The carbon dioxide in the blood of the lung capillaries has a higher concentration(46 mm) than it has in the lung alveoli(40 mm). This accounts for the diffusion of carbon dioxide out of the blood into the alveoli. By the time the blood leaves the lungs, its carbon dioxide pressure has been lowered to approximately 40 mm. Thus, in the diffusion of both these gases there is a marked diffusion gradient that determines the direction of their flow. The larger volume of inert gases, that is inspired normally passes into the blood with oxygen and carbon dioxide.