slide 1

In our investigation of the thyroid gland and pancreas, we saw how antagonistic hormones maintain homeostasis. Now let's look at an endocrine gland involved in the stress response. Located on top of the kidneys, our adrenal glands help us respond to physical and emotional stresses. In mammals, each adrenal gland is made up of two distinct regions that are actually different glands. The outer region is the adrenal cortex, and the inner region is the adrenal medulla.

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The adrenal medulla responds to nerve signals. In stressful conditions, nerve cells in the hypothalamus are stimulated. Via the sympathetic nervous system, they signal the cells of the adrenal medulla to release the hormones epinephrine and norepinephrine, or NE. Epinephrine is also known as adrenaline. These hormones trigger the fight-or-flight response—the way you react to stressful situations.

The release of epinephrine and norepinephrine gives your body a quick boost of energy. To provide extra fuel, they signal your liver to increase the breakdown of glycogen to glucose, and stimulate fat cells to release fatty acids. They also increase your heart rate and the volume of blood pumped by each beat. They increase oxygen delivery by dilating the bronchioles in the lungs.

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While the adrenal medulla responds to nerve signals, the adrenal cortex responds to endocrine signals from the hypothalamus. During a stressful situation, the hypothalamus produces releasing hormones that stimulate the anterior pituitary to release adrenocorticotropic hormone, or ACTH. As the name suggests, ACTH is a tropic hormone. When it reaches the target cells of the adrenal cortex, it stimulates them to produce and release a group of hormones called corticosteroids.

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One type of corticosteroid is the glucocorticoids. They affect glucose metabolism by increasing the effects of glucagon produced by the pancreas. More glucose is released, thereby giving the body extra fuel. Glucocorticoids also suppress the immune system. This helps explain why people who are under stress for a long time are at risk of becoming sick.

Another type of corticosteroid is the mineralocorticoids. They affect the blood's salt and water balance. They stimulate cells of the kidneys to retain sodium ions and water, instead of excreting them. This helps increase the overall blood volume of the body, which in turn raises the blood pressure. The last group of hormones we'll look at are a different type of steroid, produced by the gonads.

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The male gonads are the testes, and the female gonads are the ovaries. The gonads are the endocrine glands that produce the sex hormones, which are steroids. The testes synthesize mostly androgens, the male sex hormones. Testosterone is the main androgen. Androgens produced early in development are what cause an embryo to develop into a male. During puberty, androgens are involved in the development and maintenance of male secondary sex characteristics, including hair growth patterns and a low voice.

The ovaries produce mainly estrogens and progestins, the female sex hormones. The role of estrogen during puberty in females is equivalent to the role of androgens in males. Progestins are required to prepare and maintain the uterus for pregnancy.

Copyright 2006 The Regents of the University of California and Monterey Institute for Technology and Education