Of viscosity, vessel length and radius, which one can the body alter rapidly?

1. Rearranging the flow equation gives resistance as eight times viscosity times length, divided by pi times radius to the fourth power. 2. Only three quantities in that expression can vary: viscosity, vessel length and radius. 3. Viscosity and vessel length both change slowly, over illness or growth. 4. Radius alone can be changed quickly, by vasoconstriction and vasodilation of the smooth muscle in the wall. 5. Because radius is raised to the fourth power, even a small change in it swamps the other two. _Source: OpenStax Anatomy and Physiology (CC BY 4.0), Ch 20 "The Cardiovascular System: Blood Vessels and Circulation", section 20.2 Blood Flow, Blood Pressure, and Resistance_