Vascular beds in the airways of normal subjects and asthmatics

In humans a rich network of capillaries runs subepithelially from the central airways to the level of the peripheral bronchioles, where it anastomoses with the alveolar capillaries. The tracheobronchial capillaries converge to form venules extending to a deeper plexus of larger venules and arterioles. Transmission electronmicroscopy shows capillaries very close to the epithelial basement membrane, but not in the epithelium itself. There are two main differences in the bronchial mucosal vascular bed between healthy controls and asthmatics. In 60% of asthmatics there are endothelial gaps in the bronchial mucosal microvasculature compared to none in controls. Half of asthmatics have eosinophils adhered in the bronchial mucosal venules and capillaries, but none are seen in controls. Neutrophils are even more common than eosinophils in the bronchial mucosal capillaries and venules, but they are also found in the venules of one control out of four. The finding of inflammatory cells in the bronchial vessels suggests possible chemotaxis in asthmatics.