*The first breath of life initiates the serial opening of the alveoli. The newborn goes from a fluid-filled environment to an air-breathing, independent, extrauterine life. The lung fluid must be removed from the lungs to permit adequate movement of air.
-During delivery, fetal chest is compressed, increasing intrathoracic pressure and squeezing small amount of fluid out of the lungs.
-After birth of the newborns trunk, the chest wall recoils, creating a negative intrathoracic pressure which produces a small, passive inspiration of air that replaces the fluid in the large airways that is squeezed out.
-After this first inspiration, the newborn exhales, with crying, against a partially closed glottis, creating positive intrathoracic pressure. This distributes the inspired air throughout the alveoli and begins to establish functional residual capacity, which is the air left in the lungs at the end of normal expiration. The pos. pressure also increases absorption of fluid via the capillaries and lymphatic system.
-The negative intrathoracic pressure created when the diaphragm moves down with inspiration causes lung fluid to flow from the alveoli across the alveolar membranes into the pulmonary interstitial tissue.
-With each succeeding breath, the lungs continue to expand, stretching the alveolar walls and increasing the alveolar volume.