During measurement of pulmonary blood flow by the inert gas rebreathing technique, nitrous oxide equilibrates between alveoli and blood within seconds. This behavior illustrates that N2O transfer is:

• A Perfusion-limited ✓
• B Diffusion-limited
• C Reaction-limited
• D Ventilation-limited

Explanation

N2O is highly soluble and does not bind to anything in blood, so its partial pressure in capillary blood equilibrates with alveolar gas almost immediately, early in transit. Once equilibrium occurs, further uptake depends entirely on how much blood arrives, making the transfer perfusion-limited. Carbon monoxide is the classic diffusion-limited gas because it binds hemoglobin so avidly that capillary PCO stays near zero throughout transit.

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

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