The lungs receive blood via two sets of arteries : pulmonary arteries and bronchial arteries. Deoxygenated blood passed through the pulmonary trunk, which divides into a left pulmonary artery that enters the left lung and a right pulmonary artery that enters the right lung. (The pulmonary arteries are the only arteries in the body that carry deoxygenated blood). Return of the oxygenated blood to the heart occurs by way of the four pulmonary veins, which drain into the left atrium.
A unique feature of pulmonary blood vesels is their constriction in response to localized hypoxia (low O2 level). In all other body tissues, hypoxia diverts pulmonary blood from poorly ventilated areas of the lungs to well ventilated regions. This phenomenon is known as ventilation perfusion coupling because the perfusion (blood flow) to each area of the lungs matches the extent of ventilation (airflow) to alveoli in that area.
Bronchial arteries, which branch from the aorta, deliver oxygenated blood to the lungs. This blood mainly perfuses the walls of the bronchi and bronchioles. Connections exist between branches of the bronchial arteries and branches of the pulmonary arteries, however, and most blood returns to the heart via pulmonary veins. Some blood, however, drains into bronchial veins, branches of the azygos system, and returns to the heart via the superior vena cava.