MODULE 7
Maintaining Internal Equilibrium: he Endocrine System

The human body is a machine far more complicated than any devised by man. Even more amazing than its vast complexity is the fact that it functions so consistently for such a long time. Scientists today are just beginning to grasp how bodily functions are regulated by joint action of the nervous system and the endocrine system.

As you read the text, try to answer the following questions.

Homeostasis is the tendency of the body to maintain a balance among its internal physiological conditions. The homeostatic mechanisms that maintain this balance control such things as body temperature, blood pressure, and nutrient metabolism.

Many homeostatic mechanisms are feedback systems. Temperature control, for example, is maintained by a system that uses information from an occurrence to determine the course of subsequent occurrences. When the body senses that it is too warm, glands in the skin secrete perspiration which then evaporates to cool the body. When the body feels too cold, muscular contractions (shivering) occur, generating heat. This tendency to maintain a constant temperature is a homeostatic mechanism.

There are many homeostatic systems. Since the nervous system is the body's internal communication network, it is of major importance in the maintenance of the body. Another system that is critical to homeostasis is the endocrine system.

The endocrine system is composed of glands, with the specialized function of secreting fluids. There are two classes of glands:

Exocrine glands release their secretions through a duct. Examples of exocrine glands are sweat glands, salivary glands, and the liver.

Endocrine glands release their secretions directly into the blood stream. Endocrine secretions are called hormones.

Figure 18 shows the location of the endocrine glands.

Psychologists are particularly interested in the endocrine system because certain of its functions, or malfunctions, have a profound influence on behavior.

The thyroid gland, for example, secretes thyroxin. This hormone acts to accelerate the body's metabolic rate. An over secretion of thyroxin results in a condition known as hyperthyroidism, in which the individual is overactive and very agitated. Such a person eats more than usual since he expends energy at a higher than normal rate.

The adrenal glands are located on top of the kidneys, and consist of an inner core, the adrenal medulla, and an outer section, the adrenal cortex. The medulla secretes the hormone epinephrine, or adrenaline, a substance which has an influence very similar to that of the sympathetic division of the autonomic nervous system. In fact, secretion of epinephrine is usually brought about through neural stimulation by the sympathetic system. The outer adrenal cortex secretes hormones that regulate carbohydrate metabolism (a process vital for body growth) and the use of water by the body.

Sexual behavior is regulated by the gonads, or sex glands. In males the testes secrete hormones which bring about and maintain the secondary sex characteristics such as increased growth of body hair and lowering of the voice In females the ovaries bring on menstruation and function in the complicated processes of pregnancy.

The pituitary gland is called the master gland because its activity influences the other endocrine glands. One hormone secreted by the pituitary gland is TSH (thyroid-stimulating hormone), which stimulates the secretion of thyroxin. ACTH (adrenocorticotrophic hormone) is also secreted by the pituitary. ACTH causes the adrenal cortex to release its hormones. The pituitary gland also secretes hormones that control gonad function. In addition to acting on other glands, the pituitary secretes a growth-stimulating hormone called somatotropic hormone.

The chart below summarizes the functions of the endocrine glands.
GLAND FUNCTION
Pituitary Stimulates growth (somatotropic hormone) Simulates the gonads Stimulates the thyroid gland (TSH) Stimulates the adrenal cortex (ACTH)
Thyroid Affects the metabolic rate (thyroxin)
Adrenal medulla Reacts to stimulation from the sympathetic nervous system Mobilizes the body in stressful situations by secreting epinephrine
Adrenal cortex Regulates carbohydrate metabolism and fluid balance
Gonads (testes and ovaries) Bring about and maintain secondary sex characteristics; regulate sexual behavior; control functions in pregnancy

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MODULE 7
PROGRESS CHECK 1

Now test yourself without looking back.

1. Define homeostatic mechanism.

2. Give the major function each of the following glands.

a. Adrenal medulla

b. Thyroid

c. Adrenal cortex

d. Testes

3. Which of the following are endocrine glands?
a. Adrenal medulla
b. Ovaries
c. Sweat glands
d. Salivary glands

4. Define.

a Endocrine gland

b. Exocrine gland

5. The pituitary stimulates growth by secreting

56

6. The pituitary indirectly affects carbohydrate metabolism by secreting

7. The pituitary has an indirect effect on sexual behavior through its action on

8. Through what kind of neural action is the secretion of epinephrine controlled?

ANSWER KEY

7 OR MORE CORRECT UNIT TEST
FEWER THAN 7 CORRECT - good idea to do exercises anyhow!

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MODULE 7
EXERCISES

A mechanism that tends to maintain a balance among internal physiological conditions is a(n)_____________________mechanism.
__________________________________________10

Which of the following are actions of homeostasis?
a. Stabilize body temperature
b. Keep blood pressure within normal limits
c. Knee-jerk reflex
d. Return respiration rate to normal

_______________________________________________7

Endocrine glands secrete hormones:

a. through a duct.
b. on a selected surface.
c. directly into the bloodstream.
d. into the nervous system.

___________________________________________ 6

Which of the following are endocrine glands?
a. Pituitary
b. Sweat
c. Adrenal medulla
d. Ovaries
e. Thyroid
f. Salivary

______________________________________________________ 9

Match.

1) Controls Male sexual function_____
2) Stimulates many other endocrine glands ______
3) Governs metabolic rate _______
4) Mobilizes body in stressful situations ____
5) Affects carbohydrate metabolism______

a. Adrenal cortex
b. Thyroid
c. Pituitary
d.Salivary
e. Testes
f. Ovaries
g. Adrenal medulla

What are the two main functions of the pituitary gland?

______________________and ____________________________2

The pituitary secretes adrenocorticotrophic hormone, which affects the_______________________________
_______________________________________________________5

The thyroid gland is stimulated by TSH, which is secreted by the_______________________________________gland.

_________________________________________________1

The secretion of epinephrine in times of stress is stimulated by the action of the __________________________ nervous system.

______________________________________________________8

NOW TAKE PROGRESS CHECK 2

1 pituitary gland
2 to control the secretion of other glands; to mediate or control growth
4 1) e 2) c 3) b 4) g 5) a
5 adrenal cortex
6 c
7 a, b, d
8 sympathetic
9 a, c, d, e
10 homeostatic

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MODULE 7 PROGRESS CHECK 2

1. Name the endocrine gland(s) important in each of the following functions.
a. Stimulates thyroid gland
b. Accelerates metabolic rate
c. Affect male sexual behavior
d. Mobilizes body in stress
e. Affect female sexual behavior
f. Stimulates growth of body
9. Affects carbohydrate metabolism
h. Stimulates adrenal Cortex

2. A homeostatic mechanism is one which
3. What are two differences between endocrine glands and exocrine glands?
4. Both adrenomrticotrophic hormone (ACTH) and thyroid stimulating hormone (TSH) are secreted by the_________________________________________________ ACTH acts on the ________________________________________ while TSH increases the level of the hormone _________________________________ in the body

5. A thyroxin excess may cause

6. The adrenal medulla is stimulated by the

ANSWER KEY PAGE 62

5 OR MORE CORRECT TAKE THE UNIT TEST

FEWER THAN 5 CORRECT INSTRUCTOR CONFERENCE

YOU MAY WISH TO VISIT THE

neuropathology web site.

October 19, 2000