At which of the following points along the ventilatory cycle is alveolar pressure equal to atmospheric pressure; therefore, transpulmonary pressure equals pleural pressure? I. during inspiration II. at end-inspiration III. during expiration IV. at FRC
A. I only
B. II only
C. I, III only
D. II, IV only
?ANS: D
A. Incorrect response: See explanation D.
B. Incorrect response: See explanation D.
C. Incorrect response: See explanation D.
D. Correct response: Regional lung inflation depends on the local transpulmonary pressure, which is the difference between the pressure in the alveoli and pressure at the pleural surface. That is,
Ptranspulmonary = Ppleural Palveolar
At FRC and at end-inspiration, alveolar pressure equals atmospheric pressure. Therefore transpulmonary pressure equals pleural pressure.
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