812
UNIT FIVE
sodium, chloride, and bicarbonate ions than does extracel-
lular fl
uid. Intracellular fl
uid also has a greater concentra-
tion of protein than plasma.
Figure 21.3
shows these relative
concentrations.
PRACTICE
2
Describe the normal distribution of water in the body.
3
Which electrolytes are in higher concentrations in extracellular
F
uids? In intracellular F
uid?
4
How does the concentration of protein vary in the various body
F
uids?
Movement of Fluid Between
Compartments
Two major factors regulate the movement of water and elec-
trolytes from one fl
uid compartment to another: hydrostatic
pressure and osmotic pressure. For example, as explained in
chapter 15 (p. 579), fl
uid leaves the plasma at the arteriolar
ends of capillaries and enters the interstitial spaces because
of the net outward force of
hydrostatic pressure
(blood pres-
sure). Fluid returns to the plasma from the interstitial spaces
at the venular ends of capillaries because of the net inward
force of
colloid osmotic pressure.
Likewise, as mentioned in
chapter 16 (p. 620), fl uid leaves the interstitial spaces and
enters the lymph capillaries due to the hydrostatic pressure
of the interstitial fluid. The circulation of lymph returns
interstitial fl
uid to the plasma.
Hydrostatic pressure in the cells and surrounding inter-
stitial fl
uid is ordinarily equal and remains stable, so any
net fl
uid movement is likely to be the result of changes in
osmotic pressure
(f g. 21.4)
. Recall that osmotic pressure is
due to impermeant solutes on one side of a cell membrane.
Sodium (extracellular) and potassium (intracellular) ions
function as impermeant solutes and create an osmotic pres-
sure because of the Na
+
/K
+
pump. For example, because
most cell membranes in the body are freely permeable to
water, a decrease in extracellular sodium ion concentra-
tion causes a net movement of water from the extracellular
compartment into the intracellular compartment by osmo-
sis. The cell swells. Conversely, if the extracellular sodium
Body Fluid Composition
Extracellular fluids
generally are similar in composition,
including high concentrations of sodium, chloride, calcium,
and bicarbonate ions and lesser concentrations of potassium,
magnesium, phosphate, and sulfate ions. The blood plasma
fraction of extracellular fl
uid has considerably more protein
than do either interstitial fl
uid or lymph.
Intracellular fluid
has high concentrations of potas-
sium, phosphate, and magnesium ions. It includes a greater
concentration of sulfate ions and lesser concentrations of
Intracellular fluid
(63%)
Total body water
Interstitial fluid
Plasma
Lymph
Transcellular fluid
Extracellular fluid
(37%)
Membranes of
body cells
Extracellular
fluid
(37%)
Intracellular
fluid
(63%)
40
38
36
34
32
30
28
26
24
22
20
18
16
14
12
10
8
6
4
2
0
Liters
FIGURE 21.1
Of the 40 liters of water in the body of an average
adult male, about two-thirds is intracellular, and one-third is
extracellular.
FIGURE 21.2
Cell membranes separate F
uid in the intracellular compartment from F
uid in the extracellular compartment. Approximately two-
thirds of the water in the body is inside cells.
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