With each heartbeat there are two phases in the passage of oxygenated blood and deoxygenated blood through the heart. In the phase of relaxation, or diastole, the heart fills with blood. As the ventricles relax, valves in the aorta and in the pulmonary artery close with a dup sound and blood pours into the two atria from the venae cavae and the pulmonary veins. The mitral and tricuspid valves between the atria and ventricles open, allowing blood into the ventricles. The heart then stops filling with blood. In the phase of contraction, or systole, the heart empties and a lub sound is made by the closing of the mitral and tricuspid valves. This prevents a backflow of blood into the atria as it is pumped from the ventricles into the pulmonary artery and the aorta. Each heartbeat lasts up to eight-tenths of a second, with systole lasting for four-tenths of a second and diastole occupying the remaining time.