8
UNIT 1
Organization of the Body
1
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
Atmospheric pressure
is the force that air exerts on the surface
of the body. Breathing and gas exchange in the lungs depend
on
appropriate
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
appropriate
amounts. ±oo
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
6.
What separates living beings from nonliving objects?
7.
What name is given to all chemical reactions that occur
within body cells?
8.
Why is it necessary to be in a pressurized cabin when flying
at 30,000 feet?
For answers, see Appendix H.
Homeostasis
Define homeostasis and explain its significance.
Describe how negative and positive feedback maintain
body homeostasis.
Describe the relationship between homeostatic imbalance
and disease.
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
homeostasis
(ho
0
me-o-sta
9
sis)
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
dynamic
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.
Growth
Growth
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.
Survival Needs
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
survival needs
, include nutri-
ents (food), oxygen, water, and appropriate temperature and
atmospheric pressure.
Nutrients
Nutrients
, 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.
Oxygen
All the nutrients in the world are useless unless
oxygen
is also
available. Because the chemical reactions that release energy
from foods are
oxidative
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.
Water
Water
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,
nor-
mal body temperature
must be maintained. As body tempera-
ture drops below 37°C (98.6°F), metabolic reactions become
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