10
UNIT ONE
fl ow and enabling deeper tissues to retain heat. At the same
time, small groups of muscle cells may be stimulated to con-
tract involuntarily, an action called shivering that produces
heat, which helps warm the body.
If a person becomes overheated, the hypothalamus
triggers a series of changes that dissipate body heat. Sweat
glands in the skin secrete watery perspiration. Water evapora-
tion from the surface carries away heat, cooling the skin. At
the same time, blood vessels in the skin dilate. This allows the
blood that carries heat from deeper tissues to reach the sur-
face where more heat is lost to the outside. Chapter 6 discusses
body temperature regulation in more detail (pp. 181 and 182).
Another homeostatic mechanism regulates the blood
pressure in the blood vessels (arteries) leading away from
the heart. In this instance, pressure-sensitive areas (sensory
receptors) within the walls of these vessels detect changes
in blood pressure and signal a pressure control center in the
brain. If the blood pressure is above the pressure set point,
the brain signals the heart, causing its chambers to contract
more slowly and less forcefully. Because of decreased heart
action, less blood enters the blood vessels, and the pressure
inside the vessels decreases. If the blood pressure drops
below the set point, the brain center signals the heart to
contract more rapidly and with greater force, increasing the
pressure in the vessels. Chapter 15 (pp. 585–587) discusses
blood pressure regulation in more detail.
A homeostatic mechanism regulates the concentration of
the sugar glucose in blood. In this case, cells of an organ called
the pancreas determine the set point. If the concentration of
blood glucose increases following a meal, the pancreas detects
this change and releases a chemical (insulin) into the blood.
Insulin allows glucose to move from the blood into various
body cells and to be stored in the liver and muscles. As this
occurs, the concentration of blood glucose decreases, and as it
reaches the normal set point, the pancreas decreases its release
of insulin. If, on the other hand, blood glucose concentration
Respiratory
system
Cardiovascular
system
Digestive
system
Blood
Internal
environment
External
environment
Organic waste,
excess salts, water
Unabsorbed
matter
Nutrients,
salts, water
Cell
Extracellular
fluid
O
2
in
Urinary
system
CO
2
out
FIGURE 1.5
Our cells lie within an internal f
uid environment (extracellular f
uid). Concentrations oF water, nutrients, and oxygen in the internal
environment must be maintained within certain ranges to sustain liFe.
Stimulus
(Change occurs
in internal
environment.)
Response
(Change is corrected.)
Receptors
Effectors
(muscles or glands)
Control center
(set point)
(Change is compared
to the set point.)
FIGURE 1.6
A homeostatic mechanism monitors a particular aspect
oF the internal environment and corrects any changes back to the
value indicated by the set point.
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