Industrialised, Integrated, Intelligent sustainable Construction - I3con

SUSTAINABLE CONSTRUCTION HANDBOOK 2
Commonly, physiologists describe the lung as a multi-phase gas exchanger (Figure 6, [18, 19]).
Ventilation is only one phase, and it operates in the lung’s upper airways (the trachea and several
branches of the bronchial tree). There, gas exchange is dominated strongly by forced convection
driven by the respiratory muscles. In the lung’s terminal passages—the alveoli and alveolar ducts—
gas exchange is dominated by diffusion, and there is virtually no bulk flow of air there. Sandwiched
between these phases is an extensive region of the lung, which includes the fine bronchi and
bronchioles, where neither forced convection nor diffusion dominates flux. This mixed-regime region
is the site of the overall control of lung function. This is dramatically evident in asthma, which is a
constriction disorder of the mixed-regime airways. Small constrictions of these airways during an
asthma attack disproportionately compromises lung gas exchange in a way that similar constriction of
the upper airways do not.
Figure 6. Functional organization of the lung
This casts lung ventilation into a somewhat different perspective from how we normally view it. For
example, when respiratory exchange is elevated, as it might be during exercise, increased ventilation
(heavier breathing) does not by itself enhance respiratory flux, as most might think. Rather, increased
ventilation works its effect secondarily by enhancing gas exchange through the limiting mixed-regime
phase.
Termite colonies have a structural organization similar to the lung’s (Figure 7). The analogue to the
alveolus is the termite itself (which can be thought of as a mobile alveolus). These, in turn, are
embedded in the nest, which is organized into about a hundred galleries, each containing a fungus
comb, and each connecting to its neighbours through one or two small passageways, roughly 2-3 mm
in diameter. Altogether, the structure of the nest makes for a comparatively isolated air mass, similar
to the alveoli and alveolar ducts of the lungs. The mound air space, meanwhile, consists of a large
vertical chimney centrally, and the extensive airways of the mound reticulum, which envelop the nest
and terminates at the mound surface in innumerable small-diameter egress tunnels. These are the sites
of mound porosity, opening inward to the large vertically-oriented surface conduits on one end, and
outward through a thin porous layer of soil grains at the surface. Air movements in the mound are
strongly driven by wind. Mixing of the mound air and nest air is impeded in two ways. First, there is
the frequent stable thermal stratification that occurs when the nest is cooler than the mound (Figure 4).
Second, the nest and mound air spaces communicate only through very small channels: at the nest’s
lateral surfaces to the reticulum enveloping the nest below ground, and at the chimney from the
central part of the nest.
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