Which events occur at the beginning of ventricular systole?

• A. Opening of the semilunar valves
• B. Closure of the atrioventricular valves
• C. Atrial contraction
• D. Filling of the ventricles

Answer: (B)

Verified step by step guidance

1. Understand the cardiac cycle: The cardiac cycle consists of two main phases, systole and diastole. Systole refers to the contraction phase, while diastole refers to the relaxation phase.
2. Identify the phases of ventricular systole: Ventricular systole is the phase where the ventricles contract to pump blood out of the heart. It begins right after atrial systole (atrial contraction) and involves two main events: isovolumetric contraction and ventricular ejection.
3. Recognize the role of valves: During the cardiac cycle, the heart valves ensure unidirectional blood flow. The atrioventricular (AV) valves (tricuspid and mitral) separate the atria from the ventricles, while the semilunar valves (aortic and pulmonary) separate the ventricles from the major arteries.
4. Determine the events at the start of ventricular systole: At the beginning of ventricular systole, the ventricles start to contract, causing the pressure within them to rise. This pressure increase leads to the closure of the atrioventricular valves to prevent backflow into the atria.
5. Differentiate between valve actions: The closure of the AV valves marks the start of isovolumetric contraction, where the ventricles contract with no volume change because all valves are closed. The semilunar valves open later in the cycle when ventricular pressure exceeds arterial pressure, allowing blood to be ejected.