Which gas is transferred across the alveolar-capillary membrane entirely by perfusion limitation under normal resting conditions?

• A Nitrous oxide ✓
• B Carbon monoxide
• C Oxygen during maximal exercise
• D Carbon dioxide

Explanation

Nitrous oxide does not bind haemoglobin, so its capillary partial pressure equilibrates with alveolar gas within the first fraction of transit; uptake thereafter depends only on how much blood arrives, making it purely perfusion-limited. Carbon monoxide is the classic diffusion-limited gas because haemoglobin binds it so avidly that capillary PCO stays near zero. Oxygen is normally perfusion-limited but becomes partly diffusion-limited in heavy exercise and alveolar hypoxia.

Reference: West's Respiratory Physiology: The Essentials, 10th ed.

High-yield for: NEET PGINI-CETNExTFMGEUSMLEPLABMRCP

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