The atrioventricular (AV) valves are closed a) when the ventricles are in systole. During ventricular systole, the ventricles contract to pump blood out of the heart, which increases the pressure within the ventricles. This pressure forces the AV valves (the tricuspid and mitral valves) to close, preventing the backflow of blood into the atria. This closure is crucial for efficient blood circulation, as it ensures that blood flows from the ventricles into the arteries (the aorta and pulmonary artery) and not back into the atria.
Options b), c), and d) do not accurately describe the closing of the AV valves. Choice b) describes the movement of blood from the atria to the ventricles, which occurs when the AV valves are open during diastole, not closed. Choice c) refers to the atrial contraction, which happens prior to ventricles systole when the AV valves are open to allow blood flow into the ventricles. Lastly, choice d) mentions the ventricles distributing, which is a vague term and not a recognized phase related to the function of the AV valves.