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

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

Answer: (B)

Longer vessels create more surface area along which the blood has to push, so there is more drag or friction at the walls. In fluid dynamics of laminar flow, the resistance to flow is directly proportional to the length of the vessel: doubling the length doubles the resistance (for the same radius, viscosity, and flow rate). This means a greater pressure drop is needed to maintain the same flow, reflecting increased friction with the vessel walls. So increasing length leads to more friction, not less, and not no change.