C h a p t e r 8
Disorders of Fluid, Electrolyte, and Acid–Base Balance
165
Interstitial fluids
Arterial
end
Venous
end
Capillary filtration
pressure
Lymphatic vessel
Fluid and proteins not
reabsorbed into capillaries
Lymph Drainage.
The lymphatic
system represents an accessory sys-
tem by which fluid can be returned
to the circulatory system. Normally
the forces moving fluid out of the
capillary into the interstitium are
greater than those returning fluid to
the capillary. Any excess fluids and
osmotically active plasma proteins
that may have leaked into the inter-
stitium are picked up by vessels of
the lymphatic system and returned
to the circulation. Without the
function of the lymphatic system,
excessive amounts of fluid would
accumulate in the interstitial spaces.
3
Increased Capillary Filtration Pressure.
Edema due
to increased capillary filtration pressure is usually the
result of increased vascular volume. It is commonly
seen in conditions such as congestive heart failure
that produce fluid retention and venous congestion
(see Chapter 20). Edema does not usually become evi-
dent until the interstitial volume has been increased
by 2.5 to 3 L.
4
Dependent edema
describes an accu-
mulation of fluid in the lower parts of the body.
Because of the effects of gravity, edema that results
from increased capillary pressure commonly causes
dependent edema. For example, edema of the ankles
and feet becomes more pronounced during prolonged
periods of standing. Lower extremity edema due to an
increase in venous pressure is a common complication
of venous insufficiency resulting from thrombophle-
bitis (development of a blood clot in one of the deep
leg veins).
Decreased Capillary Colloidal Osmotic Pressure.
Plasma proteins exert the osmotic force needed to pull
fluid back into the capillary from the tissue spaces. The
plasma proteins constitute a mixture of albumin, globu-
lins, and fibrinogen. Albumin, the smallest of the plasma
proteins, has a molecular weight of 69,000; globulins
have molecular weights of approximately 140,000;
and fibrinogen has a molecular weight of 400,000.
2
Because of its lower molecular weight, 1 g of albumin
has approximately twice as many osmotically active
molecules as 1 g of globulin and almost six times as
many as 1 g of fibrinogen. Also, the concentration of
albumin (approximately 4.5 g/dL) is greater than that
of the globulins (2.5 g/dL) and fibrinogen (0.3 mg/dL).
2
Therefore, albumin has the greatest effect on the colloi-
dal osmotic pressure.
Edema caused by decreased capillary colloidal
osmotic pressure usually is the result of inadequate
production or excessive loss of plasma proteins,
mainly albumin. The plasma proteins are synthesized
in the liver. In persons with severe liver failure, the
impaired synthesis of albumin results in a decrease
in colloidal osmotic pressure. In starvation and mal-
nutrition, edema develops because there is a lack of
amino acids for plasma protein synthesis. The most
common site of plasma protein loss is the kidney. In
kidney diseases such as nephrotic syndrome, the glo-
merular capillaries become permeable to the plasma
proteins, particularly albumin (see Chapter 25). When
this occurs, large amounts of albumin are filtered out
of the blood and lost in the urine. An excessive loss of
plasma proteins also occurs when large areas of skin
are injured or destroyed, such as occurs during the
early stages of a burn.
5
Because the plasma proteins are evenly distributed
throughout the body and are not affected by the force of
gravity, edema that is caused by a decrease in capillary
colloidal osmotic pressure tends to produce generalized
edema involving the face as well the legs and feet.
Increased Capillary Permeability.
When the capil-
lary pores become enlarged or the integrity of the
capillary wall is damaged, capillary permeability is
increased. When this occurs, plasma proteins and
other osmotically active particles leak into the inter-
stitial spaces, pulling fluid out of the capillary into
the interstitium. Among the conditions that increase
Exchange
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