CO2 diffuse more easily than O2 because: Less dense

B i.e. More soluble in plasma - Carbon dioxide (CO2) is 20 times more soluble in water (= plasma) than oxygen (02), this is the reason why CO2 diffuses about 20 times more rapidly than does 02 across the alveolar-capillary barrier (although CO2 has a higher but not very different molecular weight than 02) . Rate of diffusion a solubility/ VMW - Because the transfer of carbon monoxide (CO) across blood gas barrier of lung is limited solely by diffusion, and therefore CO is the gas of choice for measuring the diffusion propeies of lunge.