Flow in the Lungs - Biofluidmechanics: An Introduction to Fluid Mechanics Macrocirculation and Microcirculation - page 259

Before exhalation begins, the lung pressure becomes equilibrated with the atmospheric
pressure again because enough atmospheric air has entered the lungs. During exhalation
the reverse occurs. The ribs move downward and inward as the diaphragm moves
upward, which pushes the parietal pleural into close contact with the visceral pleural.
This movement increases the intrapleural pressure slightly (because the pleural fluid is
not highly compressible), which acts to push the visceral pleural and hence the lungs. The
interpulmonary pressure increases by approximately 1 mmHg above atmospheric pressure
which forces air out of the lungs. Again, before the next round of inhalation begins, the
atmospheric pressure and the interpulmonary pressure equilibrate. The pleural pressure is
a measure of the elastic forces that tend to collapse or expand the lungs.
The lungs are supplied with blood through two pathways. The first supplies blood to the
lung tissue, for nutrient delivery and waste exchange with the cells that compose the lungs
(via the bronchial arteries). This is similar to the systemic circulation of every other tissue
within the body. The second pathway is used for gas exchange to oxygenate blood and is
termed the pulmonary circulation. We will not discuss the first pathway here because it is
part of the systemic circulation and was covered in Parts 2 and 3 of this textbook.
The pulmonary circulation receives blood from the right side of the heart. The right ven-
tricle pumps deoxygenated blood to the alveolar capillaries via the left and right pulmonary
arteries. These arteries enter the lungs at the level of the bronchi and branch at the same
locations that the respiratory vessels branch. Thus, the blood vessels follow the pulmonary
tree down to the level of the alveoli. Each terminal bronchiole receives blood from one ter-
minal arteriole and blood exits via one post-capillary venule. As stated earlier, each alveo-
lus is surrounded by a dense mesh of capillaries to provide a surface for gas exchange.
After the blood becomes re-oxygenated it passes into the post-capillary venule and follows
the pulmonary tree back up to the level of the bronchi. The blood is returned to the heart
via the pulmonary veins for systemic circulation. Recall that the pulmonary circulation
flows at much lower pressures than the systemic circulation. It flows at approximately one-
sixth of the systemic circulation, rarely exceeding a systolic pressure of 30 mmHg.
9.2 ELASTICITY OF THE LUNG BLOOD VESSELS AND ALVEOLI
As we have discussed in previous chapters, blood flow through vessels is highly depen-
dent on the cross-sectional area of the blood vessels. We have discussed earlier in this
chapter that during each breath, the lungs and all of the tissues and cells within the lungs
experience a deformation. Generally during inhalation the lungs experience a 20% increase
in volume (a tensile stretch) which is brought back to normal levels after expiration has
occurred (a compressive deformation). From this knowledge, we would presume that the
blood vessels in the lungs would also be subjected to large strains, and therefore, the
cross-sectional area may change significantly.
Because the pulmonary blood vessels are viscoelastic in nature, the stress-strain rela-
tionship must take into account the strain rate history. Strain rate history would be a func-
tion of loading rates and the applied loads. Because this biological material is under large
deformations, the Kirchhoff stress (S ij ) and the Green's strain (E ij ) are the most applicable
stress/strain values to use to describe the loading conditions. Experimentally, it was found