The heart is an organ whose function that hinges critically on its supply of and demand for oxygen. Supply of oxygen to the myocardium is influenced by factors such as blood oxygen content, blood pressure, coronary arterial resistance, and maximal coronary flow reserve. The last is considered the most important factor and is defined as the ratio of maximal achievable myocardial blood flow rate, by pharmacological selective coronary vessel dilation, to basal myocardial blood flow rate. Demand for oxygen by the myocardium is governed by factors such as heart rate, heart wall tension, wall thickness, and contractile state of heart muscle cells (cardiomyocytes). © 2005 Humana Press Inc.