How does lung structure affect gas exchange?

The lung is characterized morphologically by establishing a very large surface and an exceedingly thin barrier between air and blood. A model for relating these structural features to the lung's gas exchange function is first developed. It is then shown that DO2 estimated by morphometry is about two times larger than that estimated by physiology; there are possible reasons for this. Comparing animals of high activity (dog, horse) with corresponding species of lower activity (man, cow) reveals that DO2 is proportional to O2 needs. The mechanical properties of the lung are discussed which allow such a large surface with such a thin barrier to be maintained lifelong. Surfactant properties of the lining layer are important factors in stabilizing the alveolar surface. Repair processes are also essential and require metabolic activities of the cells lining the barrier. The case of adult respiratory distress syndrome is used to illustrate the consequences of severe damage to the cell linings of alveoli: the barrier is thickened, and a good part of the alveolar surface is flooded by edema fluid, so that gas exchange is severely impaired.