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

Abstract

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 V02 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%o 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. The lung's only known vital function is to provide sufficient gas-exchange surface to meet the organism's needs for oxygen uptake and carbon dioxide elimination. The importance of the relationship between oxygen consumption (VO2) and the size of the lung's gas-exchange surface area (Sa) can be appreciated from the strong interspecific conservation of a direct linear relation between Vo2 and Sa (1, 2). The size of individual gas-exchange units is inversely proportional to the species mass-specific Vo2; this allows species with a high mass-specific Vo2 to achieve sufficient Sa to meet the intensity of their metabolic needs without having a lung volume disproportionately large compared to its body mass (1). Interestingly, although the lung has sufficient Sa to sustain a short-term 10-fold increase in Vo2, as might occur with exercise (3), it responds to smaller but more prolonged (weeks to months) elevations of VO2 by increasing its gas-exchange capacity as reflected in its Sa (4-7).Pregnancy and lactation constitute long periods during which Vo2 is moderately elevated (8). Because these periods are recurrent, and rapidly so in many species, the high metabolic rate associated with reproduction may represent the "steady-state" condition during much of a female's reproductive life. The intestine adapts to reproduction-associated increased energy needs with elevations of mass and absorptive surface capacity that regress when lactation ends (9). This increased capacity occurs even though the intestine's absorptive capacity in nonpregn...

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