Respiratory System - Developmental Changes after Birth
The new-born's lungs are collapsed and need powerful breaths to inflate them; airways too are small, offering remarkable resistance to flow of air. The surface tension tends to hold the moist membranes together. The lungs of a full term baby secrete surfactant that has the unique property of exerting high surface tension while the lungs an expanded but a low surface tension while the lungs are collapsed. So after the first powerful breath which expands the lungs, breathing becomes easier, due to the reduced surface tension of collapsed lungs.