- a primary role of the respiratory system - concentration gradients are required for gas exchange to occur - when air is breathed in, it is high in oxygen concentration - the blood in the capillaries around the alveoli has a low oxygen concentration - oxygen will make more via diffusion from high to low concentration - oxygen dissolves in the moisture on the inside of the alveolus and diffuses through the membrane, through the walls of the capillaries and into the blood - the blood in capillaries around the alveoli are high in carbon dioxide - carbon dioxide diffuses out of the blood and into the air in the alveolus