Translational Research: Multi-Scale Models of the Pulmonary Circulation in Health and Disease - Multiscale Computer Modeling in Biomechanics and Biomedical Engineering - page 260

Fig. 1 a An anatomically based geometric model of the human arterial tree that fills the lung
volume obtained from imaging data (reprinted from [ 28 ], with permission from Elsevier), and b a
symmetric model of the arterial tree with the same average morphometric properties
needs to be considered with tens of branches, compared with tens of thousands of
branches in the full lung) the significant impact that structure has on function
cannot be realized. Until fairly recently, computation time limited the size of
geometric models of the pulmonary circulation and airway tree. However the rapid
advancement of computational power in recent decades has allowed significant
steps forward in imaging technologies to describe the lung geometry, as well as in
the ability to solve large systems of equations numerically over extensive and
complex domains. Therefore, the structural accuracy of geometries in which
models of blood flow can be solved has improved dramatically. Figure 1 shows a
comparison between an anatomically based model geometry and a spatially dis-
tributed symmetric model geometry (note that symmetric models are not usually
distributed spatially in this manner, nor with each pathway modeled explicitly).
Each model has the same average morphometric parameters and there are an
equivalent number of blood vessels in each tree. However, the anatomically based
model clearly captures certain aspects of the lung structure that the symmetric
model cannot.
The contribution of vascular branching structure to heterogeneity in the dis-
tribution of pulmonary perfusion and its relationship to airway structure and
function was first introduced in studies that analyzed the fractal properties of blood
flow distribution. The concept of the pulmonary circulation having a fractal
branching structure was championed by Glenny and Robertson who demonstrated
that there is a spatial correlation in local pulmonary blood flow that is independent
of the size or location of the lung region considered [ 20 , 46 ]. This implied an
important role for the pulmonary branching networks in determining regional
blood and airflow and ultimately the efficiency of the lung for gas exchange.
Combined with advances in estimating regional perfusion distribution (specifically
the use of inhaled or injected microspheres), fractal modeling of the pulmonary
circulation pointed to inconsistencies with the traditional zonal model for perfu-
sion. This led to a debate within the community concerning the importance of