Figure 1.

Composition of the respiratory epithelium in health. The human respiratory tract is divided into the upper respiratory tract including the nasal cavity, pharynx, larynx, and the lower respiratory tract that comprises the conducting airways (trachea and bronchi), the small airways (bronchioles) and the respiratory zone (the alveoli). The trachea and large airways are composed of ciliated cells, mucus‐secreting goblet cells, neuroendocrine cells and undifferentiated basal cells. Stromal cells including fibroblasts provide extracellular matrix that modulate airway epithelial cell turnover. The alveoli are comprised of alveolar macrophages and alveolar type I and type II cells.