As anyone who has ever blown bubbles as or with a child knows, if you stop blowing before the sphere is complete, it collapses and disappears. The gas exchange surfaces of our lungs, the alveoli, are also highly curved,thin-walled, moist membranes that are open to the atmosphere. They too, would tend to go poof! and disappear, save for a mixture of phospholipids and proteins that lines the epithelial surfaces of vertebrate lungs, called surfactant. In the alveoli, surfactant's primary function is to decrease surface tension to ensure that the `bubbles' remain open during successive cycles of respiratory contraction and expansion. Both neonatal and adult respiratory distress syndromes result from a lack of surfactant, leading to increased surface tension, decreased lung compliance, and lung collapse. Animal-derived surfactants have been used to treat respiratory distress syndrome in infants since the early 1990s with great success, and an increased understanding of its components, especially surfactant associated proteins,has led to the development of newer synthetic versions. Recent evidence indicates, however, that not all surfactants are created equal. Composition and function differ between vertebrate groups as well as developmentally,depending on pulmonary anatomy and respiratory physiology. These differences interested a group led by Dr Roger Spragg at UC San Diego School of Medicine,who wondered if diving animals, that have evolved to suffer repeatedly lung collapse and re-expansion, would have developed unique surfactants to facilitate such cycles.
Animals that routinely undergo lung collapse and expansion are those that dive to depths of 70 m or more; the immense hydrostatic pressure at depth is thought to completely collapse the gas exchange portions of the lung. Lung collapse prevents nitrogen from being forced into the blood under pressure and protects against both nitrogen narcosis and the bends. Oxygen, meanwhile, is stored in the blood and tissues, making the lung unnecessary. Spragg's group tested the hypothesis that diving mammals would have unique surfactants compared to terrestrial animals by sampling fluid (bronchoalveolar lavage)from juvenile sea lions, northern elephant seals and a harbor seal, and comparing their surfactant compositions to those of pigs and human patients that had been referred to UC San Diego for pulmonary artery thromboendarterectomy.
In general, the pinniped surfactants were similar in composition to terrestrial animals, though there were differences that would indicate adaptations for lung collapse and re-expansion. These included a higher concentration of phospholipid, increased cholesterol, and alterations in specific phospholipid ratios, relative to porcine surfactant, that could increase fluidity and allow rapid lung expansion. Investigation of such surfactants could provide clues to optimize therapeutic formulas for patients suffering respiratory distress syndrome.
Surprisingly, however, elephant seals, which as adults may dive to greater than 1500 m and routinely surpass 400 m depth, had surfactants with consistently higher minimum and maximum in vitro surface tensions,meaning that the surfactant to some degree had a diminished surface tension lowering function. It would be interesting to discover if this has functional significance in elephant seals, or if it is perhaps a developmental difference between juvenile and adult animals.