The lungs receive two distinct sources of blood supply: pulmonary arterial supply and bronchial arterial supply. The pulmonary arterial circulation originates from the right side of the heart. Histologically, pulmonary arteries and veins are often adjacent to bronchi and larger bronchioles. These vessels progressively divide into smaller caliber vessels and, terminally, form a network of capillaries that course through alveolar septa and function in gas exchange. These capillaries then form into venules, then veins that progressively become larger in size and exit the lung, entering the left atrium. Bronchial arteries originate from the thoracic aorta and also course along large airways, providing much of the blood supply to these structures, as well as the pleura.