Pulmonary surfactant controls the surface tension at the air-liquid interface within the lung. This system had a single evolutionary origin that predates the evolution of the vertebrates and lungs. The lipid composition of surfactant has been subjected to evolutionary selection pressures, particularly temperature, throughout the evolution of the vertebrates.
Lungs have evolved independently on several occasions over the past 300 million years in association with the radiation and diversification of the vertebrates, such that all major vertebrate groups have members with lungs. However, lungs differ considerably in structure, embryological origin, and function between vertebrate groups. The bronchoalveolar lung of mammals is a branching “tree” of tubes leading to millions of tiny respiratory exchange units, termed alveoli. In humans there are ~25 branches and 300 million alveoli. This structure allows for the generation of an enormous respiratory surface area (up to 70 m2 in adult humans). Generally, in nonmammals, lungs are baglike with either smooth walls or large, bellows-shaped respiratory units (termed faveoli) extending from the outer wall of the lung into a central air space. Birds have the most strikingly different lung structure, with a pair of small parabronchial lungs connected to a series of air sacs. Air is propelled via the airsacs, which act like bellows, in a unidirectional manner through the lung. The lungs consist of a series of tubes (parabronchi) from which emanate the very-small-diameter, rigid air capillaries, which lie in close apposition with blood capillaries and represent the site of gas exchange. Some reptiles and amphibians have a complex and dense arrangement of septate compartments. However, in contrast to mammals, the lungs of fish, amphibians, and reptiles always lack a bronchial tree, a diaphragm, and a separate pleuroperitoneal chamber, and they have respiratory units up to 100 times larger than alveoli of similar-sized mammals.
But all lungs have one common characteristic. They are internal, fluid-lined, gas-holding structures that inflate and deflate cyclically. As a result, all lungs face potential problems related to the surface tension of the fluid. Pulmonary surfactant is produced in the lung to decrease surface tension of this fluid lining (hypophase). Von Neergaard first demonstrated that the surface forces at the gas-liquid interface of the lung contribute substantially to the retractive pressure, and hence static compliance, of the lung. However, surfactant can also vary surface tension with the radius of curvature of each alveolus (or more accurately, regions within an alveolus) so that the pressures within all alveoli are maintained at similar values, permitting alveoli of different sizes to coexist. Surfactant also helps the narrowest airways to remain open, thereby reducing the resistance to air flow and controlling fluid balance in the lung. However, the alveoli, being interdependent units, do not necessarily stretch upon inflation but unpleat or unfold in a complex manner. Moreover, the many fluid-filled corners and crevices in the alveoli open and close as the lung inflates and deflates.
Surfactant in nonmammals exhibits an antiadhesive function, lining the interface between apposed epithelial surfaces within regions of a collapsed lung. As the two apposing surfaces peel apart, the lipids rise to the surface of the hypophase fluid at the expanding gas-liquid interface and lower the surface tension of this fluid, thereby decreasing the work required to separate the two surfaces. However, for surfactant to act as an antiadhesive, the respiratory tissues must “fold” in on themselves, possibly during exhalation, or when the ventilatory period is punctuated by protracted nonventilatory periods at low lung volume. These conditions occur frequently in the ventilatory pattern of nonmammals.
The pictures shows a confocal image of giant liposomes of pulmonary surfactant was magnified 40 times by Dr. Jorge Bernardino de la Serna of MEMPHYS - Center for Biomembrane Physics, Department of Biochemistry and Molecular Biology in Odense, Denmark.