AAMC FL Practice Exam

What is the relationship between blood vessel radius and flow rate?

1. Increasing radius increases flow resistance

2. Decreasing radius decreases flow rate

3. Increasing radius decreases flow

4. Increasing radius increases flow rate

The correct answer is: Increasing radius increases flow rate

The correct relationship between blood vessel radius and flow rate is that increasing the radius of a blood vessel will increase the flow rate. According to Poiseuille’s law, which delineates the behavior of fluid flow through a cylindrical vessel, the flow rate is directly proportional to the fourth power of the radius of the vessel. This means that even a small increase in the radius can lead to a significant increase in flow rate. This principle is crucial in physiology because it explains how blood flow can be regulated throughout the vascular system. When blood vessels dilate, such as during physical activity or in response to certain signals like nitric oxide, the radius increases, thereby allowing more blood to flow through with less resistance. This is vital for delivering oxygen and nutrients to tissues and organs efficiently. In contrast, decreasing the radius of a blood vessel leads to an increase in flow resistance and, consequently, a decrease in flow rate. Therefore, when assessing the dynamics of blood flow, recognizing that flow rate increases with an increase in vessel radius is fundamental to understanding cardiovascular function.