Water, Electrolytes, and
39.1 Water Metabolism
Water is the most abundant body constituent; it is 45-60%
of total body weight (Figure 39-1). In a lean person, it
accounts for a larger fraction of the body mass than in a
fat person. Since most biochemical reactions take place
in an aqueous environment, control of water balance is an
important requirement for homeostasis.
Although water permeates freely across cell mem-
branes, other solutes are less mobile because of barriers
imposed by membrane systems. These barriers give rise
to fluid pools or compartments of different but rather con-
stant composition (Figure 39-2).
Intracellular fluid makes up 30^-0% of body weight,
or about two-thirds of total body water. Potassium and
magnesium are the predominant cations. The anions are
mainly proteins and organic phosphates, with chloride and
bicarbonate at low concentrations.
Extracellular fluid contains sodium as the predominant
cation and accounts for 20-25% of body weight, or one-
third of total body water. It makes up vascular, intersti-
tial, transcellular, and dense connective tissue fluid pools.
Vascular fluid is the circulating portion, is rich in pro-
tein, and does not readily cross endothelial membranes.
Interstitial fluid surrounds cells and accounts for 18-20%
of total body water. It exchanges with vascular fluid via
the lymph system. Transcellular fluid is present in diges-
tive juices, intraocular fluid, cerebrospinal fluid (CSF),
and synovial (joint) fluid. These fluids are secretions of
specialized cells. Their composition differs considerably
from that of the rest of the extracellular fluid, with which
they rapidly exchange contents under normal conditions.
Dense connective tissue (bone, cartilage) fluid exchanges
slowly with the rest of the extracellular fluid and accounts
for 15% of total body water.
Movements of water are due mainly to osmosis and
filtration. In osmosis, water moves to the area of high-
est solute concentration. Thus, active movement of salts
into an area creates a concentration gradient down which
water flows passively. In filtration, hydrostatie pressure in
arterial blood moves water and nonprotein solutes through
specialized membranes to produce an almost protein-free
filtrate: This process occurs in formation of the renal
glomerular filtrate. Filtration also accounts for movement
of water from the vascular space into the interstitial com-
partment, which is opposed by the osmotic (oncotic) pres-
sure of plasma proteins.
Cells move ions (especially Na+ and K+) against a con-
centration gradient by a “sodium pump” that actively trans-
ports sodium across the plasma membranes (Chapter 12).
The kidneys are the major organ to regulate extracellular
fluid composition and volume. Three main processes occur
1. Formation of a virtually protein-free ultrafiltrate at the