By what process does O2 move from the alveoli into the capillary blood ?

Question

By what process does O2 move from the alveoli into the capillary blood ?

Answer 1

In this case, the simplest answer is the best one: pasive diffusion  (as opposed to facilitated diffusion, active transport or convection).

Molecular oxygen is non-polar and small, and therefore lipid soluble.  It diffuses freely across the lipid bilayers of the cells that lay between the alveolar air spaces and the blood. It therefore diffuses "down" it's concentration gradient from alveolus to blood, where it is less concentrated.

Answer 2

I think that's just simple diffusion since the concentration of oxygen is higher in the air than in your blood.