CHAPTER 25 Hormones

Biology And Society: Of Hunger and Hormones What causes hunger? In 1999, scientists discovered the hormone ghrelin, Which may induce hunger and stimulate eating. Copyright © 2007 Pearson Education Inc., publishing as Pearson Benjamin Cummings

The amount of ghrelin in the blood Tends to fluctuate during different times in a day.

Figure 25.1

Hormones: An Overview The endocrine system Hormones Is the body’s main system for internal chemical regulation. Hormones Are regulatory chemicals that affect specific sites in the body. Are made by endocrine glands.

Hormone secretion from an endocrine cell

Figure 25.2

There are two general mechanisms by which hormones trigger changes in target cells.

In the first mechanism, Hormones bring about changes without ever entering their target cells. Water-Soluble Hormone

Figure 25.3

In the second mechanism, Hormones trigger changes by binding to receptors inside the cell. Lipid-Soluble Hormone

Figure 25.4

The Human Endocrine System The human endocrine system consists of about a dozen major glands.

The major endocrine glands in humans

Figure 25.5

The actions of the major human endocrine hormones

Table 25.1

Table 25.1 (continued)

The Hypothalamus and Pituitary Gland Is the main control center of the endocrine system.

As part of the brain, the hypothalamus receives information from the nervous system and sends out appropriate responses.

Figure 25.6

The pituitary gland Receives signals directly from the hypothalamus. Consists of two parts, a posterior lobe and an anterior lobe.

The posterior lobe of the pituitary Stores and secretes hormones made in the hypothalamus.

The anterior lobe of the pituitary Synthesizes its own hormones, such as FSH, LH, and PRL, and secretes them directly into the blood.

The hypothalamus Operates through the posterior pituitary to direct the activity of the kidneys.

Figure 25.7

The anterior lobe of the pituitary also secretes growth hormone, which can have a broad effect on the body.

Figure 25.8

The Thyroid and Parathyroid Glands The thyroid gland Is located in the neck just under the larynx. Produces several hormones involved in controlling oxygen consumption, metabolism, and development.

Hypothyroidism Is a condition in which there is too little thyroid hormone in the blood. Can result from dietary deficiencies of iodine or from a defective thyroid gland.

Figure 25.9

Hyperthyroidism Is a condition in which there is too much thyroid hormone in the blood. Causes Graves’ Disease, an autoimmune disease.

The parathyroid glands Are four disk-shaped glands embedded within the thyroid. Produce hormones that control calcium homeostasis.

Calcitonin and parathyroid hormone (PTH) Have opposite effects that control calcium levels in the blood.

Figure 25.10

The Pancreas The pancreas Insulin and glucagon Produces two hormones that play important roles in managing the body’s energy supplies. Insulin and glucagon Help maintain a homeostatic balance of blood glucose and stored glucose. Are antagonists.

Control of blood glucose and stored glucose is carried out by negative feedback.

Figure 25.11

Diabetes mellitus Is a hormonal disease in which body cells are unable to absorb glucose. Includes type 1, or insulin-dependent diabetes, and type 2, or non-insulin-dependent diabetes. Can be treated.

The human body has two adrenal glands. The Adrenal Glands The human body has two adrenal glands. Each adrenal gland is two glands in one: The adrenal medulla The adrenal cortex

The hormones produced by the adrenal glands Enable the body to respond to stress. Are epinephrine (adrenaline) and norepinephrine (noradrenaline).

Stressful stimuli, whether negative or positive, activate nerve cells in the hypothalamus that send signals to stimulate the adrenal medulla.

Epinephrine and norepinephrine Are the “fight-or-flight” hormones that the adrenal medulla secretes. Ensure a rapid, short-term response to stress.

The adrenal cortex Is also stimulated by the hypothalamus. Secretes hormones that provide a slower, longer-lasting response to stress.

The short-term and long-term responses to stress

Figure 25.12 part 1

Figure 25.12 part 2

The gonads secrete sex hormones that Affect growth and development. Regulate reproductive cycles and sexual behavior.

The three categories of sex hormones are Androgens. Estrogens. Progestins.

Estrogens Maintain the female reproductive system. Promote the development of specific female features.

Progestins Are primarily involved in preparing the uterus to support a developing embryo.

Androgens Stimulate the development and maintenance of the male reproductive system.

Hormone production by the gonads is regulated by the hypothalamus and pituitary gland.

Evolution Connection: The Changing Roles of Hormones Hormones play important roles in all vertebrates. The same hormone can have different actions in different animals. Hormonal regulation was an early evolutionary adaptation. Copyright © 2007 Pearson Education Inc., publishing as Pearson Benjamin Cummings

The hormone prolactin (PRL) is a good example. It performs a variety of functions in nonmammals. It stimulates mammary glands to grow and produce milk in mammals.

Figure 25.13