Blood flows in both directions through the heart. Blood returning from the body enters the right side of the heart and exits on the left side. Blood returning from the lungs enters the heart on the left side and exits out of the top of the heart via the aorta.
Blood also flows both up and down through the heart. Blood entering the heart moves into the right or left atrium, then descends into the right or left ventricle before leaving the heart through the top to either go to the lungs or to the rest of the body.
The right side of the heart pumps blood to the lungs to pick up oxygen. The left side of the heart returns the oxygenated blood to the rest of the body. The top of the heart (the atria) receives blood, and the bottom of the heart (ventricles) pumps blood out.