The influence of blood vessel diameter on peripheral resistance is ________.

• A. Significant because resistance is inversely proportional to the fourth power of the vessel radius
• B. Insignificant and independent of radius
• C. Significant only at very large radii
• D. Negligible compared to blood viscosity

Answer: (A)

The main idea is that vessel radius has a huge effect on peripheral resistance. Poiseuille’s law tells us that resistance is proportional to 1 divided by the fourth power of the radius (R ∝ 1/r^4) for a given length and blood viscosity. Because of this 4th-power relationship, even small changes in radius cause large changes in resistance. For example, doubling the radius drops resistance by 16-fold, while halving the radius raises it 16-fold. This makes diameter changes a dominant factor in determining resistance, more influential than most changes in viscosity. The other statements are inconsistent with this relationship: radius clearly matters, it’s not limited to very large radii, and while viscosity matters, the radius’ effect is much more pronounced due to the 4th-power dependence.