Are there sex differences in the capillary blood volume and diffusing capacity response to exercise?

Previous work suggests that women may exhibit a greater respiratory limitation to exercise compared to height-matched men. Diffusion capacity (DL_CO) increases with incremental exercise, and the smaller lungs of women may limit membrane diffusing capacity (Dm) and pulmonary capillary blood volume (Vc) in response to the increased oxygen demand. We hypothesized that females would have a lower DL_CO, DL_CO relative to cardiac output (DLCO/Q), Dm, Vc, and pulmonary transit time, secondary to lower Vc at peak exercise. Sixteen females (112±12% predicted relative VO_2peak), and sixteen males (118±22% predicted relative VO_2peak) were matched for height and weight. Hemoglobin-corrected diffusing capacity (DL_CO), Vc, and Dm were determined via the multiple-F_IO₂ DL_CO technique at rest, and during incremental exercise up to 90% of VO_2peak. Both groups increased DL_CO, Vc, and Dm with exercise intensity, but women had 20% lower DLCO (p<0.001), 18% lower Vc (p=0.002), 22% lower Dm (p<0.001) compared to men across all workloads, and neither group exhibited a plateau in Vc. When expressed relative to alveolar volume (V_A), the between-sex difference was eliminated. The drop in DL_CO/Q was proportionally less in women than men, and mean pulmonary transit time did not drop below 0.3s in either group. Women demonstrate consistently lower DL_CO, Vc, and Dm compared to height-matched men during exercise; however, these differences disappear when corrected for lung size. These results suggest that after accounting for differences in lung volume, there is no intrinsic sex difference in the diffusing capacity, capillary blood volume or diffusion membrane response to exercise.

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