Blood flow through the heart is controlled by the ventricles. The capillaries open only to the forward flow of blood. Never open to backflow of blood. If the blood pressure in the atrium is high and the blood pressure in the ventricle is low, the bicuspid and tricuspid valves open. As a result, blood enters the ventricles from the atrium. When the ventricles contract with blood, the semilunar valves open and the bicuspid and tricuspid valves close. As a result blood enters the pulmonary artery and aorta. But cannot return to the atrium. Then, when the ventricular pressure decreases, the blood wants to flow back into the ventricle, but because the semilunar valve is closed, backflow does not occur. Blood flow is unidirectional as all capillaries are always open in one direction.