Piston
Venule
Arteriole
Lymphatic
capillary
Solute
molecules
(proteins)
Boundary
“Pushes”
“Pulls”
Boundary
Hydrostatic pressure (HP)
Osmotic pressure (OP)
Figure 19.17
Bulk fluid flow across capillary walls causes
continuous mixing of fluid between the plasma and the
interstitial fluid compartments, and maintains the
interstitial environment.
• Due
to
fl
uid pressing against a
boundary
• HP
“pushes” fl
uid across the
boundary
• In
bl
ood v
essels,
is due
to
bl
ood
pressure
• Due
to
nondif
fusib
le solutes that
cannot cross the boundar
y
• OP
“pulls” fl
uid across the
boundary
• In
bl
ood v
essels,
is due
to
plasma pr
oteins
The big picture
Fluid filters from capillaries at their arteriolar
end and flows through the interstitial space.
Most is reabsorbed at the venous end.
Net filtration pressure (NFP) determines the direction of
fluid movement.
Two kinds of pressure drive fluid flow:
17 L of fluid per
day is reabsorbed
into the capillaries
at the venous end.
About 3 L per day
of fluid (and any
leaked proteins) are
removed by the
lymphatic system
(see Chapter 20).
Fluid moves through
the interstitial space.
For all capillary beds,
20 L of fluid is filtered
out per day—almost 7
times the total plasma
volume!
FOCUS
Bulk Flow Across Capillary Walls
718
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