Organization of the Body
slower and slower, and ﬁnally stop. When body temperature is
too high, chemical reactions occur at a frantic pace and body
proteins lose their characteristic shape and stop functioning.
At either extreme, death occurs. Te activity of the muscular
system generates most body heat.
Appropriate Atmospheric Pressure
is the force that air exerts on the surface
of the body. Breathing and gas exchange in the lungs depend
atmospheric pressure. At high altitudes, where
atmospheric pressure is lower and the air is thin, gas exchange
may be inadequate to support cellular metabolism.
■ ■ ■
Te mere presence of these survival factors is not suﬃcient to
sustain life. Tey must be present in
much and too little may be equally harmful. For example, oxy-
gen is essential, but excessive amounts are toxic to body cells.
Similarly, the food we eat must be of high quality and in proper
amounts. Otherwise, nutritional disease, obesity, or starvation
is likely. Also, while the needs listed above are the most crucial,
they do not even begin to encompass all of the body’s needs. For
example, we can live without gravity if we must, but the quality
of life suﬀers.
Check Your Understanding
What separates living beings from nonliving objects?
What name is given to all chemical reactions that occur
within body cells?
Why is it necessary to be in a pressurized cabin when ﬂying
at 30,000 feet?
For answers, see Appendix H.
Deﬁne homeostasis and explain its signiﬁcance.
Describe how negative and positive feedback maintain
Describe the relationship between homeostatic imbalance
When you think about the fact that your body contains trillions
of cells in nearly constant activity, and that remarkably little usu-
ally goes wrong with it, you begin to appreciate what a marvelous
machine your body is. Walter Cannon, an American physiolo-
gist of the early twentieth century, spoke of the “wisdom of the
body,” and he coined the word
to describe its ability to maintain relatively stable internal condi-
tions even though the outside world changes continuously.
Although the literal translation of homeostasis is “unchang-
ing,” the term does not really mean a static, or unchanging, state.
Rather, it indicates a
state of equilibrium, or a balance,
in which internal conditions vary, but always within relatively
Because males produce sperm and females produce eggs
(ova), there is a division of labor in reproduction, and the repro-
ductive organs of males and females are diﬀerent (Figure 1.3k, l).
Additionally, the female’s reproductive structures provide the site
for fertilization of eggs by sperm, and then protect and nurture
the developing fetus until birth.
is an increase in size of a body part or the organism as
a whole. It is usually accomplished by increasing the number of
cells. However, individual cells also increase in size when not
dividing. For true growth to occur, constructive activities must
occur at a faster rate than destructive ones.
Te ultimate goal of all body systems is to maintain life. How-
ever, life is extraordinarily fragile and requires several factors.
Tese factors, which we will call
, include nutri-
ents (food), oxygen, water, and appropriate temperature and
, taken in via the diet, contain the chemical substances
used for energy and cell building. Most plant-derived foods are
rich in carbohydrates, vitamins, and minerals, whereas most
animal foods are richer in proteins and fats.
Carbohydrates are the major energy fuel for body cells. Proteins,
and to a lesser extent fats, are essential for building cell structures.
Fats also provide a reserve of energy-rich fuel. Selected minerals
and vitamins are required for the chemical reactions that go on in
cells and for oxygen transport in the blood. Te mineral calcium
helps to make bones hard and is required for blood clotting.
All the nutrients in the world are useless unless
available. Because the chemical reactions that release energy
from foods are
reactions that require oxygen, human
cells can survive for only a few minutes without oxygen. Ap-
proximately 20% of the air we breathe is oxygen. Te coopera-
tive eﬀorts of the respiratory and cardiovascular systems make
oxygen available to the blood and body cells.
accounts for 60–80% of our body weight and is the single
most abundant chemical substance in the body. It provides the
watery environment necessary for chemical reactions and the
ﬂuid base for body secretions and excretions. We obtain water
chieﬂy from ingested foods or liquids. We lose it from the body
by evaporation from the lungs and skin and in body excretions.
Normal Body Temperature
If chemical reactions are to continue at life-sustaining rates,
mal body temperature
must be maintained. As body tempera-
ture drops below 37°C (98.6°F), metabolic reactions become