From Which Chamber of the Heart Does Blood Enter the Lungs?

The blood leaves the right ventricle on its way to the lungs. Contraction of the right ventricle pushes deoxygenated blood into the pulmonary artery, which takes the blood to the lungs for oxygenation. Pulmonary veins then bring the blood back to the heart's left atrium, explains How Stuff Works.

The left atrium contracts and forces blood through the mitral valve into the left ventricle. From the left ventricle, the blood enters the aortic valve and then the aorta to be carried throughout the body, delivering oxygen to tissues. Once the oxygen is delivered, blood flows back toward the heart via veins. Blood from the upper part of the body enters the superior vena cava, and blood from the lower part enters the inferior vena cava, both of which transport deoxygenated blood to the right atrium. Contraction of the right atrium sends the blood through the tricuspid valve back into the right ventricle.