During which stage do the semilunar valves open in the cardiac cycle?

The semilunar valves open during the stage when the ventricles contract, a phase known as ventricular systole. During this stage of the cardiac cycle, the pressure within the ventricles rises as they contract to pump blood into the aorta and pulmonary artery. When the pressure exceeds that in the arteries, the semilunar valves, which are located between the ventricles and these major arteries, are forced open. This allows for the ejection of blood, enabling effective circulation throughout the body and to the lungs for oxygenation. Understanding this mechanism is crucial in studying how the heart functions as a pump and maintains blood flow.

The other choices reflect different phases of the cardiac cycle where the semilunar valves do not open. For instance, when both atria contract, they push blood into the ventricles but do not influence the semilunar valves. Atrial relaxation refers to a phase when blood fills the atria, and during this time, the focus is more on the opening and closing of the atrioventricular valves. The option concerning valves shutting does not apply here since the opening of the semilunar valves specifically occurs during ventricular contraction.