At what point does a cardiac cycle technically begin?

The cardiac cycle technically begins with the onset of ventricular systole, which is the phase where the ventricles contract. During this phase, the pressure inside the ventricles rises significantly, causing the atrioventricular valves to close and the semilunar valves to open. This leads to the ejection of blood into the aorta and pulmonary artery, marking a key transition in the cycle where the heart actively pumps blood to the surrounding circulation.

Understanding that the cardiac cycle encompasses a sequence of events—ventricular contraction (systole) and subsequent relaxation (diastole)—is crucial. Ventricular systole is pivotal because it signifies the active phase of the heart’s pumping action, thereby initiating the rhythmic sequence of contractions and relaxations that defines the cardiac cycle. While atrial and ventricular diastole, along with atrial systole, are also important components of the overall cycle, it is the start of ventricular systole that distinguishes the commencement of the cardiac cycle in terms of generating blood flow.