Exercise and alveolar diffusion

[Zuhal]

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Hi
Does anyone know why on page 564 (FA12) the exercise equilibration curve is lower than the normal O2 curve? doesnt exercise increase surface area which would increase the diffusion rate?

 

[stormchao]

You are correct that exercise leads to recruitment and expansion of the blood vessels in the lungs. However, the graph isn't specifically talking about diffusion rate, it is talking about PaO2 levels in blood as blood travels along the pulmonary capillary. Notice how under normal conditions it reaches ~95 half way along the capillary(this indicates it is perfusion limited). Now imagine you exercise and you increase your cardiac output by 3x as normal. The blood will be going through the capillary alot faster now, so it has less time to pick up O2.

I like to think of O2 as on the cusp of being between perfusion limited(normal) and diffusion limited(fibrosis). By increasing cardiac output during exercise you are increasing perfusion, which pushes it away from perfusion limited, and towards diffusion limited.