From the superior and inferior
vena cava blood enters the heart in the left atrium, The superior vena cava is a
vein specifically designed to collect the blood returning from the upper body, and the
inferior vena cava returns blood from the lower body. The deoxygenated blood of
these two veins enters the heart in the right atrium during the systole part of the
heartbeat. In this phase of the heartbeat the atria are filling, and the ventricles
are contracting in order to pump the blood. The pulmonary and aortic valves open to
allow the blood to leave the ventricles. During the diastole part of the heartbeat,
the atria contract to pump the blood into the ventricles. To allow the blood to
leave the atria and enter the ventricles, the tricuspid and mitral valves of the heart
open to allow the blood to pass through. Both of these phases of the heartbeat take
a combined eight tenths of a second.
