Oxygen is picked up by the blood from the alveoli, after dissolving in the moisture inside the lungs and then diffusing through the thin wall of the lung alveoli. Carbon dioxide moves in the opposite direction. The dissolved oxygen then moves into the haemoglobin of the red blood cells, and carbon dioxide out of the red blood cells and plasma and into the lung air space. This happens by diffusion, each molecule moving down its diffusion gradient towards the region of lowest concentration.
