Biomedical Engineering Reference
In-Depth Information
The outer surface of the lung is covered by a visceral pleura that lies next to the
parietal pleura lining the inner chest wall. The gap between the two pleura is called the
intrapleural space and is occupied by a small amount of fluid. This intrapleural fluid
provides the cohesion that helps link the pleura together. Thus, as the chest wall expands
during inspiration the lung is obliged to follow. To preserve the cohesive forces linking
the outer lung to the inner chest wall, the intrapleural gap must be kept free of air or
excessive fluid. Because the pleura membranes are modestly permeable to gases and
water, physiological processes operate to maintain the correct balance (Agostoni, 1972).
The lung is an elastic structure with an anatomical organization that promotes its
collapse to a very small volume, much like an inflated balloon. While the elastic properties
of the lung are important to bring about expiration, they also oppose lung inflation. As a
result, lung inflation depends on contraction of the respiratory muscles. How easily a lung
inflates relates to its compliance (the reciprocal of elastance). Therefore, in a lung with a
high compliance, a small pressure change would result in a large volume change, and the
work performed by the respiratory muscles to inflate the lung would be small.
If the volume of the lung (removed from the body) is plotted as a function of its
pressure, its derivative gives the compliance. A similar, albeit less practical, experiment
could be used to determine the compliance of the chest cage.
With the lungs placed inside the chest cavity and their respective pleural surfaces held
together by cohesive forces, the volume of the lung is higher than its equilibrium volume,
whereas the chest cavity volume is less than its equilibrium volume. At equilibrium, the
recoil pressure of the lung tending to deflate is opposed by an equal but opposite recoil
pressure of the chest wall tending to expand as illustrated in Figure 9-7. These equal
but opposite recoil forces are reflected by an intrapleural pressure that is slightly lower
than atmospheric pressure. Moreover, with inhalation the intrapleural pressure decreases
further, reflecting the force tending to separate the lung and chest wall, but this is prevented
by the cohesive forces.
Figure 9-8 shows separate relaxation curves for the lung and the chest cage (both
solid), along with the combined lung-chest cage relaxation curve (dotted). The slope of
each relaxation curve corresponds to the compliance for the structures. At end expiration
(point A), recoil or relaxation pressure for the lung and chest cage alone are equal but
FIGURE 9-7
Lung and chest
cage in equilibrium.
