What is the most significant source of blood flow resistance?

• A. Blood vessel diameter
• B. Blood viscosity
• C. Vessel length
• D. Blood pressure

Answer: (A)

The most significant source of blood flow resistance is the diameter of the vessel lumen. In Poiseuille-type flow, resistance is highly sensitive to radius—the relationship is proportional to 1/r^4—so even a small change in diameter produces a large change in resistance. This is why arterioles, which can constrict or dilate, play the major role in regulating resistance and thus flow to tissues. Viscosity and vessel length do affect resistance: higher viscosity or longer length increase resistance, but their effects are typically smaller in everyday physiology compared to changes in radius. Blood pressure is the force driving flow, not the resistance itself, so it does not determine how much resistance a vessel offers.