Anatomy of Pulmonary and Bronchial Circulation
- The pulmonary trunk arises from the right ventricle
- This then branches into the left and right pulmonary arteries - these pass posterolaterally to the main bronchi and follow them into the lungs.
- The pulmonary arteries give off multiple branches, generally following the bronchi so that bronchopulmonary segments have their own artery and bronchus without anastomosis.
- Pulmonary capillaries line the walls of alveoli. They form a mesh in which the holes are smaller than the vessels themselves. The capillaries have very thin walls which are fused to the basement membrane of the alveolar epithelium.
- Pulmonary veins drain oxygenated blood from the pulmonary capillaries. They are generally at the periphery of bronchopulmonary segments and drain adjacent segments.
- The large veins accompany the bronchi and arteries and drain as two veins from each lung into the left atrium. The upper vein drains the upper lobe on each side and the lower vein the lower lobe. On the right, the middle lobe is drained by the upper vein.
- The histology of the pulmonary arteries is different from systemic arteries of similar size. There is little smooth muscle tissue and a large amount of elastin in the artery walls. The walls overall are thin compared to the diameter of the vessels. This is consistent with the low pressures of the pulmonary circulation. The pulmonary veins are very thin-walled.
- The pulmonary capillaries are lined with endothelial cells which share their basement membrane with that of the type I pneumocytes lining the alveolar air space.
- The bronchial arteries arise from the thoracic aorta or from the upper intercostal arteries. There may be one or more on each side. Occasionally additional bronchial vessels arise from the descending aorta and travel in the pleural ligament.
- These follow the bronchi, forming a capillary plexus around the large bronchi, supplying the bronchial muscle coat and forming a second plexus in the mucosa. These plexi extend as far as the repiratory bronchioles where they anastomose with the pulmonary vessels.
- The deep bronchial veins drain the bronchi within the lung and join the pulmonary veins. The superficial bronchial veins drain the bronchi near the hilum outside the pleura and join the azygous on the right and accessory hemiazygous or intercostal vein on the left.
- The histology of the bronchial arteries is the same as that of other systemic arteries.