What term defines the forces that oppose blood flow through a vessel?

• A. Blood viscosity
• B. Vascular resistance
• C. Cardiac output
• D. Net pressure

Answer: (B)

Vascular resistance is the forces that oppose blood flow through a vessel. It represents the friction blood encounters as it moves, and its size is shaped mainly by the vessel radius, with smaller changes in radius causing big changes in resistance (radius to the fourth power, per Poiseuille’s law). Resistance also depends on the vessel length and the blood’s viscosity. Because flow is driven by a pressure difference, the relationship is Flow = ΔP ÷ Resistance, so higher resistance lowers flow for the same pressure gradient. Blood viscosity is a factor that increases resistance, but it’s not the overall term for opposition to flow. Cardiac output is the amount of blood the heart pumps per minute, and net pressure is the driving force, not the resistance itself.