Uptake of which gas across the alveolar-capillary membrane is limited solely by pulmonary blood flow, because partial pressure equilibration is completed within the first fraction of the capillary transit time?

• A Carbon monoxide
• B Oxygen at rest in a healthy adult
• C Oxygen during maximal exercise in a patient with pulmonary fibrosis
• D Nitrous oxide ✓

Explanation

Nitrous oxide does not bind haemoglobin, so its capillary partial pressure rises almost instantly to equal alveolar pressure; thereafter no further gradient exists and uptake depends only on how much blood arrives, making it the classic perfusion-limited gas. Carbon monoxide is the classic diffusion-limited gas because haemoglobin sequesters it and keeps capillary partial pressure near zero. Resting oxygen transfer is normally perfusion-limited, but option C describes a genuinely diffusion-limited state, which makes it the strongest distractor.

Reference: West's Respiratory Physiology: The Essentials, 11th ed.

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