PROCESSES FOR MAINTAINING WATER BALANCE
The body must maintain fluid balance to achieve homeostasis. The body
achieves this balance by adjusting fluid output to equal fluid intake so that the amount of
water in the body does not change. We have three sources of fluid intake: the fluids we
drink, water in the foods we eat, and water formed by catabolism of tissues. Fluid
output is regulated by four organs: the kidneys (which excrete urine), the lungs (which
exhale moisture, carbon dioxide, and air), the skin (which excretes perspiration), and
the intestines (which excrete moisture in the feces). The volume of urine excreted is the
most subject to change. The more fluid the person drinks, the greater the volume of
urine excreted; the less fluid consumed, the less the urine flow. This urine regulation is
accomplished by a hormone present in the blood stream.
CATEGORIES OF FLUIDS
Body water is divided into two primary types--intracellular fluid (ICF) and
extracellular fluid (ECF). The intracellular is fluid that is contained within the body's
cells. The fluid within the cells makes up about 2/3 of a person's total body water and
accounts for about 50 percent of the person's body weight. The extracellular fluid is
located in the body tissues, bones, and the body's vascular systems (blood and lymph).
The fluids discussed thus far can be depicted as shown in figure 2-1.
a. The intracellular fluid functions in three ways. It transports food within the
cells. It brings waste products from the cells so that they can be picked up and excreted
from the body. This fluid also maintains the integrity (shape and size) of the cell.
b. The second type of body fluid is the extracellular fluid. The intracellular fluid
is located outside the body cells. The extracellular fluid (ECF) comprises approximately
one-third of the water contained in the body, and it accounts for approximately 15
percent of a person's body weight. The extracellular fluid has several functions. It
carries nutrients and oxygen to the cells and waste materials from the cells. It serves to
bathe the cells in order to keep the cells moist. Two divisions or types of extracellular
fluid are interstitial fluid and intravascular fluid. The interstitial fluid surrounds cells and
it serves as a transporting medium to carry materials to and from cells. Approximately
three fourths of the extracellular fluid is contained in the interstitial fluid. Interstitial fluid
accounts for approximately 11 percent of a person's body weight. The second division
of the extracellular fluid is the intravascular fluid (one-fourth of the extracellular fluid).
The intravascular fluid is found in the body's circulatory systems. It accounts for
approximately four percent of a person's body weight (see figure 2-2).
c. Body water diffuses throughout the body without recognizing anatomic
boundaries. For example, water passes constantly across the tissue surface of the
capillaries. If we could label all the water molecules by the function they were serving at
a particular moment (interstitial, intravascular, and so forth), within another minute about
half of them would be in another location or serving another function. There is an
especially constant circulation of water (and the substance dissolved in it) between the
fluids surrounding the cells and the blood.