Smaller is better—but not too small: A physical scale for the design of the mammalian pulmonary acinus

TLDR

Oxygen transfer in the lung relies on ventilation, gas diffusion, and tissue diffusion within the acinus, where efficient design requires space‑filling surfaces that are not too large to avoid screening effects. The study shows that mammalian acini meet these design criteria, with smaller animals having more efficient acini at rest and during exercise.

Abstract

The transfer of oxygen from air to blood in the lung involves three processes: ventilation through the airways, diffusion of oxygen in the air phase to the alveolar surface, and finally diffusion through tissue into the capillary blood. The latter two steps occur in the acinus, where the alveolar gas-exchange surface is arranged along the last few generations of airway branching. For the acinus to work efficiently, oxygen must reach the last branches of acinar airways, even though some of it is absorbed along the way. This “screening effect” is governed by the relative values of physical factors like diffusivity and permeability as well as size and design of the acinus. Physics predicts that efficient acini should be space-filling surfaces and should not be too large. It is shown that the mammalian acini fulfill these requirements, small mammals being more efficient than large ones both at rest and in exercise.