The human lung is supplied by two arterial systems referred to as pulmonary and bronchial, each originating from a different side of the heart. Blood from the lungs is drained by two venous systems, pulmonary and true bronchial. The pulmonary veins drain oxygenated blood from the regions supplied by the pulmonary artery and deoxygenated blood from the airways within the lung that are supplied by the bronchial artery. The true bronchial veins serve only the perihilar region, supplied mainly by the bronchial artery, and this blood drains to the azygous system and right atrium.
The bronchial arteries arise from the aorta and supply the capillary plexus of the airway walls from the hilum to the respiratory bronchiole.
The pulmonary artery branches run with airways and their accompanying bronchial arteries in a single connective tissue sheath referred to as the bronchoarterial or bronchovascular bundle. The pulmonary artery transforms into a capillary bed only when it reaches the alveoli of the respiratory bronchiole. It supplies all capillaries in the alveolar walls that constitute the respiratory surface of the lung.
All intrapulmonary blood drains to the pulmonary veins. The veins lie at the periphery of any unit acinus, lobule, or segment. Veins receive tributaries from the alveolar capillary network, the pleura, and the airways.
Pulmonary and bronchial arteries, and hence the right and left sides of the heart, communicate through the capillary bed in the region of the respiratory bronchiole and through the intrapulmonary venous bed. Pulmonary-to-bronchial artery anastomoses are present in the walls of the larger airways but normally are closed. They open if blood ﬂow is interrupted in either system and in certain disease states such as pulmonary arteriovenous malformation.