Page 126 - Biomedical Engineering and Design Handbook Volume 1, Fundamentals

RESPIRATORY MECHANICS AND GAS EXCHANGE 103
average slope, or compliance, that characterize the
loop and have significant effects on their counterparts
in Fig. 4.7.
Insufficient surfactant levels can occur in premature
neonates whose alveolar cells are not mature enough to
produce sufficient quantities, a condition leading to respi-
ratory distress syndrome, also called hyaline membrane
2
disease. Instilling liquid mixtures, containing either nat-
ural or man-made surfactants, directly into airways via
the trachea has developed from early work in animal
9
models into an important clinical tool called surfactant
replacement therapy. The movement of these liquid
boluses through the network relies on several mecha- FIGURE 4.10 Surface area A versus surface
s
nisms, including air-blown liquid plug flow dynamics, tension s for a cycled interface containing lung
gravity, and surface tension and its gradients. 4,10,18 surfactant. There is a hysteresis area and mean slope
or compliance to the curve; compare to Fig. 4.7.
4.5 VENTILATION, PERFUSION, AND LIMITS

The lung differs from many other organs in its combina-
tion of gas-phase and liquid-phase constituency. Because
the densities of these two phases are so different, grav-
ity plays an important role in determining regional lung
behavior, both for gas ventilation and for blood perfu-
sion. In an upright adult, the lower lung is compressed
by the weight of the upper lung and this puts the lower
lung’s alveoli on a more compliant portion of their
regional P-V curve; see Fig. 4.7. Thus inhaled gas tends
to be directed preferentially to the lower lung regions,

and the regional alveolar ventilation V A decreases in a
graded fashion moving upward in the gravity field.

Blood flow Q is also preferentially directed toward
the lower lung, but for different reasons. The blood pres-
sure in pulmonary arteries, P , and veins, P , sees a
v
a
hydrostatic pressure gradient in the upright lung. These
vessels are imbedded within the lung’s structure, so they
are surrounded by alveolar gas pressure, P , which is
A
essentially uniform in the gravity field since its density
is negligible. Thus there is a region called Zone III in the
upper lung where P < P . This difference in pressures
A
a
squeezes the capillaries essentially shut and there is rel-
atively little pulmonary blood flow there. In the lower
lung called Zone I, the hydrostatic effect is large enough
to keep P > P > P and blood flow is proportional to
v
a
A
the arterial-venous pressure difference, P − P . In
a
v
between these two zones is Zone II where P > P > P .
a
A
v
Here there will be some length of the vessel that is nei-
ther fully closed nor fully opened. It is partially col-
lapsed into more of a flattened, oval shape. The physics
of the flow, called the vascular waterfall 29 or choked
23
flow 30 or flow limitation , dictates that Q is no longer FIGURE 4.11 Choked flow through a flexible
tube. (a) upstream pressure P , downstream pressure
u
dependent on the downstream pressure, but is primarily P , external pressure P , flow F, pressure P, cross-
d
ext
determined by the pressure difference P − P . sectional area A. (b) A/A versus transmural pressure
a
A
0
Figure 4.11a shows this interesting type of flexible with shapes indicated. (c) Flow versus pressure drop,
tube configuration and flow limitation phenomena where assuming P is decreased and P is fixed.
u
d