What must exist for blood to move?

• A. Blood Viscosity
• B. Pressure or Energy Gradient
• C. Temperature Gradient
• D. Capillary Pressure

Answer: (B)

Blood moves because there must be a pressure difference, an energy gradient, from one point to another. The heart creates higher pressure as it contracts, and as blood travels through the vessels the pressure falls along the circuit. That difference in pressure (ΔP) provides the driving force that pushes blood forward; flow occurs as long as a gradient exists, even though the rate is shaped by vessel resistance. Viscosity matters because it changes resistance, so it can slow or speed flow for a given gradient, but it does not by itself start movement. Temperature differences don’t drive blood flow, and capillary pressure is just one piece of the overall pressure landscape, not the sole requirement. The key idea is that a pressure or energy gradient is needed to move blood.