Phases of the cardiac cycle

The cardiac cycle consists of these phases: Isovolumetric ventricular contraction - In response to ventricular depolarization, tension in the ventricles increases. The rise in pressure within the ventricles leads to closure of the mitral and tricuspid valves. The pulmonic and aortic valves stay closed during the entire phase. Ventricular ejection - When ventricular pressure exceeds aortic and pulmonary artery pressures, the aortic and pulmonic valves open and the ventricles eject blood. Isovolumetric relaxation - When ventricular pressure falls below the pressures in the aorta and pulmonary artery, the aortic and pulmonic valves close. All valves are closed during this phase. Atrial diastole occurs as blood fills the atria. Phases of the cardiac cycle Ventricular filling - Atrial pressure exceeds ventricular pressure, which causes the mitral and tricuspid valves to open. Blood then flows passively into the ventricles. About 70% of ventricular filling takes place during this phase. Atrial systole - Known as the atrial kick, atrial systole (coinciding with late ventricular diastole) supplies the ventricles with the remaining 30% of the blood for each heartbeat.

During diastole, about 75% of the blood flows passively from the atria through the open tricuspid and mitral valves and into the ventricles even before the atria contract. Atrial contraction, or atrial kick as it's sometimes called, contributes another 25% to ventricular filling. Loss of effective atrial contraction occurs with some arrhythmias, such as atrial fibrillation. This loss results in a reduction in cardiac output. P.11Preload and afterload Phases of the cardiac cycle Preload refers to a passive stretching that blood exerts on the ventricular muscle fibers at the end of diastole. According to Starling's law, the more the cardiac muscles are stretched in diastole, the more forcefully they contract in systole.

Afterload refers to the pressure that the ventricles need to generate to overcome higher pressure in the aorta to eject blood into the systemic circulation. This systemic vascular resistance corresponds to the systemic systolic pressur During ventricular systole, the mitral and tricuspid valves are closed as the relaxed atria fill with blood. As ventricular pressure rises, the aortic and pulmonic valves open. The ventricles contract, and blood is ejected into the pulmonic and systemic circulation.