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I'm not sure if you're asking the right question or whether I interpreted you correctly. Are u asking why is it considered a right to left shunt in case of an alveolar collapse?
If yes, then imagine AN alveoli with a capillary right nxt to it. air diffuses from the alveoli into capillary. If alvelolus is taken away, then the blood flows through the capillary without any oxygen-- thus it still has venous gas values as "functionally" it is still venous blood. This gives a situation "like" right to left shunt. as if blood from right side has entered left- which if u think about it has occurred in a way.

V/Q ratio is simple.
When v/Q reaches infinity, ventilation perfusion are the maximum they can be. Now the PO2 atmospheric air will not change no matter how hard we ventilate. thus with maximal ventilation and perfusion, the maximum po2 in blood (as even perfusion is proportionally increased with ventilation to match, it is the same as the air flowing "directly" from alveolus to blood) ; will be the same as atmospheric air.

I hope I have answered your question.
Guys, if I'm wrong anywhere please do let me know!
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