The left atrium is the receiving chamber for the oxygenated blood brought to it by the four pulmonary veins (two from each lung). The blood leaves the left atrium to fill the left ventricle which pumps the blood into the aorta.
The aorta is the main artery of the systemic circulation. From it, blood is contributed to the other arteries, arterioles and capillaries to different organs of the body. Blood is collected from the organs and poured into the venae cavae which bring blood back to the right atrium. This route constitutes the systemic circulation.
