Pulmonary surfactant and evolution of the lungs.

Pulmonary surfactant has been looked for and found in 11 representatives of four vertebrate classes. The amount of surfactant, estimated by quantitative spreading as a surface film, correlates well with alveolar surface area and with the amount of saturated, mainly dipalmitoyl, phosphatidylcholine in the lung parenchyma. The quantities of other phospholipids do not correlate well with alveolar surface area.