Boundary
(capillary wall)
Boundary
(capillary wall)
Capillary
Interstitial fluid
Capillary
Interstitial fluid
How do the pressures drive fluid flow across a capillary?
Net filtration occurs at the arteriolar end of a capillary.
Net reabsorption occurs at the venous end of a capillary.
Hydrostatic pressure in capillary
“pushes” ﬂuid
out
of capillary.
Hydrostatic pressure in
interstitial ﬂuid
“pushes” ﬂuid
into
capillary.
Osmotic pressure in capillary
“pulls” ﬂuid
into
capillary.
Osmotic pressure in
interstitial ﬂuid “pulls”
ﬂuid
out
of capillary.
Hydrostatic pressure in capillary
“pushes” ﬂuid
out
of capillary. The
pressure has dropped because of
resistance encountered along the
capillaries.
Hydrostatic pressure in
interstitial ﬂuid “pushes”
ﬂuid
into
capillary.
Osmotic pressure in capillary
“pulls” ﬂuid
into
capillary.
Osmotic pressure in
interstitial ﬂuid “pulls” ﬂuid
out
of capillary.
To determine the pressure driving the
fluid out of the capillary at any given
point, we calculate the
net filtration
pressure (NFP)
––the outward pressures
(HP
c
and OP
if
) minus the inward
pressures (HP
if
and OP
c
). So,
NFP = (HP
c
+ OP
if
) – (HP
if
+ OP
c
)
= (35 + 1) – (0 + 26)
= 10 mm Hg (net outward pressure)
As a result, fluid moves from the capillary
into the interstitial space.
Again, we calculate the NFP:
NFP = (HP
c
+ OP
if
) – (HP
if
+ OP
c
)
= (17 + 1) – (0 + 26)
= –8 mm Hg (net inward pressure)
Notice that the NFP at the venous end is
a negative number. This means that
reabsorption, not filtration, is occurring
and so fluid moves from the interstitial
space into the capillary.
HP
if
= 0 mm Hg
NFP= 10 mm Hg
NFP= –8 mm Hg
HP
c
= 35 mm Hg
OP
c
= 26 mm Hg
OP
if
= 1 mm Hg
HP
if
= 0 mm Hg
HP
c
= 17 mm Hg
OP
c
= 26 mm Hg
OP
if
= 1 mm Hg
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