Biomedical Engineering Reference
In-Depth Information
the pores in the capillary walls from the interstitial space. In 1896, British physiolo-
gist Ernest H. Starling described the regulation of fluid exchange across the blood
capillaries to the tissue spaces. According to Starling, three local factors regulate the
direction and rate of fluid transfer between plasma and tissue fluids at a location:
1. The hydrostatic pressures on each side of the capillary membranes;
2. The osmotic pressures of plasma and tissue fluids across the capillary
membranes;
3. The physical properties of the capillary membranes.
Consider a blood vessel surrounded by interstial fluid (Figure 2.11). The normal
hydrostatic pressure difference (capillary blood pressure,
P
inside
, minus the pressure
of the fluid in the interstitial space,
P
outside
) across the capillary favors filtration of
water out of the capillary into the interstitial space. However, the osmotic pressure
difference across the capillary (osmotic pressure due to plasma proteins,
π
inside
, mi-
nus the osmotic pressure due to proteins in the interstial space,
π
outside
) favors the
retention. Starling's hypothesis can be used to estimate whether water at any given
point along the capillary will have a tendency to enter or leave the capillary. For
example, the blood pressure at the arterial end (32 mmHg) is significantly higher
than at the venous end (15 mmHg). The net pressure difference at the arterial end
(
10 mmHg) favors transport of water out of the capillary where as the net pres-
sure difference (
+
7 mmHg) at the venous end favors transport of water into the
capillary. Understanding these factors is important in determining the water bal-
ance throughout the body, particularly in the formation of lung edema and kidney
function. Fluid filtered from the vascular space into the interstitium then percolates
through the interstitium to enter the initial lymphatics. When the net transcapillary
filtration pressure or the permeability of the pulmonary microvascular endothelial
barrier increases, such as in acute or chronic pulmonary hypertension or in acute
respiratory distress syndrome (ARDS), an imbalance in the rate of fluid filtered into
the pulmonary interstitium and the rate of fluid exiting the lymphatic system could
occur. This results in the accumulation of fluid within the pulmonary interstitium
and leads to a state called pulmonary edema.
The volumetric flow rate (
J
v
) of water across the membrane can be calculated
using
−
Figure 2.11
Starling forces regulating the transfer of molecules.
