Biological Systems MCAT Practice Exam

What is the result of the closure of the AV valves in the heart?

1. Diastole begins

2. Systole begins

3. Blood pressure drops

4. Heart rate decreases

The correct answer is: Systole begins

The closure of the atrioventricular (AV) valves marks the transition from diastole to systole in the cardiac cycle. Specifically, this closure occurs at the beginning of ventricular contraction, which is referred to as systole. During systole, the ventricles contract to pump blood out of the heart—into the pulmonary artery from the right ventricle and into the aorta from the left ventricle. The sound produced when the AV valves close, paired with the semilunar valves opening, signifies that the ventricles are pressurized and are actively ejecting blood. This is a critical phase in the cardiac cycle, as it ensures that blood is efficiently circulated throughout the body and lungs, which is essential for delivering oxygen and nutrients to tissues. The other potential results listed involve different aspects of heart function and the cardiac cycle. For instance, diastole is the phase during which the heart chambers relax and fill with blood, which does not occur at valve closure. Blood pressure is typically elevated during systole rather than dropping, and heart rate is influenced by various factors (like autonomic nervous system input) that do not directly result from the closure of the AV valves. Thus, the closure of the AV valves initiating systole