People are watery creatures. More precisely, we are

salt-watery creatures, though not as salty as the oceans.
Water, the fluid medium of the human body, makes up
55% to 75% of the total body weight. Electrolytes are
the positive and negative ions present in body fluids. In
previous chapters we have called these salts or, in some
cases, trace elements. Many of these ions are minerals
that are already familiar to you. They each have spe-
cific functions in body fluids, and some of them are
also involved in the maintenance of the normal pH of
the body fluids. In this chapter we will first discuss
fluid–electrolyte balance, then review and summarize
the mechanisms involved in acid–base balance.

WATER COMPARTMENTS

Most of the water of the body, about two-thirds of the
total water volume, is found within individual cells and
is called intracellular fluid (ICF). The remaining
third is called extracellular fluid(ECF) and includes
blood plasma, lymph, tissue fluid, and the specialized
fluids such as cerebrospinal fluid, synovial fluid, aque-
ous humor, and serous fluid.

Water constantly moves from one fluid site in the

body to another by the processes of filtration and

osmosis. These fluid sites are called water compart-

ments(Fig. 19–1). The chambers of the heart and all

of the blood vessels form one compartment, and the

water within is called plasma. By the process of filtra-

tion in capillaries, some plasma is forced out into tis-

sue spaces (another compartment) and is then called

tissue fluid. When tissue fluid enters cells by the

process of osmosis, it has moved to still another com-

partment and is called intracellular fluid. The tissue

fluid that enters lymph capillaries is in yet another

compartment and is called lymph.

The other process (besides filtration) by which

water moves from one compartment to another is

osmosis, which, you may recall, is the diffusion of

water through a semi-permeable membrane. Water

will move through cell membranes from the area of its

greater concentration to the area of its lesser concen-

tration. Another way of expressing this is to say that

water will diffuse to an area with a greater concentra-

tion of dissolved material. The concentration of elec-

trolytes present in the various water compartments

determines just how osmosis will take place. There-

442 Fluid–Electrolyte and Acid–Base Balance

Fluid

movement

Intracellular fluid Cell

Capillary

Lymph

capillary

Lymph

Interstitial

(tissue) fluid

Plasma

Figure 19–1. Water compartments. The name given to water in each of its locations is

indicated.

QUESTION:Name two specialized fluids that are similar to tissue fluid.