Stress and the Adrenal Gland

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In 1936, a Canadian physiologist, Hans Selye, discovered that injections of a cattle ovary extract into rats (1) stimulated growth of the adrenal cortex; (2) caused atrophy of the lym-phoid tissue of the spleen, lymph nodes, and thymus; and (3) produced bleeding peptic ulcers. At first he attributed these effects to the action of a specific hormone in the extract. However subsequent experiments revealed that injections of a variety of substances—including foreign chemicals such as formaldehyde—could produce the same effects. Indeed, the same pattern occurred when Selye subjected rats to cold environments or when he dropped them into water and made them swim until they were exhausted.

The specific pattern of effects produced by these procedures suggested that the effects were due to something the procedures shared in common. Selye reasoned that all of the procedures were stressful. Stress, according to Selye, is the reaction of an organism to stimuli called stressors, which may produce damaging effects. The pattern of changes he observed represented a specific response to any stressful agent. He later discovered that stressors produce these effects because they stimulate the pituitary-adrenal axis. Under stressful conditions, there is increased secretion of ACTH from the anterior pituitary, and thus there is increased secretion of glucocorticoids from the adrenal cortex.

On this basis, Selye stated that there is "a nonspecific response of the body to readjust itself following any demand made upon it." A rise in the plasma glucocorticoid levels results from the demands of the stressors. Selye termed this nonspecific response the general adaptation syndrome (GAS). Stress, in other words, produces GAS. There are three stages in the response to stress: (1) the alarm reaction, when the adrenal glands are activated; (2) the stage of resistance, in which readjustment occurs; and (3) if the readjustment is not complete, the stage of exhaustion, which may lead to sickness and possibly death.

Glucocorticoids, such as hydrocortisone, can inhibit the immune system. For this reason, these steroids are often administered to treat various inflammatory diseases and to suppress the immune rejection of a transplanted organ. It seems reasonable, therefore, that the elevated glucocorticoid secretion that can accompany stress may inhibit the ability of the immune system to protect against disease. Indeed, studies suggest that prolonged stress results in an increased incidence of cancer and other diseases.

Selye's concept of stress has been refined by subsequent research. These investigations demonstrate that the sympathoadrenal system becomes activated, with increased secretion of epinephrine and norepinephrine, in response to stressors that challenge the organism to respond physically. This is the "fight-or-flight" reaction described in chapter 9. Different emotions, however, are accompanied by different endocrine responses. The pituitary-adrenal axis, with rising levels of glucocorticoids, becomes more active when the stress is of a chronic nature and when the person is more passive and feels less in control.

Test Yourself Before You Continue

1. List the categories of corticosteroids and identify the zone of the adrenal cortex that secretes the hormones within each category.

2. Identify the hormones of the adrenal medulla and describe their effects.

3. Explain how the secretions of the adrenal cortex and adrenal medulla are regulated.

4. Explain how stress affects the secretions of the adrenal cortex and medulla. Why does hypersecretion of the adrenal medullary hormones make a person more susceptible to disease?

Thyroid cartilage of larynx

Cricoid cartilage of larynx

Thyroid cartilage of larynx

Cricoid cartilage of larynx

List Glands And Its Secretions

Thyroid gland

Trachea

Thyroid gland

Trachea

List Glands And Its Secretions

■ Figure 11.21 The thyroid gland. (a) Its relationship to the larynx and trachea. (b) A scan of the thyroid gland 24 hours after the intake of radioactive iodine

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Essentials of Human Physiology

Essentials of Human Physiology

This ebook provides an introductory explanation of the workings of the human body, with an effort to draw connections between the body systems and explain their interdependencies. A framework for the book is homeostasis and how the body maintains balance within each system. This is intended as a first introduction to physiology for a college-level course.

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