817
CHAPTER TWENTY-ONE
Water, Electrolyte, and Acid-Base Balance
PRACTICE
14
Which electrolytes are most important to cellular functions?
15
Which mechanisms ordinarily regulate electrolyte intake?
16
By what routes does the body lose electrolytes?
particle, and one molecule of sodium chloride yields two, a
sodium ion and a chloride ion. The total number of dissolved
particles determines the osmolarity of body solutions, irrespec-
tive of the source, so the term
osmoles
is used. Thus, one mole
of glucose yields one osmole of dissolved particles, and one
mole of sodium chloride yields two osmoles. The total num-
ber of osmoles per liter gives the osmolarity of the solution.
return of tissue F
uid to the venular ends of cap-
illaries. Consequently, tissue F
uid accumulates in
the interstitial spaces.
As discussed in chapter 16 (p. 621),
lymphatic
obstructions may result from surgery or from para-
sitic infections of lymphatic vessels. Back pressure
develops in the lymphatic vessels, which interferes
disease (glomerulonephritis) that damages glom-
erular capillaries, allowing proteins to enter the
urine; or starvation, in which amino acid intake
is insu±
cient to support synthesis of plasma pro-
teins. In each of these instances, the plasma pro-
tein concentration is decreased, which decreases
plasma osmotic pressure, reducing the normal
with the normal movement of tissue fluid into
them. At the same time, proteins that the lym-
phatic circulation ordinarily removes accumulate
in the interstitial spaces, raising osmotic pressure
of the interstitial F
uid. This e²
ect attracts still more
F
uid into the interstitial spaces.
If the outF
ow of blood from the liver into the
inferior vena cava is blocked, the venous pressure in
the liver and portal blood vessels greatly increases.
This, in turn, raises pressure in liver sinusoids and
intestinal capillaries. ³luid with a high concentra-
tion of protein is exuded from the surfaces of the
liver and intestine into the peritoneal cavity. This
elevates the osmotic pressure of the abdominal
fluid, which, in turn, attracts more water into the
peritoneal cavity by osmosis. This condition, called
ascites,
distends the abdomen. It is painful.
Edema may also result from increased capil-
lary permeability accompanying
inflammation.
Recall that inflammation is a response to tissue
damage and usually releases chemicals such as
histamine from damaged cells. Histamine causes
vasodilation and increased capillary permeabil-
ity, so excess F
uid leaks out of the capillary and
enters the interstitial spaces. Table 21A summa-
rizes the factors that result in edema.
Cell
membrane
Nucleus
Excess water is added
to extracellular
fluid compartment
Solute
concentration
of extracellular
fluid compartment
decreases
Water moves into
intracellular fluid
compartment
by osmosis
1
3
2
FIGURE 21B
If excess water is added to the extracellular F
uid compartment, cells gain water
by osmosis.
TABLE
21A
|
Factors Associated with Edema
Factor
Cause
Ef
ect
Low plasma protein
concentration
Liver disease and failure to synthesize proteins; kidney disease and
loss of proteins in urine; lack of proteins in diet due to starvation
Plasma osmotic pressure decreases; less F
uid enters venular
ends of capillaries by osmosis
Obstruction of
lymph vessels
Surgical removal of portions of lymphatic pathways; certain
parasitic infections
Back pressure in lymph vessels interferes with movement of
F
uid from interstitial spaces into lymph capillaries
Increased venous
pressure
Venous obstructions or faulty venous valves
Back pressure in veins increases capillary ´
ltration and
interferes with return of F
uid from interstitial spaces into
venular ends of capillaries
InF
ammation
Tissue damage
Capillaries become abnormally permeable; F
uid leaks from
plasma into interstitial spaces
q
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