Neural regulation of bronchial blood flow.

The neural regulation of the bronchial vasculature differs from that of the general systemic circulation in that vasodilator reflexes play a major part in determining blood flow. These reflexes originate in the upper or lower airways, in carotid chemoreptors or in cardiac chemosensitive nerves; those arising in the lower airways are most potent and may increase bronchial blood flow several-fold and cause swelling of the airway mucosa. Lower airway reflexes have afferent and efferent pathways in the vagus nerves, the former including sensory C-fibers and rapidly adapting receptors, the latter involving both cholinergic and non-cholinergic transmitters. In addition, neuropeptides released from the C-fiber terminals provide a local mechanism for vasodilation independent of central reflex control. This so-called axon-reflex plays the major part in bronchial vasodilation in rodents but makes only a small contribution in larger animals. In larger animals centrally-mediated reflexes and vagal vasodilator pathways appear more important. Nevertheless, local neural vasodilation may be important in airway disease; the factors that favor its operation in animals other than rodents deserve to be explored.