The bronchial tree (arbor bronchialis) () begins at the bifurcation of the trachea by the formation of a right and a left principal bronchus (bronchus principals [dexter et sinister]). Each principal bronchus divides into lobar bronchi (bronchi lobares), formerly secondary bronchi, which is the basis for the identification of the lung lobes. These supply the various lobes of the lung and are named according to the lobe supplied. Within the lobe of the lung the lobar bronchi divide into segmental bronchi (bronchi segmentales), which are sometimes referred to as tertiary bronchi. The segmental bronchi and the lung tissue that they ventilate are known as bronchopulmonary segments (segmenta bronchopulmonalia). Ishaq (1980) studied 37 pairs of dog lungs and suggested a system for designating the bronchi. Schlesinger and McFadden (1981) discuss the morphometry of the proximal bronchial tree in six mammalian species. Adjacent bronchopulmonary segments normally communicate with each other in the dog. Various injection and reconstruction techniques have been employed to delineate these segments in the dog (). For bronchoscopic purposes Amis and McKiernan (1986) described a system of letters and numbers to identify lobar, segmental, and subsegmental bronchi on the basis of their origination and anatomic orientation. The segmental bronchi arise from the dorsal and ventral surfaces of the lobar bronchi of all the lung lobes except the right middle lobe where they arise from the cranial and caudal surfaces ().
DeLorenzi et al. (2009) endoscopically studied abnormalities of the bronchial pathways in 40 brachiocephalic dogs with stertorous breathing and clinical signs of respiratory distress. Included were 20 Pugs, 13 English Bulldogs, and 7 French Bulldogs. Bronchial collapse was a common fnding associated with laryngeal collapse. The left cranial bronchus was the most commonly affected.
The segmental bronchi usually branch dichotomously () into small bronchi. This process of branching continues until the respiratory bronchioles are formed. The bronchi are cylindrical tubes that are kept by fattened, overlapping, curved cartilages. The cartilaginous elements end when the diameter of the terminal bronchiole is reduced to 1 mm or less. In addition to the cartilages, the bronchioles have spiral bands of smooth muscle in their walls that continue peripherally on the respiratory (alveolar) bronchioles.
The respiratory bronchioles (bronchioli respiratorii) give rise to alveolar ducts (ductuli alveolares), alveolar sacs (sacculi alveolares), and pulmonary alveoli (alveoli pulmonis). The respiratory portion of the bronchial tree in the dog, including the arteries, veins, and lymphatics, was modeled and described by Miller (1900, 1937). Boyden and Tompsett (1961), in their excellent paper on the postnatal growth of the lung in the dog, point out the substantial reduction postnatally in the number of nonrespiratory branches of both the axial bronchus and the peripheral bronchioles. At birth, many nonrespiratory peripheral bronchioles lined by cuboidal epithelium are converted into respiratory units. Concomitantly, new alveoli and alveolar sacs arise along the terminal bronchioles. Boyden and Tompsett conclude that the number of nonrespiratory branches on the axial stem (dorsocaudal bronchus of the accessory lobe) is reduced from 38 rami before birth to 32 at maturity.
Loosli (1937) studied the microscopic structure of adult alveoli in several species of mammals, including the dog. He described and illustrated interalveolar communications in both normal and pathologic lungs and pointed out the signifcance of these pores in the normal mechanism of respiration and in the spread of infection.