diffusion

Open a bottle of perfume in one corner of a still room and, given a little time, you will smell it across the room — no fan required. Molecules naturally spread from where they are crowded to where they are sparse. Diffusion in the lung is exactly this: oxygen and carbon dioxide drift across the air–blood barrier on their own, simply because each gas moves from where it is more concentrated to where it is less so.

The relevant measure of crowding for a gas is its partial pressure. Oxygen has a high partial pressure in the fresh alveolar air and a lower one in the arriving venous blood, so oxygen diffuses inward; carbon dioxide is more concentrated in the blood than in the air, so it diffuses outward. The rate of this passive movement rises with the pressure difference and the surface area available, and falls as the membrane gets thicker — the relationships captured by Fick's law of diffusion.

No energy is spent; the lung does not pump gases, it merely arranges a vast, thin surface and a steep pressure gradient so diffusion can do the work. An honest caveat: oxygen crosses less readily than carbon dioxide because it is less soluble in tissue, which is why conditions that thicken or destroy the membrane tend to lower blood oxygen well before they raise carbon dioxide.