Editor-In-Chief: C. Michael Gibson, M.S., M.D. 
Respiratory epithelium is a type of epithelium found lining the upper and lower respiratory tracts, where it serves to moisten and protect the airways. It also functions as a barrier to potential pathogens and foreign objects, preventing infection by action of the ciliary escalator.
Respiratory epithelium is more specifically known as ciliated pseudostratified columnar epithelium due to the arrangement of the columnar epithelial cells; the nuclei are not aligned in the same plane and make it appear as though several layers of cells are present. In actuality, all cells make contact with the basement membrane and are therefore a single layer of cells, hence the epithelium is called pseudostratified. Goblet cells are present amongst the columnar cells and secrete a mucus which keeps the epithelium moist and traps particulate material moving through the airway.
Certain parts of the respiratory tract, such as the oropharynx, are also subject to the abrasive swallowing of food. To prevent the destruction of the respiratory epithelium in these areas it changes to stratified squamous epithelium which is better suited to the constant sloughing and abrasion. The squamous layer of the oropharynx is continuous with the esophagus.
The cilia of the respiratory epithelium beat in a concerted effort to move secreted mucus containing trapped foreign particles towards the oropharynx for either expectoration or swallowing to the stomach where the acidic pH helps to neutralize foreign material and micro-organisms. This system is collecively known as the ciliary escalator and serves two functions: to keep the lower respiratory tract sterile, and to prevent mucus accumulation in the lungs from drowning the organism.
- Histology image: 13903loa – Histology Learning System at Boston University