Surfactant and its Role in the Upper Respiratory System and Eustachian Tube

Surfactant research was originally directed toward lung mechanics, however, with growing information on the biology of the surfactant system it has expanded beyond the borders of basic physiology. The research has become interdisciplinary, not only considering aspects of lungs biology relevant for breathing, but also those aspects relevant for airway defence.

Surfactant consists mainly of phospholipids that lower the alveolar surface tension to prevent lung collapse at expiration. They also support mechanical elimination of inhaled pathogens by reducing the viscosity of airway mucus. Approximately 8-10% of surfactant is made up of proteins. Among them, specific proteins SP-A and SP-D play a crucial role in the innate defence system. They belong to collectins family and serve as the first step in immune response to inhaled pathogens. In limited extent, SP-B and SP-C are also involved in immunomodulation.

Although numerous studies have focused on the physiological function of surfactant in the lower airways, relatively little is known about its role in the upper respiratory system. Identification of lamellar bodies in ciliated epithelium of the upper airways indicates that surfactant may have a role in normal sinonasal function and pathology. Decreased levels of the main component of surfactant, phospholipids, have been implicated in atrophic rhinitis and altered levels of surfactant proteins have been observed in a number of respiratory tract diseases. The pattern of inflammation in the upper respiratory tract generally appears to parallel that in the lower airways and nowadays upper respiratory disease and lower airway disease are considered as two manifestations of one pathological process. Therefore, surfactant proteins may play a significant role in the upper respiratory tract diseases.

In addition, surfactant has been identified in the Eustachian tube where it helps to lower the opening pressure between nasopharynx and middle ear. The alterations in surfactant levels may adversely affect Eustachian tube function and contribute to chronic ear infection.

The review summarizes the current knowledge on the presence and the role of surfactant in the upper respiratory system and Eustachian tube.