Final answer:
According to Poiseuille's law, the most significant contributor to resistance in a pipe is the radius.
Step-by-step explanation:
According to Poiseuille's law, the most significant contributor to resistance in a pipe, which includes blood vessels and airways, is the radius.
Poiseuille's law states that the flow rate of a fluid through a pipe is directly proportional to the fourth power of the pipe's radius. This means that even a small change in the radius of a blood vessel or airway can have a significant impact on the resistance to flow. For example, a decrease in the radius by only 5% can reduce the flow rate to about 0.81 of its original value.