The pulmonary veins are part of the pulmonary circulation. They are vessels that carry oxygen-rich (arterial) blood from the lungs to the heart. There are four pulmonary veins, two that drain blood from the right lung and other two that drain the left lung. The pulmonary veins debouch into the left atrium bringing arterial blood to the heart. Although they are veins they carry arterial blood and not venous blood.