Pressure-volume loops of an isolated lung inflated and deflated with saline show that the inflation and deflation curves nearly superimpose, whereas loops obtained with air show marked separation between the two limbs. What explains this difference?
- A Saline eliminates the air-tissue interface, removing the surface forces responsible for hysteresis ✓
- B Saline increases tissue elastic recoil, shifting both curves to the left
- C Airway resistance dissipates more energy during saline filling
- D Surfactant is washed out by saline, abolishing lung compliance
Correct answer: A. Saline eliminates the air-tissue interface, removing the surface forces responsible for hysteresis
Explanation
Hysteresis arises mainly from surface tension forces at the air-fluid interface, which differ between inflation and deflation because surfactant molecules must be recruited during inflation. Filling the lung with saline abolishes this interface, leaving only tissue elastic forces, so the two curves merge and overall compliance rises. Saline does not wash out surfactant in this experiment, and tissue recoil alone accounts only for the smaller residual loop.
Reference: Guyton and Hall Textbook of Medical Physiology, 14th ed.
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