Physiology of the Heart: The Cardiac Cycle

During diastole, all four heart chambers relax and atrioventricular valves open while semilunar valves close. Deoxygenated blood from the vena cava fills the right atrium, and oxygenated blood from the pulmonary veins fills the left atrium. Chamber pressures remain at their lowest, promoting passive ventricular filling and preparing the heart for the next contraction.

Atrial systole, triggered by the sinoatrial node, causes the atrial muscles to contract and eject remaining blood into the ventricles. This atrial contraction, sometimes called the atrial kick, increases ventricular blood volume by approximately 15 to 25 percent, ensuring optimal filling for efficient ventricular contraction during the next phase of the cardiac cycle.

Ventricular systole begins with isovolumetric contraction, during which rising ventricular pressure closes the atrioventricular valves to prevent backflow. As pressure continues to increase, the semilunar valves open, allowing blood ejection into the pulmonary artery and aorta. When ventricular pressure drops at systole's end, semilunar valves close, marking the return to diastole.

Cardiac output is calculated by multiplying stroke volume by heart rate. In resting adults, cardiac output ranges from 4 to 6 liters per minute, with average stroke volume around 60 to 130 milliliters per beat. This measurement reflects the volume of blood a ventricle pumps per minute and varies with metabolic demands.

The left coronary artery branches into the left anterior descending and left circumflex arteries, supplying the left atrium, left ventricle, interventricular septum, and part of the right ventricle. The right coronary artery supplies the right atrium, right ventricle, posterior left ventricle wall, atrioventricular node, and bundle of His, making blockages here particularly significant for cardiac conduction.

The myocardium receives blood primarily during diastole through the coronary circulation. Deoxygenated blood from the heart muscle drains through the coronary sinus, which channels into the right atrium near the inferior vena cava, completing the blood supply cycle. During high heart rates, shortened diastole may reduce myocardial perfusion.

Chamber pressures drive valve movement throughout the cardiac cycle. Low pressures during diastole keep atrioventricular valves open and semilunar valves closed. Rising ventricular pressure during systole closes atrioventricular valves and opens semilunar valves. When ventricular pressure drops, semilunar valves close, returning the heart to diastole and completing the cycle.