An Overview of the Endocrine System

Objectives

When you have completed this section, you should be able to

• define hormone and endocrine system;
• compare and contrast the nervous and endocrine systems; and
• list the major organs of the endocrine system.

Key Point Review

1. Define the word hormone and distinguish a hormone from a neurotransmitter. Why is this an imperfect distinction?

2. Name some sources of hormones other than purely endocrine glands.

The Hypothalamus and Pituitary Gland

Objectives

When you have completed this section, you should be able to

• list the hormones produced by the hypothalamus and pituitary gland; and
• explain how the hypothalamus and pituitary are controlled and coordinated with each other.
• describe the functions of growth hormone; and
• describe the effects of pituitary hypo- and hypersecretion.

Think About It

Which of the unifying themes at the end of chapter 1 is best exemplified by the neuroendocrine reflexes that govern ADH secretion?

Think About It

How would the level of pituitary tropic hormones be affected by the surgical removal of their target organs from an experimental animal? Explain.

Key Point Review

3. What are two good reasons for considering the pituitary to be two separate glands?

4. Name three anterior lobe hormones that have reproductive functions and three that have nonreproductive roles. What target organs are stimulated by each of these hormones?

5. Briefly contrast hypothalamic control of the anterior pituitary with its control of the posterior pituitary.

6. In what sense does the pituitary "take orders" from the target organs under its command?

Other Endocrine Glands

Objectives

When you have completed this section, you should be able to

• describe the structure and location of the remaining organs of the endocrine system;
• name the hormones these endocrine organs produce and state their functions; and
• discuss some common effects of endocrine dysfunctions and hormone imbalances.

Think About It

Which could a person more easily live without–the adrenal medulla or adrenal cortex? Why?

Key Point Review

7. Name two endocrine glands that are larger in children than in adults.

8. What hormone increases the body's heat production in cold weather?

9. Name a glucocorticoid, a mineralocorticoid, and a catecholamine secreted by the adrenal gland.

10. Does the action of glucocorticoids more closely resemble that of glucagon or insulin? Explain.

11. What is the difference between a gonadal hormone and a gonadotropin?

Hormones and Their Actions

Objectives

When you have completed this section, you should be able to

• identify the chemical classes to which various hormones belong;
• describe how hormones are synthesized and transported to their target organs;
• describe the action of hormones on their target cells;
• explain how target cells adjust their sensitivity to circulating hormones;
• explain how hormones are removed from circulation after they have performed their roles; and
• explain how hormones affect each other when two or more of them stimulate the same target cells.

Think About It

Some hormones can be taken orally (such as birth-control pills and thyroxine), but insulin must be injected. Why would insulin be ineffective as an oral medication?

Think About It

During the synthesis of glycoprotein hormones, where in the cell would the carbohydrate be added? (See chapter 5.)

Think About It

Which hormone would produce the quickest effect on target cell metabolism–estradiol or insulin? Why?

Key Point Review

12. What are the three chemical classes of hormones? Name at least one hormone in each class.

13. Why do corticosteroids and thyroid hormones require transport proteins to travel in the bloodstream?

14. Explain how MIT, DIT, T₃, and T₄ relate to each other structurally.

15. What are the two locations of hormone receptors in target cells? Name one hormone that employs each receptor location.

16. How can one hormone molecule lead to the synthesis of thousands of enzyme molecules?

Eicosanoids and Other Chemical Messengers

Objectives

When you have completed this section, you should be able to

• define eicosanoid, name some of the major classes of eicosanoids, and explain their chemical origin; and
• describe several physiological roles of the prostaglandins.

Key Point Review

17. What are eicosanoids and how do they differ from neurotransmitters and hormones?

18. Distinguish between a paracrine and endocrine effect.

19. State any four functions of prostaglandins.

Stress and Adaptation

Objectives

When you have completed this section, you should be able to

• give a physiological definition of stress; and
• discuss how the body adapts to stress through its endocrine system.

Key Point Review

20. Define stress from the standpoint of endocrinology.

21. Describe the stages of the general adaptation syndrome.

22. List seven hormones that show increased secretion in the general adaptation syndrome. Describe how each one contributes to recovery from stress.

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