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Thymus Gland

The thymus gland (thi'mus gland), which lies in the mediastinum posterior to the sternum and between the lungs, is large in young children but diminishes in size with age. This gland secretes a group of hormones, called thymosins, that affect production and differentiation of certain white blood cells (T lymphocytes). The thymus gland plays an important role in immunity and is discussed in chapter 16 (p. 657).

Reproductive Glands

The reproductive organs that secrete important hormones include the ovaries, which produce estrogens and pro gesterone; the placenta, which produces estrogens, progesterone, and a gonadotropin; and the testes, which produce testosterone. These glands and their secretions are discussed in chapter 22.

Digestive Glands

The digestive glands that secrete hormones are generally associated with the linings of the stomach and small intestine. These structures and their secretions are described in chapter 17 (pp. 703-704).

Other Hormone-Producing Organs

Other organs that produce hormones include the heart, which secretes atrial natriuretic peptide (chapter 15, p. 584), and the kidneys, which secrete a hormone that stimulates red blood cell production called erythropoietin (chapter 14, p. 552).

O Where is the pineal gland located?

^9 What seems to be the function of the pineal gland?

^9 Where is the thymus gland located?

Stress and Its Effects

Because survival depends upon maintaining homeostasis, factors that change the body's internal environment are potentially life threatening. When such dangers are sensed, nerve impulses are directed to the hypothalamus, triggering physiological responses that resist a loss of homeosta-sis. These responses often include increased activity in the sympathetic division of the autonomic nervous system and increased secretion of adrenal hormones. A factor capable of stimulating such a response is called a stressor, and the condition it produces in the body is called stress.

Types of Stress

Stressors may be physical, psychological, or a combination of both.

Physical stress threatens tissues. This includes extreme heat or cold, decreased oxygen concentration, infections, injuries, prolonged heavy exercise, and loud sounds. Often physical stress is accompanied by unpleasant or painful sensations.

Psychological stress results from thoughts about real or imagined dangers, personal losses, unpleasant social interactions (or lack of social interactions), or any factors that threaten a person. Feelings of anger, fear, grief, anxiety, depression, and guilt cause psychological stress. In other instances, psychological stress may stem from pleasant stimuli, such as friendly social contact, feelings of joy or happiness, or sexual arousal. The factors that produce psychological stress vary greatly from person to person. A situation that is stressful to one person may not affect another, and what is stressful at one time may not be at another time.

Responses to Stress

The hypothalamus controls response to stress, termed the general stress (or general adaptation) syndrome. This response, evoked to stress of any kind, maintains homeostasis.

Inflammation is the immune system's generalized response to limit the effects of injury or infection. However, inflammation is painful and possibly destructive. The endocrine system keeps the immune system in check by increasing secretion by the pituitary and adrenal glands to temper inflammation. This is an example of how homeostasis operates between organ systems as well as within them.

Recall that the hypothalamus receives information from nearly all body parts, including visceral receptors, the cerebral cortex, the reticular formation, and limbic system. At times of stress, the hypothalamus responds to incoming impulses by activating the "fight or flight" response. More specifically, sympathetic impulses from the hypothalamus raise blood glucose concentration, the level of blood glycerol and fatty acids, heart rate, blood pressure, and breathing rate, and dilate the air passages. The response also shunts blood from the skin and digestive organs into the skeletal muscles and increases secretion of epinephrine from the adrenal medulla. The epinephrine, in turn, intensifies these sympathetic responses and prolongs their effects (fig. 13.35).

At the same time that sympathetic activity increases, the hypothalamus's release of corticotropin-releasing hormone (CRH) stimulates the anterior pituitary gland to secrete ACTH, which increases the adrenal cortex's secretion of cortisol. Cortisol supplies cells with amino acids and extra energy sources and diverts glucose from skeletal muscles to brain tissue (fig. 13.36). Stress can also trigger release of glucagon from the pancreas, growth hormone (GH) from the anterior pituitary, and antidiuretic hormone (ADH) from the posterior pituitary gland. Secretion of renin from the kidney may also be stimulated.

Glucagon and growth hormone help mobilize energy sources, such as glucose, glycerol, and fatty acids, and stimulate cells to take up amino acids, facilitating repair of injured tissues. ADH stimulates the kidneys to retain water. This action decreases urine output and helps to maintain blood volume—particularly important if a person is bleeding or sweating heavily. Renin, by increasing angiotensin II levels, helps stimulate the kidneys to retain sodium (through aldosterone), and through the vasoconstrictor action of an-giotension II contributes to maintaining blood pressure. Table 13.12 summarizes the body's reactions to stress.

The seventeen-year-old was visiting her family physician for the third time. She had recurrent stomach pains and seemed to have a constant respiratory infection. Unlike her previous visits, this time the woman seemed noticeably upset. Suspecting that her physical symptoms stemmed from a struggle to deal with a stressful situation, the doctor looked for signs — increased heart and respiratory rate, elevated blood pressure, and excessive sweating. He took a blood sample to measure levels of epinephrine and cortisol, while prodding her to talk about whatever was bothering her.

The young woman's cortisol was indeed elevated, which could account for her gastrointestinal pain and high blood pressure, as well as her impaired immunity. On the doctor's advice, she began seeing a psychologist. Her symptoms began to abate when she was able to discuss the source of her stress — a family member's illness and anxiety about beginning college.


Changes in internal environment

Sensory receptors stimulated

Sympathetic impulses

Sensory receptors stimulated


Sympathetic impulses


Various tissues

Blood glucose increases

_K Blood glycerol and .«_ fatty acids increase v

Adrenal medulla


Heart rate increases

Blood pressure rises

Breathing rate increases

Air passages dilate

Redistribution of blood flow

Figure 13.35

During stress, the hypothalamus helps prepare the body for fight or flight by triggering sympathetic impulses to various organs. It also stimulates release of epinephrine, intensifying the sympathetic responses.

What is stress?

Distinguish between physical stress and psychological stress.

Describe the general stress syndrome.

Life-Span Changes

General changes in the glands of the endocrine system are a decrease in size and increase in the proportion of each gland that is fibrous in nature. At the cellular level, lipofuscin pigment accumulates as the glands age. Functionally, hormone levels may change with advancing years. Treatments for endocrine disorders associated with aging are usually straightforward—supplementing deficient hormones, or removing part of an overactive gland or using drugs to block the action of an overabundant hormone.

Aging affects different hormones in characteristic ways. For growth hormone, with age the typical surge in secretion that occurs at night lessens somewhat. Lower levels of GH are associated with declining strength in the skeleton and muscles with advancing age. However, supplementing older people with GH in an attempt to duplicate the effects of exercise is dangerous, because excess hormone raises blood pressure and blood glucose levels, and may enlarge the spleen, liver, and kidneys.

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