The cardiac cycle describes the events from one heartbeat to the next. It includes three main phases: diastole, atrial systole, and ventricular systole, all driven by changes in chamber pressures and the function of heart valves.

Diastole: The Relaxation Phase

During diastole, all four heart chambers relax. The atrioventricular (AV) valves open, and the semilunar valves close. This phase sees the lowest chamber pressures, promoting ventricular filling. Venous blood enters the heart through the superior and inferior vena cava, flowing into the right atrium and then passively into the right ventricle.

Oxygenated blood from the lungs flows via the pulmonary veins simultaneously towards the left atrium and the left ventricle.

Atrial Systole: The Atrial Kick

Atrial systole occurs toward the end of diastole. The atrial muscles contract due to an electrical stimulus from the sinoatrial (SA) node. This contraction increases atrial pressure, ejecting the remaining blood into the ventricles and boosting ventricular blood volume by about 15% to 25%. This ensures they are filled for optimal efficiency in the next phase.

This phase is sometimes called the atrial kick.

Ventricular Systole: Ejection of Blood

Ventricular systole follows atrial systole and includes two sub-phases: isovolumetric contraction and ejection.

The electrical stimulus from the SA node causes ventricular contraction. Ventricular pressure rises quickly during isovolumetric contraction, closing the AV valves to prevent backflow into the atria.

As ventricular pressure increases, the pulmonic and aortic (semilunar) valves open, allowing blood to be ejected into the pulmonary artery and aorta. Blood flow starts rapidly but slows as pressures in the ventricles and arteries equalize.

At the end of systole, ventricular pressure drops, closing the semilunar valves and marking the beginning of diastole.

Cardiac Output: A Measure of Heart Function

Cardiac output, the volume of blood a ventricle pumps per minute, is a crucial indicator of heart function. In a resting adult, cardiac output ranges from 4 to 6 liters per minute, varying with metabolic needs. It is derived by multiplying stroke volume (the volume of blood released per beat) by heart rate (beats per minute).

The average resting stroke volume is about 60 to 130 milliliters.

Coronary Circulation: Blood Supply to Myocardium

The myocardium, or heart muscle, receives blood from the coronary circulation, primarily during diastole. The aorta gives rise to the left coronary artery, which splits into the left anterior descending artery and the left circumflex artery.

These supply the left atrium, left ventricle, interventricular septum, and part of the right ventricle. The right coronary artery, also originating from the aorta, supplies the right atrium, right ventricle, and a section of the posterior wall of the left ventricle. It also supplies the atrioventricular (AV) node and the bundle of His, so blockages here can significantly impact cardiac conduction.

Blood from the coronary system flows through the coronary sinus, which channels into the right atrium near the inferior vena cava, completing the blood supply cycle to and from the myocardium. During high heart rates, the duration of diastole shortens, potentially reducing myocardial perfusion.