THE ANATOMY AND PHYSIOLOGY OF THE BRONCHIAL CIRCULATION

The origin and distribution of the bronchial vasculature vary consider ably between and among species both at the macro- and microvascular le vel. Bronchial vessels usually originate from the aorta or intercostal arteries, entering the lung at the hilum, branching at the mainstem b ronchus to supply the lower trachea, extrapulmonary airways, and suppo rting structures; this fraction of the bronchial vasculature drains in to the right heart via systemic veins. Bronchial vessels also supply t he intrapulmonary airways as far as the level of the terminal bronchio les where they form extensive anastomoses with the pulmonary vasculatu re; this systemic-to-pulmonary blood drains via pulmonary veins to the left heart. Repeated arborization of the bronchial artery along the l ength of the tracheal bronchial tree results in a vast increase in the total surface area of the vascular bed. The tracheal bronchial vascul ature consists of a continuous dense network of subepithelial capillar ies that converge to form venules extending to a deeper plexus of larg er venules and arterioles on the adventitial side of the smooth muscle . Innervation is under the control of vasodilatory parasympathetic ner ves that release acetylcholine and vasoactive intestinal polypeptide; vasoconstrictor sympathetic nerves that release norepinephrine and neu ropeptide 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 pressu re also influence the distribution of blood flow through the tracheal bronchial vasculature.