What is the name of the force that opposes blood flow?

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Resistance is the force that opposes blood flow. In circulation, blood moves because a pressure difference drives it from higher to lower pressure, but the movement is checked by friction between the moving blood and the vessel walls. This frictional opposition is what we call vascular resistance. The relationship is often summed up as ΔP = Q × R, meaning for a given flow, higher resistance requires a higher pressure difference, and vice versa. Vessel radius has a huge effect—via Poiseuille’s law, small changes in radius dramatically change resistance, so narrowing vessels markedly reduce flow. Blood viscosity and vessel length also contribute to resistance. Gravity and hydrostatic pressure can influence local pressures in the body, but they aren’t the general opposing force to flow. Flow rate is the result that comes from the balance between driving pressure and resistance, not the opposing force itself.