The Kidneys Regulate the Composition
of Body Fluids
the electrolyte composition of various
fluid compartments in the body.
the sources of water gain and loss and
the mechanisms involved in controlling blood
ater is by far the largest single component
of the body, making up 45-75% of total body
mass, depending on age and gender. In lean
adults, body fluids make up between 55% and
60% of total body mass. Babies are about 75% water, as they
have low bone mass and little fat. Water content declines
with age and may be only 45% of body weight in old age.
Body fluids are distributed in three main compart-
ments within the body, called fluid compartments. About
two-thirds of body fluid is intracellular fluid (ICF), the
fluid within cells. The other third, called extracellular
fluid (ECF) is outside cells and includes all remaining
body fluids, notably interstitial fluid and blood plasma,
along with other extracellular fluids (Figure 15.9).
the role of the renin-angiotensin-al-
dosterone system in NaCl reabsorption, blood
volume, and blood pressure regulation.
how parathyroid hormone affects the
kidneys to regulate Ca2+ levels in the blood.
Fluid can travel among the three major fluid com-
partments in two ways, as shown by the black arrows
in Figure 15.9. First, fluid can be exchanged between
cells and their surrounding interstitial fluid because the
plasma membrane of each cell is a selectively perme-
able barrier. Second, fluid can be exchanged between
blood plasma in the capillaries and interstitial fluid. Ex-
changes take place constantly at both levels throughout
the body to maintain fluid balance.
The body is in fluid balance when the
amounts of water and solutes are present, in the right pro-
portions, in the various fluid compartments. We’ll take a
look at those solutes next.
D i s t r i b u t i o n o f w a t e r in v a r i o u s c o m p a r t m e n t s o f t h e b o d y •
F i g u r e 1 5 . 9
Distribution of body fluids in
Total body mass (female)
Total body mass (male)
Interstitial fluid also includes lymph,
CSF, synovial fluid, aqueous and
vitreous humor (eyes), pleural,
peritoneal, and pericardial fluids
Water continuously exchanges
between fluid compartments.