Normally, the coronary circulation supplies the myocardium with the oxygen and substrate needed for it to meet its metabolic requirements at both rest and during stress such as physical exercise etc. When a coronary artery is occluded beyond 75% of its luminal diameter, but is not totally occluded, the metabolic needs of the myocardium supplied by that vessel can be met at rest but not during stress. When the vessel is totally occluded, as shown here, the oxygen and substrate (depicted here as green balls) available to the muscle becomes inadequate at rest as well as during stress, anaerobic metabolism ensues and the end products of metabolism (depicted as yellow balls), normally carried away by venous outflow, accumulate in the extracellular space. This sequence of events defines myocardial ischemia.