Essentials of Anatomy and Physiology

(avery) #1

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?
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