Red and blue balls represent oxygenated and oxygen-depleted, or deoxygenated, blood, respectively. By following the movement of these balls, you get a picture of how blood flows from major blood vessels into the left and right atria and then through the atrioventricular valves into the ventricles. From there, the blood is pushed through the semilunar valves into major blood vessels leading out of the heart. To take a closer look, click on the "Right heart with blood" link.