Comparison between the vascular beds of upper and lower airways.

The nasal and tracheobronchial circulations are both innervated by parasympathetic cholinergic, sympathetic adrenergic, and sensory neurones. All three systems contain neuropeptides. Structurally the systems are similar, with subepithelial networks and deeper networks of sinuses or sinusoids. The nose in addition has arteriovenous anastomoses, not described for the tracheobronchial tree. Parasympathetic nerves release acetylcholine and vasoactive intestinal polypeptide, both dilators; sympathetic nerves release noradrenaline and neuropeptide Y, both constrictors; and the sensory nerves release substance P, neurokinin A and calcitonin gene-related peptide, all dilators. The last are important in axon reflexes. Many inflammatory mediators also act on both vascular beds in response to tissue inflammation or damage. Both vascular beds are under reflex control, probably acting mainly via the sympathetic system. In particular, peripheral chemoreceptors cause vasoconstrictions in the nose and trachea, and pulmonary C-fibre receptors and cardiac receptors cause vasodilations in the nose and lower airways. Reflexes from carotid sinus baroreceptors and slowly adapting pulmonary stretch receptors seem to be inactive on the nasal and tracheal vascular beds.