Which one of the following gases diffuses across the alveoli-capillary membrane by a diffusion-limited transport process?
Explaination: Answer: D. A diffusion-limited transport process is one in which the alveolar gas does not reach equilibrium with the end-pulmonary capillary blood. When diffusional equilibrium does not occur, the amount of gas transferred is proportional to the concentration gradient and the permeability of the gas to the alveolar-capillary interface. Carbon monoxide (CO) is transported by a diffusion-limited process because it is avidly bound to hemoglobin. So much CO binds to hemoglobin that the partial pressure of CO in the capillary blood remains near zero. As a result, the concentration gradient from alveolar gas to capillary blood remains constant and the amount of CO diffusing across the alveolar capillary interface depends only on the permeability of the gas. In contrast, all of the other gases reach equilibrium with the capillary blood, and, therefore, the amount of those gases that diffuses across the alveolar capillary membrane is dependent on the amount of blood passing thro
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