Biomedical Engineering Reference
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Fig. 4 A schematic of a model of pulmonary perfusion that bridges the relevant spatial scales in
the lung. Arteries and veins are connected by a 'ladder-like' structure of arterioles, venules, and
capillaries that cover the alveoli in the acinar unit
4 Whole Organ Models: Bridging Spatial Scales
Macro- and micro-scale models of the pulmonary circulation can provide impor-
tant insight regarding individual aspects of its function, however there is a sig-
nificant limitation to these models, as boundary conditions must be prescribed at a
spatial scale at which it is difficult to obtain pressure or flow measurements.
A model that bridges the gap between these spatial scales is then necessary to
understand how small scale phenomena emerge into whole lung function. The first
steps toward an anatomically-based whole lung model were taken by Clark et al.
[ 31 ], who incorporated serial and parallel micro-circulatory connections in a
model of the pulmonary acinus. This model connects the arteries that accompany
the respiratory bronchioles to their corresponding veins via the capillary network
across several alveolar septa in what has been termed a 'ladder-like' structure
(Fig. 4 —gray box). There are multiple pre-capillary vessel generations in the
acinus, and this model assumed that these vessels branch in a regular dichotomy,
connected at each generation by capillary sheets (the sheet flow model described in
Sect. 3.1 ). The model showed that this ladder-like structure may provide func-
tional benefits to the lung: it predicted that the serial-parallel capillary connections
provided a 17 % lower pulmonary vascular resistance than simple parallel capil-
lary connections, when coupled to a symmetric model of the macro-vasculature.
As well as predicting a lower total pulmonary vascular resistance than previous
models, the ladder-like geometrical structure was able to predict stratification of
intra-acinar blood flow similar to that observed experimentally many years pre-
viously [ 77 - 80 ]. This stratification of blood flow may play an important role in the
matching of perfusion to oxygen supply from air.
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