Essentials of Anatomy and Physiology

The result of all of these mechanisms working to-
gether is that all aspects of body functioning, that is, of
metabolism, are kept within normal limits, a steady
state or equilibrium. This is homeostasis.
In the chapters to come, you will find many more
examples of homeostasis. As you continue your study

of the human body, keep in mind that the proper func-

tioning of each organ and organ system contributes to

homeostasis. Keep in mind as well that what we call

the normal values of metabolism are often ranges, not

single numbers. Recall that normal body temperature

is a range: 97to 99F (36to 38C). Normal pulse

Organization and General Plan of the Body 11

Cells decrease

energy

production

Metabolic

rate

decreases

Bacteria

White blood cells

Hypothalamus

Heat gain

mechanisms

Key:

Stimulates Inhibits Leads to

Cells increase

heat

production

Fever

Metabolic

rate

increases

Cells increase

energy

production

Thyroid gland

Thyroxine

increases

Stimulates

thyroid

gland

Thyroxine

decreases

Thyroid gland

No longer

stimulates

thyroid gland

Metabolic

rate

increases

Hypothalamus

and pituitary gland

A

B

Hypothalamus

and pituitary gland

Figure 1–3. Feedback mechanisms. (A) The negative feedback mechanism of regulation

of metabolic rate by thyroxine. (B) The positive feedback mechanism triggered by a fever.

See text for description.

QUESTION:For each mechanism, where is the source of the “brake” or inhibition?