Sexual dimorphism in the architecture of the lung's gas-exchange region.

The lung's only vital function is to provide sufficient gas-exchange surface area (Sa) to meet the organism's needs for oxygen uptake (VO2) and carbon dioxide elimination. A direct linear relation between Sa and VO2 and an inverse linear relation between the size of the lung's gas-exchange units and the species mass-specific VO2 are strongly conserved across species. Within species, Sa increases in response to prolonged (weeks) elevation of VO2. We now report sex-dependent deviations from these relationships that seem to anticipate the need for increased gas-exchange capacity engendered in females by the metabolic demands of pregnancy and lactation. We found that although VO2 almost doubled in rats during pregnancy and lactation, Sa was the same in age-matched virgin, pregnant, and lactating females. However, at the onset of sexual maturity, virgin female rats and mice had higher mass-specific Sa than males of the same species although mass-specific VO2 was identical, within species, in both sexes. In addition, even though mass-specific VO2 was identical in males and females, alveoli were 30% and 50% smaller in female rats and mice, respectively, than males of the same species. We suggest the greater mass-specific Sa and smaller alveoli in females in spite of identical mass-specific VO2 as males were selected for evolutionarily; they help females meet the metabolic demands of reproduction without adding to the energy demands of these periods a requirement to form additional lung.