Originally Posted by zig zag
What is perfusion limited gas exchange ??
It is a pretty simple concept. The more blood you provide the more gas you will exchange. An example of oxygen. Blood with little partial pressure of oxygen enters the alveolar capillaries and almost instantaneously equilibrates with the alveolar oxygen partial pressure. Once there is equilibrium there is no more gas exchange. The only way you can exchange more oxygen is by providing new blood with low partial pressure of oxygen, by perfusing the alveolar capillaries. Therefore, perfusion is whats limiting the gas exchange.
Contrast that to CO, creates almost no partial pressure, virtually all of CO that gets diffused into the blood is bound to Hb. Gas exchange is diffusion limited because the only thing that is limiting gas exchange is diffusing capacity of the lungs. By the way, that is why we use CO to measure diffusing capacity (DLCO).
I do not know why those two gases behave so differently, could be because CO has much higher affinity for Hb than does oxygen but I never bothered to go much deeper than what I have explained.