b. Therefore, all other fluids are extracellular. The extracellular fluids are found
in two different compartments.
(1) The tissue fluid is located among but not within the cells of the body. It is
therefore called interstitial or intercellular fluid.
(2) In some systems, fluids serve as a vehicle to carry items around the
body. These systems are called circulatory systems. The circulating fluid is called the
plasma--the non-cellular component of blood.
Within the fluids of the body, there are certain chemicals known as electrolytes.
Electrolytes are chemicals that dissociate ("break up") into ions ("charged particles")
when they are dissolved. To maintain life and good health, electrolytes must be in
balance. That is, they must be present in certain proportions and concentrations in
each fluid compartment.
As we have mentioned, water is the main constituent of the human body.
a. Some Physical Characteristics. Water has several important physical
characteristics that make it extremely useful to the body.
(1) First, it is a fluid. Therefore, it has the capacity to flow.
(2) Secondly, it is often called the "universal solvent." This refers to its ability
to dissolve so many substances within itself. Thus, water is an excellent vehicle for the
(3) Water is very useful in the temperature control mechanisms of the body.
This is because of its heat-carrying capacity and its tendency to remove large numbers
b. Sources. Water thirst and water satiation is controlled by special centers in
the hypothalamus of the brain. The human body obtains water in two primary ways:
(1) Most items that humans drink or eat consist largely of water.
(2) A second source of water is metabolic oxidation. This water is referred to
as metabolic water. As various food substances are oxidized within the individual cell,
water is one of the main by-products.