During expiration, why does gas flow out from the lungs to the atmosphere?
a. Palv is less than at the airway opening.
b. Palv is the same as at the airway opening.
c. Palv is greater than at the airway opening.
d. Airway pressure is greater than Palv.
ANS: C
As expiration begins, the thorax recoils and Ppl starts to rise. As pleural pressure rises, alveolar pressure also increases. The transpulmonary pressure gradient narrows and alveoli begin to de-flate. As the alveoli become smaller, alveolar pressure exceeds that at the airway opening.
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