According to Poiseuille's law, which of the following statements is true if we wish to maintain a constant flow of gases?

a. Alveolar recruitment has its greatest effect on flow.
b. Halving the tube radius will require a 16-time increase in driving pressure.
c. Large jumps in driving pressure are need-ed to overcome airway narrowing.
d. The driving pressure varies directly with the airway circumference.

ANS: B
For gas flow to remain constant, delivery pressure must vary inversely with the fourth power of the airway's radius. Reducing the radius of a tube by half requires a 16-fold pressure increase to maintain a constant flow. To maintain ventilation in the presence of narrowing airways, large in-creases in driving pressure may be needed. The energy necessary to generate these pressures can markedly increase the work of breathing.