Human hearts are not symmetrical with the right auricle being a little larger than the left, but having thinner walls. The right auricle holds about two ounces of blood and is subdivided into the atrium (or sinus venosus) and anterior, a smaller appendix auriculae, resembling a dog's ear in shape. Oxygen-depleted blood is carried by the superior vena cava from the upper half of the body and the inferior vena cava from the lower half, to the right auricle. Blood next enters the right ventricle and is then pumped via the pulmonary artery under pressure to the lungs for releasing carbon dioxide and receiving fresh oxygen. Fresh blood returns to the heart via the four pulmonary veins to the left auricle where it is then pumped to the longer left ventricle. Blood leaves the heart via the aorta that has a valve for use throughout the body.