ABSTRACT
The epithelial cells of the respiratory system are composed of numerous
types, which vary as a function of location. Each cell type provides for spe-
cific needs required at each site along the upper and lower airways and
alveoli. The primary function of epithelial cells is to provide an appropriate interface with the environment, which will allow a number of processes to
be completed. One of these is to condition the air through warming and
humidification during transport through the nasal cavity and tracheobron-
chial tree. In these same regions, a combination of epithelial cells work in
concert to provide a protective lining layer (mucous) and movement of this
layer along the airways (ciliary beating). The epithelium beyond the con-
ducting airways serves to facilitate the rapid passage and exchange of gases
from alveolar air spaces to the blood by forming a portion of the air-toblood tissue barrier. Specific epithelial cells of the distal lung also secrete
a complex of phospholipids, proteins, and carbohydrates (known as surfac-
tant) to form a liquid lining layer over the alveolar surfaces to create a
low surface tension at the air-tissue interface. Surfactant maintains open
alveolar air spaces during ventilation and the appropriate conditions to pre-
vent desiccation of delicate epithelial cells. Both surfactant functions
derived from the epithelium are critical in facilitating gas exchange. In
essence, the role of the epithelium is to provide a highly effective barrier
while also maintaining conditions that will not subject the respiratory tract to damage due to its interaction with the external environment.
