Coronary artery blood flow

In contrast to the other vascular beds in the body, the heart receives its blood supply primarily during ventricular relaxation, or diastole, when the left ventricle is filling with blood. This effect results because the coronary ostia lie near the aortic valve and become partially occluded when the aortic valve opens during ventricular contraction or systole. However, when the aortic valve closes, the ostia are unobstructed, allowing blood to fill the coronary arteries. Because diastole is the time when the coronary arteries receive their blood supply, anything that shortens diastole, such as periods of increased heart rate or tachycardia, also decreases coronary blood flow. In addition, the left ventricular muscle compresses the intramuscular vessels during systole. During diastole, the cardiac muscle relaxes, and blood flow through the left ventricular capillaries is no longer obstructed. Coronary artery blood flow Cardiac veins Similar to other parts of the body, the heart has its own veins, which remove oxygen-depleted blood from the myocardium. About 75% of the total coronary venous blood flow leaves the left ventricle by way of the coronary sinus, an enlarged vessel that returns blood to the right atrium. Most of the venous blood from the right ventricle flows directly into the right atrium through the small anterior cardiac veins, not by way of the coronary sinus. A small amount of coronary blood flows back into the heart through the thebesian veins, minute veins that empty directly into all chambers of the heart. Cardiac physiology In this section, you'll find descriptions of the cardiac cycle; cardiac muscle innervation; depolarization and repolarization; and normal and abnormal impulse conduction. The cardiac cycle Coronary artery blood flow The cardiac cycle includes the cardiac events that occur from the beginning of one heartbeat to the beginning of the next. The cardiac cycle consists of ventricular diastole, or relaxation, and ventricular systole, or contraction. During ventricular diastole, blood flows from the atria through the open tricuspid and mitral valves into the relaxed ventricles. The aortic and pulmonic valves are closed during ventricular diastole. (See Phases of the cardiac cycle, page 10.)