Clearance and recycling of pulmonary surfactant.

In a steady state the rate of secretion of pulmonary surfactant lipids and proteins into the alveolar airspace must be balanced by the rate of removal. Several potential pathways for clearance have been identified including uptake by alveolar type II cells, which also synthesize and secrete surfactant components, uptake by other epithelial cells, and internalization by alveolar macrophages. A small amount of surfactant moves up the airways and through the epithelium-endothelium barrier into the blood. Some of the surfactant lipids and proteins that are cleared from the alveolar airspace appear to be "recycled" in that they appear in the lamellar body, a surfactant secretory granule found in the type II cell. Some surfactant lipids are degraded, probably intracellularly, and the degradation products are reutilized to synthesize new lipids. Several factors have been shown to affect internalization by the type II cell and/or alveolar clearance including the surfactant proteins, lipids, and known stimuli of surfactant secretion. Surfactant proteins may be involved in regulating pool size by modulating both secretion rates and uptake rates, possibly by a receptor-mediated process, although such receptors have not yet been identified or isolated. Clearance of surfactant lipids from the alveolar airspace is more rapid than clearance from the whole lung, and these two processes may be regulated by different factors. Elucidation of the factors that fine tune the balance between synthesis, secretion, and clearance of the lipid and protein components of surfactant awaits further investigation.