Figure 2: Airway vascular tone and blood flow regulate airway smooth muscle reactivity. The airway vasculature is densely innervated by sympathetic (tyrosine hydroxylase expressing) neurons which provide a basal level of adrenergic vascular tone. Soluble and insoluble particles in the airway wall are actively cleared by the submucosal vasculature. Increased blood flow is associated with increased clearance, and this can significantly modify airway smooth muscle reactivity to bronchoactive agents which are deposited onto, or generated within, the airway wall. See text for additional details.