Review the concept of diffusion and Fick's law. Understand the concept of perfusion limited vs diffusion limited gas exchange. Describe the measurement of diffusing capacity. At rest the red blood cell only takes around 0.75 seconds to transit the pulmonary capillary and this may impose a limit on the time available for diffusion. Gases which are highly soluble or react chemically with blood constituents tend to be diffusion limited, because large quantities can be taken up by the blood and no appreciable back pressure develops to reach equilibrium.
Carbon monoxide is such a gas because it rapidly binds with hemoglobin.
Inert and poorly soluble gases such as nitrous oxide rapidly reach equilibrium as only small quantities can be taken up. High altitude or low alveolar oxygen pressure. Thickening of the alveolar capillary membrane. Increased pulmonary blood flow. While carbon dioxide is more soluble than oxygen and diffuses 20 times more rapidly, it also is more soluble in the blood and, in sum, its rate of equilibration is comparable to that of oxygen.