Objectives
Identify the functions of some of the hormones secreted by endocrine glands.
Summarize the negative feedback mechanism controlling hormone levels in the body.
Contrast the roles of steroid and amino acid hormones.

Vocabulary
endocrine glands
pituitary gland
hypothalamus
target tissues
receptor
negative feedback system
adrenal glands
thyroid gland
parathyroid glands

Imagine yourself lined up as quarterback behind these offensive linemen. Could you see over everyone’s head? Do you wonder how tall you will end up being? Because you continue growing until about age 25, you may still be a few inches shy of your adult height. What controls your growth? In part, chemical messages within your body. They can affect how long and at what rate you grow.

Control of the Body
Internal control of the body is directed by two systems: the nervous system, which you will learn more about later, and the endocrine system. The endocrine system is made up of a series of glands, called endocrine glands, that release chemicals directly into the bloodstream. These chemicals act as messengers to relay messages to other parts of the body, as shown in Figure 35.10. Whereas the nervous system produces an immediate response, the endocrine system induces gradual change. Let’s take one of the football players as an example: While his nervous system is directing his legs to run in order to catch a forward pass, his endocrine system is controlling the rate at which he grows. The first response is instant; the second takes years.

Word Origin
endocrine
From the Greek words endo, meaning "within," and krinein, meaning "to separate." The endocrine glands secrete hormones into the blood.

Figure 35.10
The diagram shows the principal human endocrine glands. The top label indicates the name of the gland, the middle label indicates the type of hormone(s) secreted, and the bottom label tells the action of the gland/hormone.

Interaction of the nervous system and endocrine system
Although endocrine glands are found throughout the body, most of them are controlled by the action of the pituitary gland (puh TEW uh ter ee), the master endocrine gland. Because there are two control systems within the body—nervous and endocrine—there must be coordination between them. The hypothalamus is the portion of the brain that controls the pituitary gland. The pituitary gland is located in the skull just beneath the hypothalamus, and the two are connected by nerves and blood vessels, as shown in Figure 35.11. The hypothalamus sends messages to the pituitary, which then releases its own chemicals, or stimulates other glands to release theirs. Other endocrine glands under control of the pituitary include the thyroid gland, the adrenal glands, and glands associated with reproduction.

Figure 35.11
The hypothalamus connects the nervous system and the endocrine system. The pituitary gland regulates most of the other glands of the endocrine system.

Endocrine control of the body
The chemicals secreted by the endocrine glands into the bloodstream are called hormones. Recall that a hormone is a chemical released in one part of an organism that affects another part. Hormones convey information to other cells in your body, giving them instructions regarding your growth, development, and behavior. Once released by the glands, the hormones travel in the bloodstream and then attach to specific binding sites found on the plasma membranes, or in the nuclei, of target tissue cells. These binding sites are called receptors.

Example of endocrine control
Human growth hormone (hGH) is a good example of an endocrine system hormone. When your body is actively growing, blood glucose levels are slightly lowered as the growing cells use up the sugar. This low blood glucose level is detected by the hypothalamus, which stimulates the production and release of hGH from the pituitary into the bloodstream. hGH binds to receptors on the plasma membranes of liver cells, stimulating the liver cells to release glucose into your blood. Your cells need the glucose in order to continue growing. Figure 35.12 summarizes the control of hGH by the pituitary gland. You can further investigate growth rate in humans by doing the BioLab at the end of this chapter.

Figure 35.12
The hypothalamus and pituitary gland control the amount of hGH hormone in your blood.

Negative Feedback Control
The amount of hormone released by an endocrine gland is determined by your body’s demand for that hormone at a given time. In this way, the endocrine system ensures that the appropriate amounts of hormone are present in the system at all times.

How do your endocrine glands know when your body needs a certain hormone? The endocrine system is controlled by a self-regulating system called the negative feedback system. The negative feedback system is a system in which the hormones, or their effects, are fed back to inhibit the original signal. The thermostat in your home is controlled by a similar negative feedback system. It maintains the room at a set temperature. When the temperature drops, the thermostat senses the lack of thermal energy and signals the heater to increase its output. When the thermal energy of the room rises again to a certain point, the thermostat no longer stimulates the heater, which shuts off. When the temperature drops again, the process repeats itself. In this negative feedback system, the increase in temperature "feeds back" to signal the thermostat to stop stimulating thermal energy production.

Feedback control of hormones
The majority of endocrine glands operate under negative feedback systems. A gland synthesizes and secretes its hormone, which travels in the blood to the target tissue where the appropriate response occurs. Information regarding the hormone level or its effect on the target tissue is fed back, usually to the hypothalamus or pituitary gland, to regulate the gland’s production of the hormone.

Control of blood water levels
Let’s take a look at an example of a hormone that is controlled by a negative feedback system. After working out in the gym and building up a sweat, you are thirsty. This is because sweating has reduced the water content of your blood. The hypothalamus, which is able to sense the concentration of water in your blood, determines that your body is dehydrated. In response, it stimulates the pituitary gland to release antidiuretic (ANT ih di uh reht ihk) hormone (ADH). ADH reduces the amount of water in your urine. It does so by binding to receptors in kidney cells, promoting their absorption of water and reducing the amount of water excreted in urine. Information regarding blood water levels is constantly fed back to the hypothalamus so it can regulate the pituitary’s release of ADH. If the body becomes overhydrated, the hypothalamus stops stimulating release of ADH.

Control of blood glucose levels
Another example of a negative feedback system involves the regulation of blood glucose levels. Unlike most other endocrine glands, the pancreas is not controlled by the pituitary gland. When you have just eaten and your blood glucose levels are high, your pancreas releases the hormone insulin. Insulin signals liver and muscle cells to take in glucose, thus lowering blood glucose levels. When blood glucose levels become too low, another pancreatic hormone, glucagon, is released. Glucagon binds to muscle and liver cells, signaling them to release glucose. Learn more about glucose storage and release by doing the Problem-Solving Lab on this page.
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Problem-Solving Lab 35-3 Interpreting the Data
What are the effects of glucagon and insulin during exercise? Exercise represents a special example of rapid fuel mobilization in the body. The body must gear up to supply great amounts of glucose and oxygen for muscle metabolism. The glucose use in a resting muscle is generally low but changes dramatically with exercise. Within ten minutes of beginning exercise, glucose uptake from the blood may increase by fifteenfold; within 60 minutes, it may increase by thirtyfold.

Analysis
The graph here shows the effects of prolonged exercise on blood insulin and glucagon levels in humans.

graph

Thinking Critically
Explain why glucagon concentration goes up and insulin concentration goes down during exercise, and how these actions help get glucose to the body cells.
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Hormone Action
Once hormones are released by an endocrine gland, they travel to the target tissue and cause a change. Hormones can be grouped into two basic types according to how they act on their target cells: steroid hormones and amino acid hormones.

Action of steroid hormones
Hormones that are made from lipids are called steroid hormones. Steroid hormones are lipid-soluble and therefore diffuse freely into cells through their plasma membranes. There they bind to a hormone receptor inside the cell. The hormone-receptor complex then travels to the nucleus where it activates the synthesis of specific messenger RNA molecules. The mRNA molecules move out to the cytoplasm where they activate the synthesis of the required proteins. This process is shown in Figure 35.13.

Figure 35.13
Steroid hormones enter cells, bind to a receptor, which in turn binds to DNA. The action of these hormones results in the production of specific proteins.

Action of amino acid hormones
The second group of hormones is made from amino acids. Recall that amino acids can be strung together in chains and that proteins are made from long chains of amino acids. Some hormones are short chains of amino acids and others are large chains. These hormones, once secreted into the bloodstream, bind to receptors embedded in the plasma membrane of the target cell. From there, they activate ion channels in the membrane, or route signals from the surface of the membrane down a pathway to activate enzyme pathways inside the cell. The enzyme pathways, in turn, alter the behavior of other molecules inside the cell, as shown in Figure 35.14. In this way, the hormone is able to control the actions inside the target cell.

Figure 35.14
When an amino acid hormone binds to the receptor on the cell membrane, it can control internal cell functions.

Adrenal Hormones and Stress
You are sitting in math class and the teacher is about to hand out the semester test. Because this test is an important one, you have spent many hours studying for it. Like most of your classmates, you are a little nervous as the test is being passed down the row. Your heart is beating fast and your hands are a little sweaty. As you review the first problem, however, you begin to calm down because you know how to solve it.

The adrenal glands play an important role in preparing your body for stressful situations. The adrenal glands, located on top of the kidneys, consist of two parts—an inner portion and an outer portion. The outer portion secretes steroid hormones, including glucocorticoids (glew ko KOR tuh koydz) and aldosterone (ahl duh STEER ohn). These steroid hormones cause an increase in available glucose and raise blood pressure. In this way, they help the body combat stresses such as fright, temperature extremes, bleeding, infection, disease, and even test anxiety.

Word Origin
adrenal
From the Latin words ad, meaning "attached to," and ren, meaning "the kidneys." The adrenal glands are located on top of the kidneys.

The inner portion of the adrenal gland secretes two amino acid hormones: epinephrine (ep uh NEF run)—often called adrenaline—and norepinephrine. Recall the fight-or-flight response discussed in the animal behavior chapter. During such a response, the hypothalamus relays impulses to the nervous system, which in turn stimulates the adrenal glands to increase their output of epinephrine and norepinephrine. These hormones increase heart rate, blood pressure, and rate of respiration; increase efficiency of muscle contractions; and increase blood sugar levels. If you have ever had to perform in front of a large audience, you may have experienced these symptoms, referred to collectively as an "adrenaline rush." This is how the body prepares itself to face or flee a stressful situation.

Other Hormones
The thyroid gland, located in the neck, regulates metabolism, growth, and development. The main metabolic and growth hormone of the thyroid is thyroxine. This hormone affects the rate at which the body uses energy and determines your food intake requirements.

The thyroid gland secretes calcitonin (kal suh TONE un)—a hormone that regulates calcium levels in the blood. Calcium is a mineral the body needs for blood clotting, formation of bones and teeth, and normal nerve and muscle function. Calcitonin binds to the membranes of kidney cells and causes an increase in calcium excretion. Calcitonin also binds to bone-forming cells, causing them to increase calcium absorption and synthesize new bone.

Another hormone involved in mineral regulation, the parathyroid hormone (PTH), is produced by the parathyroid glands, which are closely associated with the thyroid gland. It increases the rate of calcium, phosphate, and magnesium absorption in the intestines and causes the release of calcium and phosphate from bone tissue. It also increases the rate at which the kidneys remove calcium and magnesium from urine and return them to the blood. The overall effect of parathyroid hormone and calcitonin hormone interaction in the body is shown in Figure 35.15. Take a closer look at thyroid and parathyroid tissue by completing the MiniLab on this page.

As you can see, hormones associated with the endocrine system are responsible for controlling many different functions in your body, including your behavior and development.
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MiniLab 35-2 Observing
Compare thyroid and parathyroid tissue Although their names seem somewhat similar, the thyroid and parathyroid glands perform rather different functions within the body.

1. Copy the data table.
2. Use low power magnification to examine a prepared slide of thyroid and parathyroid endocrine gland tissue. Note: Both tissues appear on the same slide. Therefore you must move the slide slowly on your microscope stage while viewing through the eyepiece to help locate each tissue type.
3. Diagram A is a photograph of thyroid and parathyroid tissue. Use it as a guide in locating the two types of endocrine gland tissue under low power and in answering certain analysis questions.
4. Now locate each type of gland tissue under high power magnification. Draw what you see in the space provided in the table below. Then use what you learned in the chapter to identify the names of the hormones produced by each gland.

Data Table

Analysis
1. Compare the microscopic appearance of parathyroid tissue to that of thyroid tissue.
2.
   a. Which tissue type contains follicles (large liquid storage areas)?
   b. What may be present within the follicles?
   c. Are follicles composed of cells? Could they produce the hormone
       associate with this gland tissue? Explain your answer.
   d. Hypothesize what the function may be for the thin layer of tissue
       that surrounds each follicle.
3. How might you explain the fact that both thyroid and parathyroid tissue can be seen on the same microscope slide?
4. Explain how both glands are associated with bone management.
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Understanding Main Ideas
1. How does a steroid hormone affect its target cell?
2. Explain how the nervous system helps to control the endocrine system.
3. How does the negative feedback system work to control hormone levels in the blood?
4. What glands and hormones are involved in stress reactions?

Thinking Critically
5. Hormones continually make adjustments in blood glucose levels. Why must blood  glucose levels be kept fairly constant?

Skill Review
6. Comparing and Contrasting What effects do calcitonin and parathyroid hormone have on blood calcium levels? For more information, refer to Thinking Critically in the Skill Handbook.