The pulmonary veins are blood vessels that carry oxygen-rich blood from the lungs to the heart. There are four main pulmonary veins, two from each lung, which enter the left atrium of the heart. This process is essential for delivering oxygen to the body after it has been absorbed in the lungs. Unlike most veins, which carry deoxygenated blood, the pulmonary veins are unique because they transport oxygenated blood. This oxygen-rich blood is then pumped from the left atrium into the left ventricle, where it is sent out to the rest of the body through the aorta.