946
UNIT 4
Maintenance of the Body
24
weather), skin cells deprived of oxygen and nutrients begin to
die. Tis extremely serious condition is
frostbite
.
2.
Shivering.
Shivering—involuntary shuddering contrac-
tions—is triggered when brain centers controlling muscle
tone are activated and muscle tone reaches sufficient levels
to alternately stimulate stretch receptors in antagonistic
muscles. Shivering raises body temperature because skel-
etal muscle activity produces large amounts of heat.
3.
Increase of metabolic rate.
Cold stimulates the adre-
nal medulla to release epinephrine and norepinephrine
in response to sympathetic nerve stimuli, elevating the
vessels of the skin causes strong vasoconstriction. Tis re-
stricts blood to deep body areas and largely bypasses the
skin. Because a layer of insulating subcutaneous (fatty) tis-
sue separates the skin from deeper organs, this reduces
heat loss from the shell dramatically and lowers shell tem-
perature toward that of the external environment.
Homeostatic Imbalance 24.7
Restricting blood flow to the skin is not a problem for a brief
period, but if it is prolonged (as during exposure to very cold
Activates
heat-loss
center in
hypothalamus
Sweat glands secrete
perspiration, which is
vaporized by body heat,
helping to cool the body
Skin blood vessels
dilate: capillaries
become flushed with
warm blood; heat radiates
from skin surface
Body temperature
decreases: blood
temperature
declines and
hypothalamus
heat-loss center
“shuts off”
Stimulus
Increased body
temperature;
blood warmer
than hypothalamic
set point
Homeostasis:
Normal body temperature (35.8°C–38.2°C)
Stimulus
Decreased body
temperature;
blood cooler than
hypothalamic set
point
Body temperature
increases: blood
temperature rises
and hypothalamus
heat-promoting
center “shuts off”
Activates heat-
promoting center
in hypothalamus
Skeletal muscles
activated when more
heat must be generated;
shivering begins
Skin blood vessels constrict, which
diverts blood from skin capillaries
to deeper tissues, minimizing
overall heat loss from skin
surface
IMBALANCE
IMBALANCE
Figure 24.26
Mechanisms of body temperature regulation.
previous page 980 Human Anatomy and Physiology (9th ed ) 2012 read online next page 982 Human Anatomy and Physiology (9th ed ) 2012 read online Home Toggle text on/off