Blood from the body enters the right atrium of the heart via the vena cava. It then travels into the right ventricle by a combination of gravity and contraction of the right atrium. After the right atrium has contracted, the right ventricle will contract to push blood out of the heart into the pulmonary arteries.Blood is then oxygenated in the lungs and returns to the heart via pulmonary veins which drain into the left atrium. This again contracts to push blood into the left ventricle, which subsequently contracts. Blood is pushed out into the aorta and supplies the body with oxygen.