The heart is constantly pumping blood around the body.
blood entering the lungs is deoxygenated and high in C02
The air in the alveoli has a high oxygen concentration and in comparison is low in C02
This sets up a concentration gradient.
This causes oxygen to diffuse into the RBC and C02 to diffuse out in order to try balance the concentration.
This balance is maintained by breathing, taking away the C02 in the lungs and bringing in fresh O2 and the constant pumping of blood, bringing more deoxygeneate blood into the lungs
I thinks it's because of the blood supply - it's always circulating so there's always a steep concentration gradient of more oxygen in the lungs compared to the blood and vice versa for carbon dioxide.
I'm not sure but that's what I would go with.
Answered Sat 15th September, 2012 @ 14:53 by Joanne