There are essentially two blood supplies for the lungs--nutrient blood and functional blood. Nutrient blood is carried by the bronchial arteries from the thoracic aorta. Nutrient blood provides nourishment and oxygen to the tissues of the lung.
Functional blood is actually involved in the respiratory exchange of gases between the alveoli and the capillaries. Functional blood is brought to and from the lungs by the pulmonary cycle of the cardiovascular system.
The pulmonary cycle originates in the right ventricle of the heart. Contraction of the right ventricle forces the blood into the pulmonary arch, which divides into the right and left pulmonary arteries to their respective lungs.
Paralleling the branching of the respiratory tree, the arteries divide and subdivide within the lungs. These arteries lead to capillaries in the vicinity of the alveoli. The walls of these capillaries are thin enough to accommodate the passage of gases to and from the alveolus.
The blood, now saturated with oxygen, is collected by the pulmonary venous system. The blood is deposited ultimately into the left atrium of the heart.