Human heart. Illustration of a partly-dissected heart showing the internal structure of its left side, with arteries and veins that service it. As a pump the heart circulates blood around the body. The left side receives oxygenated blood from the lungs via pulmonary veins (short, red). Blood enters heart chambers on this side: atrium first, then a thick-walled ventricle, separated by white valve to prevent backflow. This blood is pumped to the body through the red ascending aorta. Backflow is prevented by another valve (white). Vena cava veins (blue) collect used body blood before it too is pumped through the heart, the right side, to the lungs via pulmonary arteries (light purple).