“[H]ow much surface area is there in the normal lung? This wasn’t an easy question to answer. Researchers who had looked at lung tissue through the microscope said the surface area was huge — maybe the size of a football field if you spread it all out. But physiologists who estimated the energy that’s used to inflate the sacs came up with a much, much smaller area. “That huge discrepancy between two supposedly correct methods bothered me,” Clements says.
In 1953, he suggested an explanation for the discrepancy. Maybe there was a substance in the lung that made the alveoli easier to inflate and keep inflated. A few years later, Clements and others found it — a slippery substance known as a surfactant that reduces surface tension in the alveolar membranes. Soon scientists showed that a lack of surfactant is involved in human lung disease.”
A breath of fresh air: npr.org.