Figure 7.01. Perfusion-limited (nitrous oxide = N2O & oxygen) transfer of oxygen vs. diffusion-limited transfer (carbon monoxide = CO).
Since CO has a very high affinity for hemoglobin (Hb, 230-times that of oxygen), it combines with Hb almost as fast as it diffuses across the alveolar-capillary membrane. Thus, its partial pressure hardly rises at all in the blood.
In contrast, N2O does not combine with Hb or anything else in the blood, so its partial pressure rises very rapidly to attain the same value as in the alveolar gas. Therefore, its diffusion ceases since there is no longer a gradient driving it - its diffusion can only continue if the gradient is restored by bringing in fresh blood. Thus the diffusion of N2O is dependent on blood flow while diffusion of CO depends on the properties of the "membrane" itself, i.e. its surface area and thickness characteristics.
Oxygen transfer is more dependent on blood perfusion rate, in contrast to CO where transfer is dependent on the diffusion properties of the membrane (surface area and thickness). Thus, when pulmonary diffusing capacity is measured clinically, CO is used as the test gas.
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