Essentials of Anatomy and Physiology

sweating. Excess body heat evaporates sweat on the

skin surfaces, rather than overheating the body’s

cells, and because of water’s high heat of vaporiza-

tion, a great deal of heat can be given off with the

loss of a relatively small amount of water.

WATER COMPARTMENTS

All water within the body is continually moving, but
water is given different names when it is in specific
body locations, which are called compartments (Fig.
2–4).

Intracellular fluid (ICF)—the water within cells;
about 65% of the total body water

Extracellular fluid (ECF)—all the rest of the water
in the body; about 35% of the total. More specific
compartments of extracellular fluid include:
Plasma—water found in blood vessels
Lymph—water found in lymphatic vessels
Tissue fluid or interstitial fluid—water found in
the small spaces between cells
Specialized fluids—synovial fluid, cerebrospinal
fluid, aqueous humor in the eye, and others
The movement of water between compartments in

the body and the functions of the specialized fluids

will be discussed in later chapters.

OXYGEN

Oxygen in the form of a gas (O 2 ) is approximately

21% of the atmosphere, which we inhale. We all know

that without oxygen we wouldn’t survive very long,

but exactly what does it do? Oxygen is important to us

because it is essential for a process called cell respira-

tion, in which cells break down simple nutrients such

as glucose in order to release energy. The reason we

breathe is to obtain oxygen for cell respiration and to

exhale the carbon dioxide produced in cell respiration

(this will be discussed in the next section). Biologically

useful energy that is released by the reactions of cell

respiration is trapped in a molecule called ATP

(adenosine triphosphate). ATP can then be used for

cellular processes that require energy.

CARBON DIOXIDE

Carbon dioxide (CO 2 ) is produced by cells as a waste

product of cell respiration. You may ask why a waste

product is considered important. Keep in mind that

“important” does not always mean “beneficial,” but it

Some Basic Chemistry 29

Figure 2–4. Water compartments, showing the names water is given in its different loca-

tions and the ways in which water moves between compartments.

QUESTION:Which of the fluids shown are extracellular fluids?

Fluid

movement

Intracellular fluid Cell

Capillary

Lymph

capillary

Lymph

Interstitial

(tissue) fluid

Plasma