Pulmonary Surfactant in Health and Disease

¹ Cardiovascular Research Institute, University of California School of Medicine

The airspaces of the lung may be compared to several hundred million bubbles blown on airways, and the surface tension necessary to prevent alveolar emptying can be estimated. If the alveolar surface were made of a transudate of plasma, we would expect the alveoli to be empty at the end of expiration. Instead, current evidence indicates that the surface tension of the lung varies widely and alveoli do not collapse because the surface tension can be very low. However, if the surface tension is high due to an abnormal surface, or if the distending pressure is maintained low, we would expect focal atelectasis. Although a very low surface tension must be necessary in the lung at low distending pressures, the lung extracts and apparently the lung, cannot maintain a low surface tension for a prolonged period. Occasional increase of the surface area, as normally occurs with a deep breath. may replenish the surface, but during shallow breathing with a low distending pressure, the lung surface area decreases with eventual atelectasis. This atelectasis is not due to airway collapse and does not need to follow the distribution of a bronchus.