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PART 3 Integration and Control Systems

glycogen to glucose in the liver and the release of glucose into theblood, thus providing skeletal muscles with a source of energy.Because epinephrine and glucagon have short half-lives, they canrapidly adjust blood glucose levels for varying conditions ofactivity.During sustained activity, glucose released from the liver andother tissues is not adequate to support muscle activity, and thedanger exists that blood glucose levels will become too low to sup-port brain function. A decrease in insulin prevents the uptake ofglucose by most tissues, thus conserving glucose for the brain. Inaddition to other functions, epinephrine, glucagon, cortisol, andGH cause an increase in fatty acids, triglycerides, and ketones inthe blood. Because GH increases protein synthesis and slows thebreakdown of proteins, muscle proteins are not used as an energysource. Consequently, glucose metabolism decreases, and lipidmetabolism in skeletal muscles increases. At the end of a longrace, for example, muscles rely to a large extent on lipid metabo-lism for energy.

18.8 Hormones of the

Reproductive System

LEARNING OUTCOMES

After reading this section, you should be able to

A. List and describe the functions of the hormonessecreted by the testes and ovaries.

B. Explain how the anterior pituitary regulates secretionby the testes and ovaries. C. Explain how the placenta acts as a temporary endocrinegland.

Predict 11

Explain why long-distance runners may not have much of a “kick” leftwhen they try to sprint to the finish line.

ASSESS YOUR PROGRESS

52. Describe the hormonal effects that occur immediatelyafter a meal to cause nutrients to move into cells andbe stored.

53. What occurs hormonally 1–2 hours after a meal thatcauses stored materials to be released and usedfor energy?

54. During exercise, how does sympathetic nervous systemactivity regulate blood glucose levels? Name fivehormones that interact to ensure that the brain and themuscles have adequate energy sources during exercise,and explain the role of each.

All aspects of reproduction, including puberty, menstruation,gamete formation, and pregnancy, are under control of reproduc-tive hormones. Reproductive hormones are secreted primarilyfrom the ovaries, testes, placenta, and pituitary gland (table 18.12).These hormones are discussed in chapter 28. The main endocrineglands of the male reproductive system are the testes. The func-tions of the testes depend on the secretion of FSH and LH fromthe anterior pituitary gland. The main hormone secreted by thetestes is testosterone, an androgen. Testosterone regulates theproduction of sperm cells by the testes and the development andmaintenance of male reproductive organs and secondary sexualcharacteristics. The testes secrete another hormone, called inhibin, which inhibits the secretion of FSH from the anteriorpituitary gland.The main endocrine glands of the female reproductive sys-tem are the ovaries. Like the testes, the functions of the ovariesdepend on the secretion of FSH and LH from the anterior pitu-itary gland. The main hormones secreted by the ovaries are estro-gen and progesterone. These hormones, along with FSH andLH, control the female reproductive cycle, prepare the mammaryglands for lactation, and maintain pregnancy. Estrogen and pro-gesterone are also responsible for the development of the female

TABLE 18.12

Hormones of the Reproductive Organs