The anatomy and physiology of the bronchial circulation.

The origin and distribution of the bronchial vasculature vary considerably between and among species both at the macro- and microvascular level. Bronchial vessels usually originate from the aorta or intercostal arteries, entering the lung at the hilum, branching at the mainstem bronchus to supply the lower trachea, extrapulmonary airways, and supporting structures; this fraction of the bronchial vasculature drains into the right heart via systemic veins. Bronchial vessels also supply the intrapulmonary airways as far as the level of the terminal bronchioles where they form extensive anastomoses with the pulmonary vasculature; this systemic-to-pulmonary blood drains via pulmonary veins to the left heart. Repeated arborization of the bronchial artery along the length of the tracheal bronchial tree results in a vast increase in the total surface area of the vascular bed. The tracheal bronchial vasculature consists of a continuous dense network of subepithelial capillaries that converge to form venules extending to a deeper plexus of larger venules and arterioles on the adventitial side of the smooth muscle. Innervation is under the control of vasodilatory parasympathetic nerves that release acetylcholine and vasoactive intestinal polypeptide; vasoconstrictor sympathetic nerves that release norepinephrine and neuropeptide Y; and sensory nerves that release substance P, neurokinin A, and calcitonin gene-related peptide, all of which are vasodilators. Mechanical factors such as the downstream pressure and alveolar pressure also influence the distribution of blood flow through the tracheal bronchial vasculature.