The respiratory epithelium is the lining of the airway, extending from the nasal cavity through the branching respiratory tree to the terminal air sacs of the lungs. Together, the respiratory epithelium serves as a leak-proof barrier that transports, filters and conditions the air we breathe. Different types of respiratory epithelial cells line the airways, each uniquely suited to its location and function.
Stratified Squamous Epithelial Cells
The respiratory epithelium is dominated by stratified squamous epithelial cells. The epithelium is several layers thick, offering a fortified barrier of protection. Squamous cells are cuboidal at the base of the epithelium and become rounded and flattened as they reach the surface. This type of epithelium is well suited to withstand abrasion and the harsh conditions of the nasal passages and the throat.
Ciliated Columnar and Cuboidal Epithelial Cells
Moving down the airway into the trachea and the large branches of the airways of the lungs (the bronchi), the respiratory epithelium is predominantly comprised of ciliated columnar epithelial cells. These tall slender cells are anchored on the base of the epithelium, which is one layer thick. As the airways branch and get smaller, the respiratory epithelial cells get shorter. Thus, as the air sacs of the lungs are approached, the respiratory epithelium is formed primarily of cuboidal epithelial cells.
The surfaces of most columnar and cuboidal respiratory epithelial cells are covered with short, hair-like appendages called cilia. Remarkably, the cilia on all respiratory epithelial cells beat in coordinated rhythmic waves toward the outside. Ciliary waves keep the airways clean by sweeping captured dust, particles and chemicals up and out of the respiratory tract.
Seromucous Glands and Goblet Cells
The epithelium of the upper airway and all but the terminal branches of the airways in the lungs are interspersed with seromucous glands and goblet cells. These cells manufacture and release mucin, a slippery protein-rich substance that coats the epithelial surface. Mucin protects the respiratory epithelium from drying, adds moisture to inhaled air, and captures particles and chemicals, which are then swept out by ciliary waves.
The airways end with structures called pulmonary alveoli or air sacs. This is where the work of the lungs—delivering oxygen to the bloodstream and releasing carbon dioxide—takes place. The respiratory epithelium in the alveoli consists of type I and type II alveolar cells. Type I alveolar cells are extremely thin, flat cells that conduct the exchange of gases occurring with every breath. Type II alveolar cells produce and secrete surfactant, which coats the surface of the alveoli. Surfactant allows the alveoli to expand without bursting during inhalation and keeps the walls of the alveoli from sticking together during exhalation.
Damage and Renewal of the Respiratory Epithelium
The cells of the respiratory epithelium are regularly replaced with new cells. These cells can be damaged by pollutants including cigarette smoke and ozone. Repetitive damage to the respiratory epithelium is a major contributing factor to the development of lung cancer.