In which part of the heart does ventricular systole begin?

Ventricular systole begins in the ventricles, which are the two lower chambers of the heart responsible for pumping blood to the lungs and the rest of the body. During this phase of the cardiac cycle, the ventricles contract following the initiation of electrical impulses that travel through the heart's conduction system. As the ventricles contract, they create a high-pressure environment that forces blood out of the heart: the right ventricle sends deoxygenated blood to the lungs, while the left ventricle sends oxygen-rich blood to the systemic circulation.

The other areas listed, such as the atria, sinoatrial node, and atrioventricular node, play important roles in the overall sequence of heart contractions but are not the sites where ventricular systole initiates. The atria contract first to fill the ventricles, the sinoatrial node acts as the primary pacemaker initiating the heartbeat, and the atrioventricular node serves as a critical relay point for the electrical signal between the atria and ventricles. However, the actual contraction of the ventricles, which defines ventricular systole, originates in the ventricles themselves.