Extravascular Coronary 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 deformation of those vessels by the mechanical motion of the beating heart. Smooth muscle in the coronary bed is controlled by cardiac nerves and local metabolic processes in the heart. This smooth muscle is the effector for a rapid and efficient control system that ensures that the heart receives an appropriate blood flow under a wide variety of blood pressures and contractile states. An in-depth discussion of that control can be found elsewhere in this book.