According to Poiseuille's law, reducing the radius of an airway to one-third will increase its resistance how many fold?
High-Yield Explanation
Poiseuille's law states that during laminar flow, airway resistance is inversely propoional to the 4th power of the radius, other things being equal. Therefore, a reduction in the radius by a factor of 3 increases the resistance by 34, that is, 81.