Nitrous oxide reaches complete equilibration between alveolar gas and end-capillary blood well before blood traverses one third of the pulmonary capillary, whereas carbon monoxide fails to equilibrate even along the full capillary length. This difference exists because:
- A N2O does not bind hemoglobin, so its uptake is limited only by blood flow, while CO binding keeps its capillary partial pressure near zero ✓
- B N2O has a much lower solubility coefficient in blood than CO
- C CO diffuses through the alveolar membrane far more slowly than N2O
- D N2O causes pulmonary vasodilation, shortening transit time
Correct answer: A. N2O does not bind hemoglobin, so its uptake is limited only by blood flow, while CO binding keeps its capillary partial pressure near zero
Explanation
Transfer is diffusion-limited when equilibration is incomplete along the capillary. CO binds avidly to hemoglobin, so its partial pressure in capillary blood stays near zero and equilibrium never occurs, making its uptake depend on membrane properties. N2O does not bind hemoglobin, equilibrates rapidly, and its uptake thereafter depends only on cardiac output, so it is perfusion-limited. Both gases diffuse readily across the membrane.
Reference: West's Respiratory Physiology: The Essentials, 10th ed.
High-yield for: NEET PGINI-CETNExTFMGEUSMLEPLABMRCP
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