During exercise and exposure to extreme envi-ronments, the body is faced with tremendous demands that require a multitude of physiologi- cal adjustments. Energy production must increase and metabolic by-products must be cleared. Cardiovascular and respiratory function must be constantly adjusted to match the demands placed upon these and other body systems, such as those regulating temperature.

While the body’s internal environment is in a constant state of flux even at rest, during exercise these well-orchestrated changes must occur rapidly and frequently.

The more rigorous the exercise, the more difficult it is to maintain homeostasis. Much of the regulation required during exercise is accomplished by the nervous system. But another physiological system affects virtually every cell, tissue, and organ in the body. It constantly monitors the body’s internal environment, noting all changes that occur and responding quickly to ensure that homeostasis is not dramatically disrupted.

It is the endocrine system that exerts this control through the hormones it releases. Because we cannot cover all aspects of endocrine control during exercise, the focus will be on hormonal control of metabolism and body fluids.

The Endocrine SystemThe Endocrine System As the body transitions from a resting to an active state,

the rate of metabolism must increase to provide the necessary energy. This requires the coordinated integration of many physiological and biochemical systems. Such integration is possible only if all of the involved tissues, organs, and systems can efficiently communicate. Although the nervous system is responsible for much of this communication, fine-tuning the physiological responses to any disturbance in homeostasis is primarily the responsibility of the endocrine system.

The endocrine and nervous systems, often collectively called the neuroendocrine system, work in concert to initiate and control movement and all of the physiological processes that support exercise. The nervous system functions quickly, having short-lived, localized effects, whereas the endocrine system responds more slowly but has longer-lasting effects.

The endocrine system is defined as all tissues or glands that secrete hormones. The major endocrine glands are illustrated in figure. Endocrine glands typically secrete their hormones directly into the blood where they act as chemical signals throughout the body. When secreted by the specialized endocrine cells,

Hormone are transported via the blood to specific target cells—cells that possess specific hormone receptors. On reaching their destinations, hormones can control the activity of the target tissue. Now hormones are more broadly defined as any chemical that controls and regulates the activity of certain cells or organs. Some hormones affect many body tissues, whereas others target very specific cells of the body.

Hormones

Hormones are involved in most physiological processes, so their actions are relevant to many aspects of exercise and sport.

Chemical Classification of Hormones

Hormones can be categorized as two basic types: steroid hormones and nonsteroid hormones. Steroid hormones have a chemical structure similar to cholesterol, since most are derived from cholesterol. For this reason, they are soluble in lipids so they diffuse rather easily through cell membranes.

This group includes the hormones secreted by:- the adrenal cortex (such as cortisol and aldo-

sterone), • The ovaries (estrogen and progesterone), • the testes (testosterone), and • the placenta (estrogen and progesterone).

Nonsteroid hormones are not lipid soluble, so they cannot easily cross cell membranes. The nonsteroid hormone group can be subdivided into two groups: protein or peptide hormones and amino acid–derived hormones. The two hormones produced by the thyroid gland (thyroxine and triiodothyronine) and the two from the adrenal medulla (epinephrine and norepi- nephrine) are amino acid hormones. All other non- steroid hormones are protein or peptide hormones.

Most hormone secretion is regulated by a negative feedback system. Secretion of a hormone causes some change in the body, and this change in turn inhibits further hormone secretion.

Negative feedback is the primary mechanism through which the endocrine system maintains homeostasis. for example, when the plasma glucose concentration is high, the pancreas releases insulin. Insulin increases cellular uptake of glucose, lowering plasma concentration of glucose. When plasma glucose concentration returns to normal, insulin release is inhibited until the plasma glucose level increases again.

Endocrine glands and Endocrine glands and Their HormonesTheir Hormones

Anterior pituitary Growth hormone (GH) Target Organ- All cells in the body Function- Promotes development and enlargement

of all body tissues until maturation; increases rate of protein synthesis; increases mobilization of fats and use of fat as an energy source; decreases rate of carbohydrate use

Thyrotropin (TSH) hormone Target organ- Thyroid gland Functions- Controls the amount of thyroxin and

triiodothyronine produced and released by the thyroid gland.

Adrenocorticotropin (ACTH) Target organ- Adrenal cortex Functions- Controls the secretion of hormones

from the adrenal cortex

Prolactin hormone Target organ- Breasts Function-Stimulates milk production by the

breasts

Follicle-stimulating hormone (FSH) Target organ- Ovaries, testes Functions- Initiates growth of follicles in the

ovaries and promotes secretion of estrogen from the ovaries; promotes development of the sperm in the testes.

Luteinizing hormone (LH) Target organ- ovaries and testis Functions- Promotes secretion of estro- gen and

progesterone and causes the follicle to rupture, releasing the ovum; causes testes to secrete testosterone

Posterior pituitaryPosterior pituitary

Antidiuretic hormone (ADH or vasopressin) Target organ- Kidneys Functions- Assist in controlling water excretion

by the kidneys; elevates blood pressure by constricting blood vessels

ThyroidThyroid

Thyroxine (T4) and triiodothyronine (T3) Target organ- all cell in the body Functions- Increase the rate of cellular

metabolism; increase rate and contractility of the heart

ParathyroidParathyroid

Parathyroid hormone (PTH or parathor- mone) Target organ- Bones, intes- tines, and kidneys Functions- Controls calcium ion con- centration in

the extracellular fluid through its influence on bones, intestines, and kid- neys

Adrenal medullaAdrenal medulla

Epinephrine Target organ- Most cells in the body Functions- Stimulates breakdown of glycogen in

liver and muscle and lipolysis in adipose tissue and muscle; increases skeletal muscle blood flow; increases heart rate and contractility; increases oxygen consumption

Norepinephrine Target organ- most cells in the body Functions- Stimulates lipolysis in adi- pose tissue

and in muscle to a lesser extent; constricts arterioles and venules, thereby elevating blood pres- sure

Adrenal cortexAdrenal cortex Mineralocorticoids (aldosterone) Target organ- kidneys Functions- increase sodium retention and

potassium excretion through the kidneys Glucocorticoids (cortisol) Target organ- most cells of the body Functions - Control metabolism of carbo-

hydrates, fats, and proteins; exert an anti-inflammatory action

Androgens and estrogens Target organ- Ovaries, breasts, and testes Functions- Assist in the development of female

and male sex charac- teristics

PancreasPancreas

Insulin Target organ- all cells in the body Functions-Controls blood glucose levels by

lowering glucose levels; increases use of glucose and synthesis of fat

Glucagon Target organ- all cells in the body Functions- Increases blood glucose; stimulates the

breakdown of protein and fat

KidneyKidney

Renin Target organ- Adrenal cortex Functions- Assists in blood pressure control Erythropoietin (EPO) Target organ- bone marrow Functions- Stimulates erythrocyte production

TestesTestes

Testosterone Target organ- sex organs and muscles Functions- Promotes development of male sex

characteristics, including growth of testes, scrotum, and penis, facial hair, and change in voice; promotes muscle growth

OvariesOvaries

Estrogens and progesterone Target organs- Sex organs and adipose tissue Functions- Promote development of female sex

organs and char- acteristics; increase storage of fat; assist in regulating the menstrual cycle

EFFECT OF EXERCISE ON HORMONAL SECRETION

Metabolic Rate

Exercise, particularly heavy weightlifting, stimulates the release of luteinizing hormone from your anterior pituitary gland, and luteinzing hormone triggers testosterone production. Exercise that involves intense bursts of energy also stimulates the release of thyroxine from your thyroid gland. Exercise can help you control or reduce your weight because testosterone and thyroxine speed up your metabolism.

Blood Sugar Insulin is a hormone that regulates your glucose, or blood

sugar, by transporting it to muscles and tissues that use glucose for energy. Excessive insulin in your blood reduces your sensitivity to insulin and can lead to diabetes. More glucose stays in the blood when insulin sensitivity goes down, and high blood glucose can cause nausea, vomiting, shortness of breath, organ failure, circulation problems and can lead to coma if left untreated. Exercise might increase your insulin sensitivity by reducing blood concentrations of insulin. Blood insulin levels begin decreasing after 10 minutes of aerobic exercise, and weight training might increase your sensitivity to insulin at rest.

Blood Flow The adrenal medulla releases epinephrine during

exercise and increases epinephrine levels at higher exercise intensities. Epinephrine increases the amount of blood that your heart pumps. Epinephrine also enhances your ability to use muscles during exercise by widening blood vessels, which lets your muscles get more oxygen-rich blood. Thyroxine secretions during exercise increase the amount of blood in your body by about 30 percent, and these secretions might remain elevated for around five hours.

Psychological Effects

The effects of exercise on your endocrine system might positively affect your mental state. Exercise-induced testosterone might increase confidence and libido. Conversely, low testosterone levels might inhibit your motivation, self-confidence, concentration and memory. Your pituitary gland may produce a large increase in blood endorphin levels shortly after exercise begins. Endorphins block your sensitivity to pain, and can reduce tension or anxiety by inducing a sense of euphoria.

Pituitary Gland

During exercise, the pituitary gland releases human growth hormone, which tells the body to increase bone, muscle and tissue production.

Thyroid Gland

When you start exercising, the thyroid gland (located at the base of the neck) sends out hormones that regulate the body’s temperature, heart rate and blood pressure. It also regulates the alertness and focus that are needed to work at a high intensity.

Adrenal Gland

Located at the top of the kidneys, the adrenal glands are responsible for the release of cortisol into the bloodstream. Cortisol levels control blood pressure, glucose and acts as an anti-inflammatory agent. The adrenal glands also release aldosterone, a hormone that regulates hydration levels, the speed of the heart and the strength of contractions. It also turns stored carbohydrates into energy.

Pancreas

Insulin regulates glucose, or blood sugar, by transporting it to the muscles and tissue that use glucose for energy. Excessive insulin in the blood reduces your sensitivity to insulin and can cause diabetes, which is also linked to overweight and obesity. Exercise improves insulin sensitivity and reduces the reliance on insulin injections

EFFECT OF EXERCISE ON HORMONAL SECRETION