Conducting Airways
The nasal cavity is divided longitudinally by a cartilaginous
nasal septum. Two turbinate bones divide each of
the two halves of the nasal cavity into three meatuses:
dorsal, middle, and ventral (Figure 21.1). The length of
the nasal cavity varies between different breeds.
The trachea is relatively short and divides posteriorly
into two primary bronchi, one for the left lung and one
for the right lung. A separate, small bronchus branches
from the trachea leading to the right cranial (apical)
lobe. Bronchi branch into the left cranial (apical) lobe,
left and right middle (cardiac) lobes, left and right
caudal (diaphragmatic) lobes, and the right accessory
(intermediate) lobes as outlined in Figure 21.2. The
finest branches of the tubular system are the bronchioles,
each dividing into alveolar ducts and alveoli.
The vestibular region of the nasal cavity is lined
with stratified squamous epithelium. The rest of the
nasal cavity is lined by ciliated pseudostratified epithelium
with mucus-producing goblet cells. This ciliated
epithelium is covered by a bilayered mucus coating
produced by the goblet cells and mucosal glands, and
this epithelium continues through the pharynx, larynx,
trachea, and bronchi. As the bronchioles approach
the alveoli, the epithelium is reduced in height, becoming
more squamous. Alveoli are lined by very flat,
single-layered epithelial cells (type I pneumocytes)
and by a small percentage of cuboidal epithelial cells
(type II pneumocytes). Type II pneumocytes produce