Biomedical Engineering Reference
In-Depth Information
production being stimulated by increased shear stress. Therefore, a complete
model of pulmonary blood flow would include important structural influences,
passive influences due to blood pressure and gravity, as well as active mechanisms
that translate from the cellular to the full organ scale.
2 The Macrocirculation
The pulmonary vasculature provides a direct route from the heart to the gas
exchange surface. The branching structure of the pulmonary vasculature, like that
of the airways, allows efficient gas exchange through its bifurcating tree-like
structure that terminates in a very large alveolar-capillary surface area. After
circulating through the systemic circuit to deliver oxygen and nutrients to the body
tissue, blood returns to the right atrium of the heart via the vena cavae, and then to
the RV. The pulmonary trunk emerges from the RV through the pulmonary valve,
and is the conduit for deoxygenated blood to enter the pulmonary circuit. The left
and right main pulmonary arteries enter into the hila of the lungs and divide into
branches that accompany the lobar, segmental and subsegmental bronchi. Arterial
branches continue to bifurcate into successively smaller daughter branches in a
pattern that closely follows that of the bronchi. That is, each bronchus or bron-
chiole (airway) has a corresponding accompanying artery. The venous network
forms a similar branching structure, but it is located in between the paired arterial-
bronchial branches. A single main vein emerges from each of the lobes, and four
veins feed back into the left atrium of the heart. The pulmonary vessels do,
however, branch more frequently than the airways, giving rise to additional
'supernumerary vessels' that do not accompany an airway branch [
42
]. The
functional role of these numerous additional vessels is not known. Morphometric
cast-based studies have meticulously measured the geometric properties of the
pulmonary arterial and venous vessels [
37
,
43
-
45
] providing useful information
for constructing anatomically-based models of this complex system.
2.1 Modeling the Macrocirculation: From In Vivo
Imaging to In Silico Experiments
As with any simulation study, the first task in modeling the pulmonary circulation
is to develop the geometric domain over which to solve a set of mathematical
equations. These computational meshes can range vastly in their complexity. With
respect to modeling the pulmonary macrocirculation, by far the most predominant
method has been to represent the branching arterial and venous trees as symmetric
structures; that is, branches within each generation have equal diameters and
lengths. While this simplifies calculations tremendously (only a single pathway
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