202
Dynamics
of
the
Vascular System
sphincters serve to adjust the amount of blood flow to meet the demands
in tissues.
Starling’s hypothesis describes the filtration and absorption
of
fluid
across capillary walls.
The capillary wall is known to consist of
endothelium with a basement membrane that is highly permeable to
allow fluid exchange.
Several factors govern such exchange: (1)
hydrostatic capillary blood pressure, p,, (2) osmotic pressure
of
plasma
proteins,
n,,
(3)
hydrostatic interstitial fluid pressure, pi and (4) osmotic
pressure,
n,,
of the proteins in the interstitial fluid. These define the
Starling’s hypothesis:
(7.2.4)
pc
-PI
=rc
-XI
under equilibrium. In other words, the difference in fluid pressure in the
capillary and the surrounding interstitial tissue equals the difference in
osmotic pressure between the capillary blood and the extravascular fluid.
Depending on the physiological demand, the microcirculatory system
adjusts itself rapidly and efficiently. For this reason, the trans-capillary
flow is given by
a
modification
of
eqn. (7.2.4) to:
(7.2.5)
where
k,
is capillary permeability and
S
is the cross-sectional area
available for the exchange.
Capillary permeability is in the order of
10.3’
g-’cm2s. Several methods are available to measure this flow,
including the micro-occlusion and electro-optical techniques and the
use
of optical dye.
7.2.2
Some Pressure-Related Mechanical Characteristics
of
the
Microcirculation
Pressure measurement techniques in the microvessels were mostly based
on the method originally designed by Wiederhielm et al. (1964).
Zweifach and colleagues have refined the technique and performed
extensive
measurements
(e.g.
Zweifach,
1974 and Zweifach and
Lipowsky,
1977).
Fig. 7.2.1 illustrates
such an intravascular
