Nitrous oxide reaches complete equilibrium between alveolar gas and pulmonary capillary blood within the first fifth of transit through a resting pulmonary capillary. Its overall transfer across the lung is therefore limited primarily by:

• A Thickness of the blood-gas barrier
• B Binding affinity of nitrous oxide for plasma proteins
• C Pulmonary blood flow (perfusion) ✓
• D Alveolar surface area available for diffusion

Explanation

Because nitrous oxide is highly blood soluble and does not bind to anything, its partial pressure equilibrates very early in the capillary, after which no further gradient exists; additional uptake requires delivering more mixed venous blood, making transfer perfusion limited. Carbon monoxide behaves oppositely: avid hemoglobin binding maintains the gradient along the whole capillary, making it diffusion limited, which is precisely why it suits DLCO measurement.

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

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