C h a p t e r 8
Disorders of Fluid, Electrolyte, and Acid–Base Balance
163
A third, usually minor, subdivision of the ECF com-
partment is the
transcellular compartment.
It includes
the cerebrospinal fluid and fluid contained in the var-
ious body spaces, such as the peritoneal, pleural, and
pericardial cavities, and joint spaces. Normally, only
about 1% of ECF is in the transcellular space. This
amount can increase considerably in conditions such
as ascites, in which large amounts of fluid are seques-
tered in the peritoneal cavity. When the transcellular
fluid compartment becomes considerably enlarged, it
is referred to as a
third space,
because this fluid is
not readily available for exchange with the rest of
the ECF.
Capillary/Interstitial Fluid Exchange
The transfer of water between the vascular and inter-
stitial compartments occurs at the capillary level.
There are four main forces that control the movement
of water between the capillary and interstitial spaces:
(1) the
capillary filtration pressure
, which
pushes
water
out of the capillary into the interstitial spaces; (2) the
capillary colloidal osmotic pressure
, which
pulls
water
back into the capillary; (3) the
interstitial
or
tissue
hydrostatic pressure
, which
opposes
the movement of
water out of the capillary; and (4) the
interstitial col-
loidal osmotic pressure
, which
pulls
water out of the
capillary into the interstitial spaces.
1–3
Normally, the
combination of these four forces is such that only a
small excess of fluid remains in the interstitial compart-
ment. This excess fluid is removed from the interstitium
by the lymphatic system and returned to the systemic
circulation.
Capillary filtration
refers to the movement of water
through capillary pores because of hydrostatic pressure,
rather than an osmotic force. The capillary filtration
pressure (about 30 to 40 mm Hg at the arterial end,
10 to 15 mm Hg at the venous end, and 25 mm Hg
in the middle) is the pressure pushing water out of the
capillary into the interstitial spaces.
2
It reflects the arte-
rial and venous pressures, the precapillary (arterioles)
and postcapillary (venules) resistances, and the force of
gravity.
2
A rise in arterial or venous pressure increases
capillary pressures throughout the body, whereas the
force of gravity increases the capillary pressure in the
dependent or lower parts of the body. In a person who is
standing absolutely still, the weight of blood in the vas-
cular column causes an increase of 1 mm Hg pressure
for every 13.6 mm of distance from the heart.
2
Since
this pressure results from the weight of water, it is called
hydrostatic pressure.
In the adult who is standing abso-
lutely still, the pressure in the veins of the feet can reach
90 mm Hg.
The
capillary colloidal osmotic or oncotic pressure
(about 28 mm Hg) is the osmotic pressure generated by
the plasma proteins that are too large to pass through the
pores of the capillary wall.
2
The term
colloidal osmotic
pressure
differentiates this type of osmotic pressure from
the osmotic pressure that develops at the cell membrane
from the presence of electrolytes and nonelectrolytes.
Because plasma proteins do not normally penetrate
the capillary pores and because their concentration is
greater in the plasma than in the interstitial fluids, it
is capillary colloidal osmotic pressure that pulls fluids
back into the capillary.
The interstitial fluid pressure (about −3 mm Hg)
and interstitial colloidal osmotic pressure (about 8 mm
Hg) contribute to movement of water into and out
of the interstitial spaces.
2
The
interstitial hydrostatic
fluid pressure
, which is normally negative, contributes
to the outward movement of water into the intersti-
tial spaces. The
interstitial colloidal osmotic pressure
,
which reflects the small amount of plasma proteins that
normally escape into the interstitial spaces from the
capillary, also pulls water out of the capillary into the
tissue spaces.
The lymphatic system represents an accessory route
whereby fluid from the interstitial spaces can return to
the circulation. More importantly, the lymphatic sys-
tem provides a means for removing plasma proteins
and osmotically active particulate matter from the tis-
sue spaces, neither of which can be reabsorbed into the
capillaries.
Edema
Edema can be defined as palpable swelling produced by
an increase in interstitial fluid volume. The physiologic
mechanisms that contribute to edema formation include
factors that: (1) increase the capillary filtration pressure,
(2) decrease the capillary colloidal osmotic pressure,
(3) increase capillary permeability, or (4) produce
obstruction to lymph flow. The causes of edema are
summarized in Chart 8-1.
CHART 8-1
Common Causes of Edema
Increased Capillary Pressure
Increased vascular volume (e.g., heart failure, kidney
disease)
Venous obstruction (e.g., thrombophlebitis)
Liver disease with portal vein obstruction
Acute pulmonary edema
Decreased Colloidal Osmotic Pressure
Increased loss of plasma proteins (e.g., protein-
losing kidney diseases, extensive burns)
Decreased production of plasma proteins (liver
disease, malnutrition)
Increased Capillary Permeability
Inflammation
Allergic reactions (e.g., hives, angioneurotic edema)
Malignancy (e.g., ascites, pleural effusion)
Tissue injury and burns
Obstruction of Lymphatic Flow
Malignant obstruction of lymphatic structures
Surgical removal of lymph nodes
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