Respiratory phys: V/Q, perfusion/diffusion, A-a

[kevmore]

I am having some trouble putting the respiratory physiology concepts of V/Q, diffusion/perfusion limited, and A-a gradient together, although I feel like I have some understanding of each individually. As I understand it:

Normally:
- V/Q = 0.8 overall, with differences between the apex and base
- Oxygen gas exchange is perfusion-limited
- A-a gradient = 0-15 mmHg

Questions:
- If oxygen diffusion occurs fully (and respiration is perfusion-limited) then why does a A-a gradient exist (rather than being 0)? Is the oxygen partial pressure in the alveoli and pulmonary capillaries the same, normally? I understand that physiological shunting contributes a small amount, when considering the arterial oxygen in the A-a gradient. Are there other contributors normally to the A-a gradient? What about V/Q mismatch, since V/Q<1 overall and in the bases?
- If V/Q < 1, then is there a diffusion limitation? If so, then why is oxygen gas exchange perfusion limited normally.
- Why is V/Q of 1 ideal, if delivery is approximately V = 200 mL O2/L and blood capacity = 200mL O2/L, and there is already substantial oxygen content in the venous blood

Can anyone clarify?

Thanks!

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