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?