Physiologic control. Anatomy and physiology of the airway circulation.

Both for the nose and the lower airways there is an extensive subepithelial capillary network. That for the nose is fenestrated, and this is true for the tracheobronchial tree of rats, guinea pigs, and hamsters, and for that of human asthmatics. However, healthy humans, dogs, and sheep have capillaries without fenestrations except for those close to neuroepithelial bodies and submucosal glands. Deeper in the mucosa there is a capacitance system of vessels, conspicuous in the nose but present also in the lower airways of rabbits and sheep and, to a lesser extent, in those of dogs and humans. Both for the nose and the lower airways, parasympathetic nerves are vasodilator, sympathetic nerves are vasoconstrictor, and sensory nerves are able to release dilator neuropeptides. Most inflammatory and immunologic mediators are vasodilator. A conspicuous difference between the nasal and lower airway vasculatures is the presence of arteriovenous anastomoses only in the former. Countercurrent mechanisms also exist in the nose to increase its efficiency in air conditioning, but they have not been established for the trachea. The pulmonary vasculature could be part of such a system for the bronchi. Distension of the airway vasculature thickens the mucosa, probably both by vascular distension and by edema formation. The latter can lead to exudation into the airway lumen. These processes have not been well quantitated, and the balance sheet of capillary and capacitance vessel volumes, interstitial liquid volume, and exudate volume needs to be worked out in physiologic and pathologic conditions.