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