During expiration, why does gas flow out from the lungs to the ...

[clippers!]

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.

Question 2

What happens during normal inspiration? 1. The Ppl increases further below atmospheric pressure. 2. The transpulmonary pressure gradient widens. 3. Palv drops below that at the airway opening.
 
  a. 1 and 2 only
  b. 2 and 3 only
  c. 1 only
  d. 1 and 3 only

[rnehls]

Answer to Question 1

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.

Answer to Question 2

ANS: B
As the alveoli expand, their pressures fall below the pressure at the airway opening. This nega-tive (i.e., sub-atmospheric) transrespiratory pressure gradient causes air to flow from the airway opening to the alveoli, increasing their volume.