In which of the following types of ventilation is alveolar expansion during inspiration due to an increase in alveolar pressure?

1. Negative-pressure ventilation
2. Positive-pressure ventilation
3. Spontaneous ventilation
a. 1 and 2 only
b. 2 and 3 only
c. 2 only
d. 1, 2, and 3

ANS: C
Gas flows into the lungs because pressure at the airway opening (Pawo) is positive and alveolar pressure (Palv) is initially zero or less positive.