What effect does increasing the luminal diameter of a blood vessel have on friction?

• A. Increases friction
• B. Decreases friction
• C. No change
• D. Increases resistance

Answer: (B)

When a liquid flows through a tube, the friction it experiences along the wall and the overall resistance to flow depend strongly on the tube’s radius. In laminar flow through a circular vessel, resistance to flow is inversely proportional to the fourth power of the radius (Poiseuille’s law). That means even a modest increase in diameter greatly lowers resistance. Wall shear stress, which is a measure of friction at the wall, also decreases as radius increases because the velocity gradient at the wall is reduced for a larger tube.

So, increasing luminal diameter reduces friction (and resistance). For example, doubling the radius makes the resistance drop by a factor of 16. The other options don’t fit because they imply more friction or no change, which contradicts how radius controls flow resistance and wall shear.