Cerebral gas exchange: perfusion and diffusion limitations.

The simultaneous washout of argon and methane from brain tissue stores was studied in anesthetized dogs that were rendered either hypocapnic or hypercapnic. The washout pattern can be analyzed in terms of two compartments with different rate constants. The two tracers were eliminated at the same rate from any one compartment in either experimental condition. Because argon and methane have the same blood-brain partition coefficient, and a ratio of molecular weights of 2.5, we conclude that their transport into or from the brain is not limited by diffusion over a wide range of cerebral blood flows.