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From Birds to Humans

New Concepts on Airways Relative to Alveolar Surfactant

Department of Neonatology, Faculty of Medicine, Eberhard-Karls-University, Tübingen, Germany; Adult Intensive Care Unit, Royal Brompton & Harefield NHS Trust and Unit of Critical Care, National Heart & Lung Institute, Faculty of Medicine, Imperial College of Science, Technology & Medicine, Royal Brompton Hospital, London, United Kingdom; and Departments of Cellular and Molecular Physiology, Pediatrics, and Obstetrics and Gynecology, Pennsylvania State University College of Medicine, Hershey, Pennsylvania

Address correspondence to: Wolfgang Bernhard, M.D., Ph.D., Department of Neonatology, Faculty of Medicine, Eberhard-Karls-University, Calwer Straße 7, D-72076 Tübingen, Germany. E-mail: wolfgang.bernhard{at}med.uni-tuebingen.de

Pulmonary surfactant is a surface-active mixture of phospholipids and specific proteins that lines the epithelial surfaces of mammalian lungs. In the alveoli, its main function is to reduce surface tension to ensure that these structures can remain open during respiratory cycles of contraction and expansion. However, surfactant is also present in the conducting airways, even though they are relatively rigid and do not need a system capable of rapidly lowering surface tension in response to compression. This has raised the question whether there is a difference in composition and function between airway and alveolar surfactant. Interest in this question has been stimulated further by the recognition that surfactant also has important functions in the immune defenses of the respiratory tract. In this review, we describe differences that have been reported between human airway and alveolar surfactant. In addition, we draw parallels between human airway surfactant and surfactant from the lungs of birds. The latter are tubular and rigid and do not undergo cycles of contraction and expansion, thus more resembling the human conducting airways than alveoli. Using this as a model, we propose a new hypothesis to explain structural and functional differences between human airway and alveolar surfactant. We suggest that the molecular composition of surfactant is adapted to differences in the architecture of pulmonary surfaces and to the dynamics of surface area changes during respiration.

Abbreviations: phosphatidylcholine, PC • surfactant protein, SP

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