Compartmental Modeling - Introduction to Biomedical Engineering - page 378

Unlike the cell membrane, hydrostatic pressure exists across the capillary wall that
changes as we move from the arteriole to the venule ends. The hydrostatic pressure at
the arteriole end is 30 mm Hg and at the venule end is 10 mm Hg. The pressure drop occurs
approximately linearly from the arteriole to the venule end of the capillary.
Summing all the pressures at the arteriole end gives the following:

Filtration Pressure mm Hg

Hydrostatic Pressure þ 30

Interstitial Colloid Osmotic Pressure þ 8

Plasma Colloid Osmotic Pressure 28

Net Outward Pressure þ10

Thus, a net outward pressure of 10 mm Hg exists that moves fluids out of the capillaries
into the interstitial volume. Some refer to this pressure as the filtration pressure. The
amount of fluid moved is approximately 5 percent of the total plasma.
Summing all the pressures at the venule end gives the following:

Reabsorption Pressure mm Hg

Hydrostatic Pressure þ 10

Interstitial Colloid Osmotic Pressure þ 8

Plasma Colloid Osmotic Pressure 28

Net Outward Pressure 10

Thus, a net inward pressure of 10 mm Hg exists that moves fluids out of the interstitial
volume into the capillaries. Some refer to this pressure as the reabsorption pressure. Almost
all of the fluid that flows into the interstitial volume from the capillaries flows back into the
capillaries from the interstitial volume. The remainder left in the interstitial volume flows
into the lymphatic fluid, from which it returns to the plasma.
Mathematically, at either end of the capillary, the flow rate of fluid is given by

Q A ¼ D p A RTc C þ RTc I
R m

ð 7 : 26 Þ

Q V ¼ D p V RTc C þ RTc I
R m

ð 7 : 27 Þ

where Q A is the flow rate at the arteriole end into the interstitial fluid, Q V is the flow rate at

the venule end into the capillary,

D p A

is the hydrostatic pressure at the arteriole end,

D p V

is

the hydrostatic pressure at the venule end, c C is the concentration of proteins in the

plasma/capillary, and is the concentration of proteins in the interstitial fluid.

With the rate of plasma protein leakage into the interstitial volume given by V P , and
the rate of fluid that moves from the interstitial volume into lymphatic volume given by
V L , the change in interstitial volume, denoted V I , is given by
V I ¼ Q A Q V þ V P V L
¼ D p A RTc C þ RTc I
R m

c I

D p V RTc C þ RTc I
R m

ð 7 : 28 Þ

þ V P V L