  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 entropy spring-like 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, $\Delta S$, is related to the
hysteresis area, $\Delta A$, enclosed by the $P-V$ curve during a breathing
cycle, namely, $\Delta S=\Delta A/T$, where $T$ is the body temperature.
Although $\Delta A$ is highly dependent on the disease, such formula applies to
healthy as well as diseased lungs, regardless of the disease stage. Finally, we
use {\it ansatzs} to predict analytically the entropy produced by the fibrotic
and emphysematous lungs.
