Increasing the length of a vascular segment will have what effect on flow resistance?

• A. Decrease
• B. No change
• C. Increase
• D. Initially increase then decrease

Answer: (C)

Increasing the length of a vascular segment increases flow resistance. This follows Poiseuille’s law for laminar flow in a cylindrical tube, where resistance R is proportional to the fluid’s viscosity η and the length L, and inversely proportional to the fourth power of the radius r: R ∝ (ηL)/(r^4). With viscosity and radius held constant, longer vessels add more frictional losses along the path, so resistance rises linearly with length. Because flow rate Q equals the pressure difference ΔP divided by resistance (Q = ΔP/R), a longer vessel means less flow for the same driving pressure. The other options don’t fit this direct, linear relationship: increasing length does not decrease resistance, and there isn’t a scenario where resistance initially increases then decreases in this static, straightforward model.