11
CHAPTER ONE
Introduction to Human Anatomy and Physiology
Although most feedback mechanisms in the body are
negative, some changes stimulate further change. A process
that moves conditions away from the normal state is called a
positive feedback mechanism.
Positive feedback mechanisms may be important to
homeostasis and survival. In blood clotting, for example,
certain chemicals stimulate more clotting, which minimizes
falls too low, the pancreas detects this change and secretes a
different chemical (glucagon) that releases stored glucose into
the blood. Chapter 13 (pp. 509–511) discusses regulation of
blood glucose concentration in more detail (see F g. 13.36).
Human physiology offers many other examples of homeo-
static mechanisms, which all work by the basic mechanism
just described. Just as anatomical terms are used repeatedly
throughout this book, so the basic principles of physiology
apply in all organ systems.
Receptors
Thermostat in
room detects change.
Receptors
Thermostat in
room detects change.
too high
too low
Normal room
temperature
Thermostat
set point
Control center
Thermostat detects
deviation from set
point and signals
effectors.
Control center
Thermostat detects
deviation from set
point and signals
effectors.
Stimulus
Room temperature rises
above normal.
Effectors
Heater turns off; air
conditioner turns on.
Response
Room temperature
returns toward
set point.
Effectors
Heater turns on;
air conditioner
turns off.
Stimulus
Room temperature
decreases.
Response
Room temperature
returns toward set point.
Receptors
Thermoreceptors
send signals to the
control center.
too high
too low
Normal body
temperature
37
°
C (98.6
°
F)
Control center
The hypothalamus
detects the deviation
from the set point and
signals effector organs.
Control center
The hypothalamus
detects the deviation
from the set point and
signals effector organs.
If body temperature
continues to drop,
control center signals
muscles to contract
involuntarily.
Stimulus
Body temperature
rises above normal.
Effectors
Skin blood vessels
dilate and sweat glands
secrete.
Response
Body heat is lost to
surroundings, temperature
drops toward normal.
Receptors
Thermoreceptors
send signals to the
control center.
Effectors
Skin blood
vessels constrict
and sweat glands
remain inactive.
Stimulus
Body temperature
drops below normal.
Effectors
Muscle
activity
generates
body heat.
Response
Body heat is conserved,
temperature rises toward normal.
FIGURE 1.7
A thermostat signals an air conditioner and a furnace
to turn on or oF
to maintain a relatively stable room temperature. This
system is an example of a homeostatic mechanism.
FIGURE 1.8
The homeostatic mechanism that regulates body
temperature.
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