The Bronchial Circulation Provides a Blood Supply to Airways, Large Vessels, and in Some Species the Visceral Pleura
The bronchial circulation, which receives approximately 2% of the output of the left ventricle, originates from two sources: the Iironchoesophageal artery and a branch of the bicarotid trunk, the right apical bronchial artery.
The former supplies the airways and the interlobular septa of most of the lung; the latter supplies the airways of the right apical lobe. Bronchial arteries follow the tracheobronchial tree to the terminal bronchioles and form a peribronchial plexus in the connective tissue along the length of the airways. Branches from this plexus penetrate the smooth muscle layer of the bronchial wall and form a Subepithelial vascular plexus that serves to warm inhaled air. Branches are also given off to form the vasa vasorum (nutrient blood vessels) of pulmonary vessels. At the level of the terminal bronchiole, bronchial vessels anastomose with the pulmonary circulation. There are few anastomoses between bronchial and pulmonary arteries; most anastomoses are present at the capillary or venular level.The extensiveness of the bronchial blood supply to the pleura varies among species. In cattle, sheep, pigs, and horses, the bronchial artery provides blood flow to the visceral pleura; in dogs, cats, and monkeys, it does not. The bronchial blood flow to the large extrapulmonary airways drains into the azygos vein; venous drainage of the intrapulmonary bronchial circulation enters the pulmonary circulation.
Although the bronchial circulation provides nutrient blood flow to many lung structures, the king does not die if the bronchial circulation is obstructed. The extensive anastomoses between bronchial and pulmonary vessels provide pulmonary blood flow to bronchial vessels. Similarly, when the pulmonary circulation is obstructed, the bronchial circulation proliferates and maintains blood flow to the lung. The bronchial circulation also proliferates when the airways are inflamed.
Inflow pressure to the bronchial circulation is systemic arterial pressure, but outflow pressure varies, depending on whether venous drainage is through the azygos vein or pulmonary circulation. Changes in pressure in both the systemic and the pulmonary vascular bed affect the magnitude of bronchial blood flow. Increasing systemic pressure increases flow, but increasing pulmonary vascular pressures (downstream pressure) reduces and may even reverse flow. Under hypoxic conditions, bronchial arteries dilate; in contrast, pulmonary arteries constrict under these conditions.