Adaptations to maximize respiratory gas exchange are reflected in the values of Fick’s law of diffusion Q = DA(P1 – P2)/L. Such adaptations have to do one or more of the following:
Increase the surface area for gas exchange (A)
Maximize the partial pressure difference driving diffusion (P1 – P2)
Minimize the diffusion path length (L)
Minimize the diffusion that takes place in an aqueous medium (maximize D)
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With these factors in mind, we will look at four different respiratory systems: those of insects, fishes, birds, and mammals.
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In insects, efficient gas exchange is facilitated by an internal network of air passages.
In fishes, countercurrent ventilation and perfusion facilitate efficient gas exchange.
In birds, anterior and posterior air sacs act as bellows to maintain a continuous unidirectional flow of air through the respiratory system.
Tidal volume, inspiratory reserve volume, and expiratory reserve volume can be measured directly, and residual volume can be determined using the helium dilution method.