The bifurcation of the trachea in the mediastinum gives rise to the right and left main (principal) bronchi (Fig. 2.26).
The right main bronchus is wider and more steeply inclined than the left (Fig. 2.27). As a consequence, inhaled foreign bodies are more commonly found in the right main bronchus. The main bronchi give rise to lobar (secondary) bronchi, which are confined to their respective lobes. On the right, the upper lobe bronchus arises outside the hilum in the lung root, whereas on the left, the lobar bronchi arise entirely within the lung. In each lobe, further subdivision occurs into segmental (tertiary) bronchi, which are constant in position and supply specific portions of lung called bronchopulmonary segments. Each lobe consists of a definite number of these segments. Within individual segments, the bronchi further subdivide into bronchioles, then respiratory bronchioles, which in turn lead into the alveolar ducts and alveoli. Bronchial arteries derived from the descending thoracic aorta accompany and supply the major bronchi. Venous return from the bronchi is through bronchial veins that terminate in the azygos venous system (p. 63).
The right and left pulmonary arteries divide into branches that correspond to and accompany the subdivisions of the bronchi within the lungs. The bronchi and pulmonary arteries lie centrally in the bronchopulmonary segments. The arteries ultimately give rise to pulmonary capillaries in the alveolar walls. Oxygenated blood drains from these capillaries into tributaries of the pulmonary veins that occupy intersegmental positions. These vessels empty into two pulmonary veins, which usually emerge separately through each hilum (Figs 2.24 & 2.25) and drain into the left atrium.
The pulmonary plexus, most of which lies behind the lung root, contains both sympathetic and parasympathetic fibres, which accompany the bronchi into the lung. Sympathetic nerves originate in the upper thoracic ganglia of the sympathetic trunk and supply smooth muscle in the walls of the bronchi and pulmonary blood vessels. The parasympathetic fibres are derived from the vagus nerves and supply bronchial smooth muscle and mucous glands.