hormone levels? Apply Concepts In a small group, brainstorm examples of positive and negative feedback that you may see in everyday life. For example, a thermostat that controls the temperature of a room is an example of a negative feedback mechanism. Share your examples with the rest of the class. Explain why each is an example of a feedback mechanism. 8. Explain Why are insulin and glucagon antagonistic hormones? What Controls the Production of Hormones? The body has to have a way of controlling how much of a hormone is in the blood at a given time. For example, imagine what would happen if the amount of glucagon or cortisol in the blood never changed. The body would keep producing glucose. Eventually, the amount of glucose in the blood would get too high, and the person would die. The body uses feedback mechanisms and antagonistic hormones to control hormone levels in the blood. In a feedback mechanism, one step in a series of events controls a previous step in the series. Antagonistic hormones are pairs of hormones that have opposite effects on the body. FEEDBACK MECHANISMS There are two main kinds of feedback mechanisms in the body: positive feedback and negative feedback. Most hormone systems in the body use negative feedback. Type of feedback Description Example Positive Negative A change in the body causes more change in the same direction. A change in the body prevents more change in the same direction. A hormone controls the release of eggs in women. A small amount of the hormone causes more of the hormone to be made, until an egg is released. The thyroid gland produces several hormones. The presence of these hormones in the blood causes the thyroid to stop making and releasing them. ANTAGONISTIC HORMONES Antagonistic hormones work in pairs to control body systems. For example, remember that glucagon causes the amount of glucose in the blood to increase. A hormone called insulin is an antagonistic hormone to glucagon. Insulin causes the amount of glucose in the blood to decrease. These two hormones act to control the level of glucose in the blood. When glucose levels are high, the body secretes insulin, which lowers glucose levels. When glucose levels are low, the body secretes glucagon, which increases glucose levels. Interactive Reader 442 Endocrine System

5 Section 1 Review SECTION VOCABULARY antagonistic hormone a hormone that counteracts the effect of another hormone endocrine gland a ductless gland that secretes hormones into the blood feedback mechanism a cycle of events in which information from one step controls or affects a previous step hormone a substance that is made in one cell or tissue and that causes a change in another cell or tissue located in a different part of the body second messenger a molecule that is generated when a specific substance attaches to a receptor on the outside of a cell membrane, which produces a change in cellular function target cell a specific cell to which a hormone is directed to produce a specific effect 1. Identify What are the four major functions of the endocrine system? 2. Describe Which cells can produce and secrete hormones? 3. Explain Hormones move in the blood, so they come into contact with many different cells. Why don t they affect every cell they come into contact with? 4. Describe Fill in the blank spaces in the flowchart below to describe how aminobased hormones affect cells. An amino-based hormone binds to a the outside of a target cell. on The causes enzymes in the cell to produce molecules called. The other enzymes cause a change in the cell. The activates other enzymes within the cell. 5. Compare How is positive feedback different from negative feedback? Interactive Reader 443 Endocrine System

6 CHAPTER 39 SECTION 2 Endocrine System Major Endocrine Glands KEY IDEAS As you read this section, keep these questions in mind: Which two glands control the endocrine system? Which glands regulate metabolism? How is the adrenal gland involved in responding to stress? Which glands and hormones regulate reproduction? READING TOOLBOX Define As you read this section, underline words you don t know. When you figure out what the words mean, write the words and their definitions in your notebook. What Controls the Endocrine System? Remember that endocrine glands produce and secrete hormones. Two of these glands the hypothalamus and the pituitary gland work with the nervous system to control the endocrine system. Both the hypothalamus and the pituitary gland are located in the brain. The hypothalamus affects both the nervous system and the endocrine system. It controls feelings of hunger and thirst. It also produces hormones that affect the pituitary gland and other parts of the body. Hypothalamus Anterior pituitary Posterior pituitary 1. Compare Give one difference between the anterior pituitary gland and the posterior pituitary gland. The anterior pituitary gland produces several important hormones. Some of them, called tropins, control hormone production in other glands. The others affect body processes, such as cell growth and division. The posterior pituitary gland does not produce hormones. However, it does store and release hormones that are made by the hypothalamus. These hormones affect other parts of the body, such as the kidneys. Interactive Reader 444 Endocrine System

7 SECTION 2 Major Endocrine Glands continued Which Glands Regulate Metabolism? Remember that metabolism is all of the chemical reactions that occur in the body. The thyroid gland, parathyroid glands, pancreas, and pineal gland control metabolism. Each has a different function. Gland Location Role Thyroid throat produces hormones that control metabolic rate, protein production, growth, development, and reproduction Parathyroid glands throat, below the thyroid produce a hormone that increases the amount of calcium in the blood Pancreas abdomen produces hormones that control the level of sugar in the blood Pineal gland brain produces the hormone that controls the sleep cycle How Does the Endocrine System Respond to Stress? Think about how you feel when someone scares you. Your heart beats faster. You may breathe harder. Your hands may start to sweat or shake. These are all reactions to stress. Two glands above your kidneys, the adrenal glands, control responses like these. Each adrenal gland has two parts: the adrenal medulla and the adrenal cortex. The adrenal medulla controls short-term responses to stress, which generally occur quickly and do not last very long. The adrenal cortex controls long-term responses to stress, which generally happen slowly and last a longer time. 2. Infer A person has Type I diabetes. His body cannot control the amount of sugar in his blood. Which gland do you think Type I diabetes affects? Explain your answer. 3. Identify Which glands control the body s responses to stress? Adrenal gland Adrenal cortex Kidney Adrenal medulla 4. Label On the figure, label the functions of the adrenal cortex and the adrenal medulla. Interactive Reader 445 Endocrine System

8 SECTION 2 Major Endocrine Glands continued 5. Describe What does epinephrine do? 6. Compare Give one similarity and one difference between gonadotropins and sex hormones. HORMONES PRODUCED BY ADRENAL GLANDS The adrenal medulla produces epinephrine and norepinephrine. Epinephrine increases heart rate and blood flow to the muscles and brain. It also causes the liver to produce more glucose so that the muscles have more energy available. Norepinephrine increases blood pressure to make sure muscles get enough oxygen and nutrients. This results in the fight-or-flight response humans feel when they are under sudden stress. The adrenal cortex produces several different hormones called corticosteroids. These hormones increase blood pressure and glucose levels. This helps a person s body respond to severe or long-term stress. However, if the stress lasts too long, corticosteroids can make the person more likely to get sick. What Hormones Control Reproduction? There are two main types of hormones that control reproduction: gonadotropins and sex hormones. Gonadotropins are produced by the pituitary gland. They control the functions of the gonads the ovaries and the testes. They cause the gonads to mature during puberty. They also cause the gonads to produce sperm and egg cells. Sex hormones are produced by the gonads. There are three types of sex hormones: androgens, estrogens, and progesterone. Type of hormone Description Effects Androgens Estrogens Progesterone male sex hormones; mainly produced by the testes female sex hormones; mainly produced by the ovaries female sex hormone; produced by the ovaries and by the placenta during pregnancy control the development and function of the male reproductive system; causes the development of male characteristics, such as facial hair work with progesterone to control the development and function of the female reproductive system works with estrogens to control the female reproductive system; causes changes in the uterus to prepare it for pregnancy Interactive Reader 446 Endocrine System

9 Section 2 Review SECTION VOCABULARY androgen a type of hormone that regulates the sexual development of males and that stimulates development of secondary sex characteristics in males epinephrine a hormone that is released by the adrenal medulla and that rapidly stimulates the metabolism in emergencies, decreases insulin secretion, and stimulates pulse and blood pressure; also called adrenaline estrogen a hormone that regulates the sexual development and reproductive function of females norepinephrine a chemical that is both a neurotransmitter produced by the sympathetic nerve endings in the autonomic nervous system and a hormone secreted by the adrenal medulla to stimulate the functions of the circulatory and respiratory systems (abbreviation, NE) progesterone a steroid hormone that is secreted by the corpus luteum of the ovary, that stimulates changes in the uterus to prepare for the implantation of a fertilized egg, and that is produced by the placenta during pregnancy 1. Identify Which two glands control the endocrine system? 2. Describe What are four functions of the thyroid gland? 3. Compare Give one similarity and one difference between epinephrine and norepinephrine. 4. Explain How can corticosteroids be both helpful and harmful to a person? 5. Describe What do androgens do? Interactive Reader 447 Endocrine System