Figure 1.

In fetal life, lung and respiratory airways are filled with fluid (panel (A)). Immediately after birth, intense respiratory efforts generating intense pressures extrude lung fluid to the interstitium, and closure of the glottis and surfactant distribution in the surface of the alveoli contributes to establishing a functional residual capacity (panel (B)). Ongoing respiratory efforts once the FRC has been established require less positive inspiratory pressures (PIP) and positive-end expiratory pressures (PEEP) to maintain an adequate gas exchange (panel (C)).