Introduction to Human Anatomy and Physiology
The body maintains homeostasis through a number of
self-regulating control systems, or
homeostatic mecha-
These mechanisms share the following three compo-
g. 1.6)
which provide information about speciF
conditions (stimuli) in the internal environment. A
receptor may be a molecule or a cell.
2. A
control center,
which includes a
set point,
tells what
a particular value should be (such as body temperature
at 98.6°±).
such as muscles or glands, which elicit
responses that alter conditions in the internal environment.
A homeostatic mechanism works as follows. If the
receptors measure deviations from the set point, effectors
are activated that can return conditions toward normal. As
conditions return toward normal, the deviation from the
set point progressively lessens, and the effectors gradually
shut down. Such a response is called a
negative feedback
ah-tiv fe
bak) mechanism, both because the devia-
tion from the set point is corrected (moves in the opposite or
negative direction) and because the correction reduces the
action of the effectors. This latter aspect is important because
it prevents a correction from going too far.
To better understand this idea of maintaining a stable
internal environment, imagine a room equipped with a fur-
nace and an air conditioner. Suppose the room temperature
is to remain near 20°C (68°±), so the thermostat is adjusted
to a set point of 20°C. A thermostat is sensitive to tempera-
ture changes, so it will signal the furnace to start and the air
conditioner to stop whenever the room temperature drops
below the set point. If the temperature rises above the set
point, the thermostat will cause the furnace to stop and the
air conditioner to start. These actions maintain a relatively
constant temperature
in the room
(f g. 1.7)
A similar homeostatic mechanism regulates body tem-
perature in humans
(f g. 1.8)
. The “thermostat” is a temper-
ature-sensitive region in a control center of the brain called
the hypothalamus. In healthy persons, the set point of this
body thermostat is at or near 37°C (98.6°±).
If a person is exposed to a cold environment and the
body temperature begins to drop, the hypothalamus senses
this change and triggers heat-conserving and heat-generating
activities. Blood vessels in the skin constrict, reducing blood
for the amoeba
(f g. 1.4)
. It survives and reproduces as long
as its lake or pond environment is of a tolerable temperature
and composition, and the amoeba can obtain food. With a
limited ability to move, the amoeba depends upon the condi-
tions in its lake or pond environment.
In contrast to the amoeba, humans are composed of 50
to 100 trillion cells in their own environment—our bodies.
Our cells, as parts of organs and organ systems, interact
in ways that keep this
internal environment
constant, despite an ever-changing outside environment.
Anatomically the internal environment is inside the body,
but consists of fl
uid that surrounds cells, called the
cellular fl
(see chapter 21, p. 811). The internal envi-
ronment protects our cells (and us!) from external changes
that would kill isolated cells such as the amoeba
g. 1.5)
The body’s maintenance of a stable internal environment
is called
sis), and it is so impor-
tant that it requires most of our metabolic energy. Many of
the tests performed on Judith R. during her hospitalization
(as described in this chapter’s opening vignette on page 2)
assessed her body’s return to homeostasis.
Requirements of Organisms
A chemical substance
For metabolic processes, as a medium
for metabolic reactions,
to transport
substances, and to regulate body
A form of energy
To help regulate the rates of
metabolic reactions
Various chemical
To supply energy and raw materials for the
production of necessary substances and for
the regulation of vital reactions
A force
Atmospheric pressure for breathing;
hydrostatic pressure to help circulate
A chemical substance
To help release energy from food substances
The amoeba is an organism consisting of a single, but
complex, cell (100×).
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