The Extravascular Resistance

Two phenomena determine the heart’s resistance to coronary blood flow — the caliber of the resistance vessels as determined by smooth muscle in the walls of the coronary vessels and external compression of those vessels by the mechanical motion of the beating heart. The smooth muscle is controlled by cardiac nerves and local metabolic processes in the heart and is the effector for a rapid and efficient control system which matches blood flow to metabolic requirements under a wide variety of hemodynamic and contractile states.