does mean “significant.” If the amount of carbon di-
oxide in the body fluids increases, it causes these fluids
to become too acidic. Therefore, carbon dioxide must
be exhaled as rapidly as it is formed to keep the
amount in the body within normal limits. Normally
this is just what happens, but severe pulmonary dis-
eases such as pneumonia or emphysema decrease gas
exchange in the lungs and permit carbon dioxide to
accumulate in the blood. When this happens, a person
is said to be in a state of acidosis, which may seriously
disrupt body functioning (see the sections on pH and
enzymes later in this chapter; see also Box 2–1: Blood
Gases).
CELL RESPIRATION
Cell respirationis the name for energy production
within cells and involves both respiratory gases, oxy-
gen and carbon dioxide. Many chemical reactions are
involved, but in its simplest form, cell respiration may
be summarized by the following equation:
Glucose (C 6 H 12 O 6 ) 6O 2 →6CO 2 6H 2 O AT P heat
This reaction shows us that glucose and oxygen
combine to yield carbon dioxide, water, ATP, and heat.
Food, represented here by glucose, in the presence of
oxygen is broken down into the simpler molecules
carbon dioxide and water. The potential energy in the
glucose molecule is released in two forms: ATP and
heat. Each of the four products of this process has a
purpose or significance in the body. The carbon diox-
ide is a waste product that moves from the cells into
the blood to be carried to the lungs and eventually
exhaled. The water formed is useful and becomes part
of the intracellular fluid. The heat produced con-
tributes to normal body temperature. ATP is used for
cell processes such as mitosis, protein synthesis, and
muscle contraction, all of which require energy and
will be discussed a bit further on in the text.
We will also return to cell respiration in later chap-
ters. For now, the brief description just given will suf-
fice to show that eating and breathing are interrelated;
both are essential for energy production.TRACE ELEMENTS
Trace elementsare those that are needed by the body
in very small amounts. When they are present in food
or nutritional supplements, we often call them miner-
als, and examples are iron, cobalt, and zinc. Although
they may not be as abundant in the body as are carbon,
hydrogen, or oxygen, they are nonetheless essential.
Table 2–2 lists some of these trace elements and their
functions (see also Box 2–2: Nitric Oxide).30 Some Basic Chemistry
BOX2–1 BLOOD GASES
oxygen falls below the normal range, oxygen will
be administered; if blood carbon dioxide rises
above the normal range, blood pH will be corrected
to prevent serious acidosis.
Damage to the heart may also bring about a
change in blood gases, especially oxygen. Oxygen
is picked up by red blood cells as they circulate
through lung capillaries; as red blood cells circulate
through the body, they release oxygen to tissues.
What keeps the blood circulating or moving? The
pumping of the heart.
A mild heart attack, when heart failure is unlikely,
is often characterized by a blood oxygen level that
is low but still within normal limits. A more severe
heart attack that seriously impairs the pumping of
the heart will decrease the blood oxygen level to
less than normal. This condition is called hypoxia,
which means that too little oxygen is reaching tis-
sues. When this is determined by measurement of
blood gases, appropriate oxygen therapy can be
started to correct the hypoxia.A patient is admitted to the emergency room with
a possible heart attack, and the doctor in charge
orders “blood gases.” Another patient hospitalized
with pneumonia has “blood gases” monitored at
frequent intervals. What are blood gases, and what
does measurement of them tell us? The blood gases
are oxygen and carbon dioxide, and their levels
in arterial blood provide information about the
functioning of the respiratory and circulatory
systems. Arterial blood normally has a high con-
centration of oxygen and a low concentration of
carbon dioxide. These levels are maintained by
gas exchange in the lungs and by the proper circu-
lation of blood.
A pulmonary disease such as pneumonia inter-
feres with efficient gas exchange in the lungs. As a
result, blood oxygen concentration may decrease,
and blood carbon dioxide concentration may
increase. Either of these changes in blood gases
may become life threatening for the patient, so
monitoring of blood gases is important. If blood