The expression in brackets represents the NDF
If the NDF is positive, filtration occurs (J > 0),
and if it is negative, reabsorption occurs (J < 0).
For a given NDF, the rate of fluid movement (J)
is determined by the product of KF and A.
Capillary Exchange Model
Capillary fluid exchange can be modeled as
shown in Figure 8.9. This model assumes that
the following values remain constant along cap-
illary length: P. = 1 mm Hg, nc = 25 mm Hg, n. =
6 mm Hg, and O = 1. According to Equation 8-4,
if Pc is 30 mm Hg at the entrance to the capil-
lary and falls linearly to 15 mm Hg at the end
of the capillary, the NDF changes from +10 at
the entrance of the capillary to -5 at the end of
the capillary. Filtration occurs along most of the
length of the capillary wherever NDF is greater
than zero. Reabsorption occurs where NDF is
less than zero, which is near the venular end of
the capillary. Net fluid movement is zero at the
point along the capillary where NDF = 0. Exper-
imental studies have shown that the hydraulic
conductivity in single capillaries increases sev-
eralfold from the arteriolar to the venular end
of the capillary. Therefore, significant reabsorp-
tion can still occur at the distal end of a capillary
when the NDF is only slightly negative.
This model is highly simplified because it
assumes that P., n , and n. remain constant,
which does not occur in vivo. As fluid leaves
the arteriolar end of the capillary, nc increases,
P. increases, and n decreases. These changes
oppose the filtration. For most capillaries, the
fraction of fluid filtered from the capillary (fil-
tration fraction) is <1%, so P n
and n do not
change appreciably. Renal capillaries, however,
are different because the filtration fraction in
these capillaries is very high (~20%), which
leads to significant increases in plasma oncotic
pressure. In nonrenal capillaries, if capillary
permeability is increased, or if capillary hydro-
static pressure is increased to high levels by
venous occlusion or heart failure, the increase
in filtration can lead to significant changes in
P., nc, and n in a manner that opposes and
therefore limits the net filtration of fluid.
Lymphatics (not shown in Fig. 8.9) pick
up excess filtered fluid and transport it out
of the tissue. W hen net filtration increases,
Capillary Length
■ FIGURE 8.9 Model of capillary fluid exchange.
Assuming that Pi = 1, nc = 25, ni = 6 mm Hg, and
o = 1, and assum ing th a t capillary h yd ro sta tic pres-
sure (Pc) at the beginning and end of the capillary
are 30 mm Hg and 15 mm Hg, respectively, the net
driving force [NDF = (Pc - Pi) - (nC
- ni)] is greater
than zero along most of the length of the capillary,
which causes filtration
to occur. Near the
venular end of the capillary, the
is less than
zero and reabsorption
lymphatic flow also increases. The lymphat-
ics, therefore, along with the dynamic changes
in P
and n help to maintain a proper
state of interstitial hydration and thereby pre-
vent edema from occurring.
Finally, it is important to note that there
is considerable heterogeneity among capil-
laries in terms of filtration and reabsorption.
Some capillaries may filter along most or all
of their length, whereas others may display
reabsorption along most of their length. Fur-
thermore, this can change depending on the
balance of hydrostatic and oncotic forces,
which can vary under different physiological
and pathophysiological conditions. With arte-
riolar vasodilation or increased venous pres-
sure, capillaries may filter along most or all of
their length. Inflammation is accompanied by
arteriolar vasodilation and increased capillary
permeability, along with increased permeabil-
ity of small postcapillary venules, which can
become the major site of fluid filtration under
inflammatory conditions.
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