Cardiac Output, Blood Flow, and Blood Pressure 457whereP (^) c 5 hydrostatic pressure in the capillary
p (^) i 5 colloid osmotic pressure of the interstitial (tissue) fluid
P (^) i 5 hydrostatic pressure of interstitial fluid
p (^) p 5 colloid osmotic pressure of the blood plasma
The expression to the left of the minus sign represents the
sum of forces acting to move fluid out of the capillary. The
expression to the right represents the sum of forces acting to
move fluid into the capillary. Figure 14.9 provides typical val-
ues for blood capillaries in skeletal muscles. Notice that the sum
of the forces acting on the capillary is a positive number at the
arteriolar end and a negative number at the venular end of the
capillary. Examination of figure 14.9 reveals that this change is
caused by the decrease in hydrostatic pressure (blood pressure)
within the capillary as blood travels from the arteriolar to the
venular end. The positive value at the arteriolar end indicates
that the Starling forces that favor the filtration of fluid out of
the capillary predominate. The negative value at the venular end
indicates that the net Starling forces favor the return of fluid to
the capillary. Fluid thus leaves the capillaries at the arteriolar end
and returns to the capillaries at the venular end ( fig. 14.9 , top ).
This “classic” view of capillary dynamics has been modified
in recent years by the realization that the balance of filtration and
reabsorption varies in different tissues and under different condi-
tions in a particular capillary. For example, a capillary may be open
or closed off by precapillary muscles that function as sphincters.
When the capillary is open, blood flow is high and the net filtration
force exceeds the force for the osmotic return of water throughout
the length of the capillary. The opposite is true if the precapillary
sphincter closes and the blood flow through the capillary is reduced.
Through the action of the Starling forces, plasma and inter-
stitial fluid are continuously interchanged. The return of fluid to
the vascular system at the venular ends of the capillaries, how-
ever, does not exactly equal the amount filtered at the arterio-
lar ends. Approximately 85% to 90% of the filtrate is returned
directly to the blood capillaries; the remaining 10% to 15% is
returned to the blood by way of the lymphatic system. This
amounts to about 1–2 L of interstitial fluid per day, containing
20–30 g of protein per liter that enters lymphatic capillaries.
As may be recalled from chapter 13 (see fig. 13.36), lymphatic
capillaries are blind-ended, highly permeable vessels that drain
their contents (now called lymph) into lymphatic vessels, which
ultimately return this fluid to the venous system.
Causes of Edema
Excessive accumulation of interstitial fluid is known as
edema. This condition is normally prevented by a proper bal-
ance between capillary filtration and osmotic uptake of water
and by proper lymphatic drainage. Edema may thus result from
- high arterial blood pressure, which increases capillary
pressure and causes excessive filtration; - venous obstruction —as in phlebitis (where a thrombus
forms in a vein) or mechanical compression of veins
Exchange of Fluid Between
Capillaries and Tissues
The distribution of extracellular fluid between the plasma and
interstitial compartments is in a state of dynamic equilibrium.
Tissue fluid is not normally a “stagnant pond;” rather, it is a con-
tinuously circulating medium, formed from and returning to the
vascular system. In this way, the tissue cells receive a continu-
ously fresh supply of glucose and other plasma solutes that are
filtered through tiny endothelial channels in the capillary walls.
Filtration results from blood pressure within the capillaries.
This hydrostatic pressure, which is exerted against the inner capil-
lary wall, is equal to about 37 mmHg at the arteriolar end of sys-
temic capillaries and drops to about 17 mmHg at the venular end
of the capillaries. The net filtration pressure is equal to the hydro-
static pressure of the blood in the capillaries minus the hydrostatic
pressure of tissue fluid outside the capillaries, which opposes filtra-
tion. If, as an extreme example, these two values were equal, there
would be no filtration. However, the hydrostatic pressure of inter-
stitial fluid is normally kept low by the removal of fluid through
drainage into lymphatic vessels (chapter 13; see fig. 13.36).
The magnitude of the tissue hydrostatic pressure varies from
organ to organ. With a hydrostatic pressure in the interstitial fluid
of 1 mmHg, as it is outside the capillaries of skeletal muscles,
the net filtration pressure would be 37 2 1 5 36 mmHg at the
arteriolar end of the capillary and 17 2 1 5 16 mmHg at the
venular end.
Glucose, comparably sized organic molecules, inorganic
salts, and ions are filtered along with water through the capillary
pores. The concentrations of these substances in interstitial (tissue)
fluid are thus the same as in plasma. The protein concentration of
interstitial fluid (2 g/100 ml), however, is less than the protein con-
centration of plasma (6 to 8 g/100 ml). This difference is due to
the restricted filtration of proteins through the capillary pores. The
osmotic pressure exerted by plasma proteins—called the colloid
osmotic pressure of the plasma (because proteins are present as
a colloidal suspension)—is therefore much greater than the col-
loid osmotic pressure of interstitial fluid. The difference between
these two osmotic pressures is called the oncotic pressure. The
colloid osmotic pressure of the interstitial fluid is sufficiently low
to be neglected, so the oncotic pressure is essentially equal to the
colloid osmotic pressure of the plasma. This value has been esti-
mated to be 25 mmHg. Because water will move by osmosis from
the solution of lower to the solution of higher osmotic pressure
(chapter 6), this oncotic pressure favors the movement of water
into the capillaries.
Whether fluid will move out of or into the capillary depends
on the magnitude of the net filtration pressure, which varies
from the arteriolar to the venular end of the capillary, and on the
oncotic pressure. These opposing forces that affect the distribu-
tion of fluid across the capillary are known as Starling forces,
and their effects can be calculated according to this relationship:
Fluid movement is proportional to:
( p^ c 1 p^ i ) 2 ( p^ i 1 p^ p )
(Fluid out) 2 (Fluid in)