This fluid must go when breathing begins at birth. Of a total of about 50ml, l5ml are lost passively from the pharynx and upper air passages during expulsion through the birth canal. Finally, as air enters and blood flow increases in the lung, the rest of the fluid enters the blood. Liquid-filled lungs are very resistant to being filled with air. It is 10 to 15 times more difficult to take the first breath and fill the lungs with air as it is to take a breath after the lungs are filled with air.
Premature babies meet such resistance with every breath.
One reason is that two organs that have high metabolic rates, the liver and the brain, are a disproportionately large part of the infant's total weight. It was discovered in 1955 that the insides of alveoli of adult lungs are coated with a surfactant. The discovery was made by Richard Pattle who worked with the Chemical Defense Experimental Establishment in Britain. In 1957, John Clements, then with the U.S. Army Chemical Center at Edgewood, MD, described how the lung surfactant changes the surface tension during inhalation and expiration. As air enters, the alveolus expands and the surfactant is diluted into the greater area so surface tension increases, the surface tension decreases as the air is let out and the surfactant concentration increases.