Explain why the atrioventricular valves open and close during the cardiac cycle.

Pressure is related to volume, and a larger volume means a lower pressure. The pressure in the atria is low initially as it is relaxed (diastole). Blood flows from high to low pressure, and this means that the atria fill with blood. The atria contract (systole) so the pressure in the atria starts to rise, as the volume decreases. The pressure is higher in the atria compared to the ventricles and there is a pressure gradient across the atrioventricular valves. These valves therefore open. Blood enters the ventricles. The atria start to relax again and pressure in the atria decreases. Now the ventricles start to contract, and their volume decreases. The pressure gradient across the atrioventricular valves is reversed and the valves close.