The gas occurs by simple diffusion through the cells.
capillaries
the alveolus
in the leaf
in the leave
gas exchange occurs between pulmonary capillaries and the alveoli in the lungs.
gas exchange occurs between the thin walls of the alveoli and the thin walls of the capillaries
in their gills.
In the alveoli
lungs
--it is in the alveoli where the exchange of gases occur.. :))
Between the avioli and the lungs.