34.1 The Endocrine System

The endocrine system is made up of glands that release hormones into the blood. Hormones deliver messages throughout the body.

Steroid hormones can easily cross cell membranes. Once inside the nucleus, they change the pattern of gene expression in target cells.

Nonsteroid hormones bind to receptors on cell membranes and cause the release of secondary messengers that affect cell activities.

• hormone (978)

• endocrine gland (979)

• target cell (978)

• prostaglandin (980)

• exocrine gland (979)

34.2 Glands of the Endocrine System

The pituitary gland secretes hormones that directly regulate many body functions or control the actions of other endocrine glands.

The hypothalamus controls the secretions of the pituitary gland.

The adrenal glands release hormones that help the body prepare for—and deal with—stress.

Insulin and glucagon help to keep the blood glucose level stable.

The thyroid gland has a major role in regulating the body's metabolism.

The two functions of gonads are the production of gametes and the secretion of sex hormones.

Like most systems of the body, the endocrine system is regulated by feedback mechanisms that function to maintain homeostasis.

• pituitary gland (982)

• norepinephrine (983)

• releasing hormone (983)

• thyroxine (985)

• corticosteroid (983)

• calcitonin (985)

• epinephrine (983)

• parathyroid hormone (985)

34.3 The Reproductive System

In females, the effects of the sex hormones include breast development and a widening of the hips. In males, they result in the growth of facial hair, increased muscular development, and deepening of the voice.

The main functions of the male reproductive system are to produce and deliver sperm.

The main function of the female reproductive system is to produce egg cells. The system also prepares the female's body to nourish an embryo.

Chlamydia is the most common bacterial STD in the United States. Viral STDs include hepatitis B, genital herpes, genital warts, and AIDS.

• puberty (988)

• ovary (991)

• testis (989)

• menstrual cycle (991)

• scrotum (989)

• ovulation (993)

• seminiferous tubule (989)

• corpus luteum (993)

• epididymis (989)

• menstruation (993)

• vas deferens (989)

• sexually transmitted disease (994)

• semen (990)

34.4 Fertilization and Development

The fusion of a sperm and egg cell is called fertilization.

Gastrulation produces the three cell layers of the embryo. Neurulation leads to the formation of the nervous system.

During the fourth, fifth, and sixth months after fertilization, the tissues of the fetus become more complex and specialized.

During the last three months before birth, the organ systems of the fetus mature, and the fetus grows in size and mass.

• zygote (995)

• neurulation (997)

• blastocyst (996)

• placenta (998)

• implantation (996)

• fetus (998)

• gastrulation (997)

Think Visually Trace the outline of Figure 34–1. Without referring to the Figure, label as many endocrine glands as you can.