During the cardiac cycle, which condition marks the start of ventricular systole?

• A. When the pressure in the ventricles exceeds the pressure in the arteries
• B. When atrial pressure exceeds ventricular pressure
• C. When the ventricles relax
• D. During passive filling of the ventricles

Answer: (A)

The start of ventricular systole is marked by the ventricle generating enough pressure to push blood into the arteries. When ventricular pressure rises above the pressure in the aorta and pulmonary artery, the semilunar valves open and blood is ejected into the arteries. This opening of the outflow valves signals the onset of the ventricular ejection phase of systole. Prior to this, during isovolumetric contraction, the AV valves have closed but the semilunar valves remain closed; backflow is prevented because the pressure hasn’t yet surpassed arterial pressure. The other conditions describe diastolic events or the phase before ejection, when the ventricles are filling or relaxing.