Abstract
We investigate analytically the production of entropy during a breathing
cycle in healthy and diseased lungs. First, we calculate entropy
production in healthy lungs by applying the laws of thermodynamics to
the well-known transpulmonary pressure-volume (P-V) curves of the lung
under the assumption that lung tissue behaves as an entropic spring
similar to rubber. The bulk modulus, B, of the lung is also derived from
these calculations. Second, we extend this approach to elastic recoil
disorders of the lung such as occur in pulmonary fibrosis and emphysema.
These diseases are characterized by particular alterations in the P-V
relationship. For example, in fibrotic lungs B increases monotonically
with disease progression, while in emphysema the opposite occurs. These
diseases can thus be mimicked simply by making appropriate adjustments
to the parameters of the P-V curve. Using Clausius's formalism, we show
that entropy production, AS, is related to the hysteresis area, AA,
enclosed by the P-V curve during a breathing cycle, namely, Delta S=Delta A/T, where T is the body temperature. Although AA is highly
dependent on the disease, such formula applies to healthy as well as
diseased lungs, regardless of the disease stage. Finally, we use an
ansatz to predict analytically the entropy produced by the fibrotic and
emphysematous lungs.
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