Dynamics of pulmonary water before and after birth

In fetal life an electrochemical gradient across the pulmonary epithelium causes liquid to flow from the microcirculation of the lungs through the interstitium into potential air spaces. That liquid is rich in chloride (greater than 150 meq/l) and almost free of protein (less than 0.03 g/dl). At birth rapid reversal of the direction of liquid flow is essential for a smooth transition from placental to pulmonary gas exchange. Removal of liquid from the lungs begins even before birth. In fetal rabbits and lambs, a substantial decrease in extravascular lung water occurs late in gestation and during labor. When breathing begins, transpulmonary pressure inflates the lungs and displaces residual liquid from terminal respiratory units into the interstitium. Fluid accumulates in connective tissue spaces surrounding large pulmonary blood vessels, distant from sites of gas exchange. The concentration of protein in the interstitium decreases, thereby increasing the transvascular gradient of protein osmotic pressure. Air entry into the lungs also decreases hydraulic pressure in the pulmonary microcirculation and increases pulmonary blood flow, which expands microvascular surface area for fluid exchange in the lungs. These developments facilitate reabsorption of water into the pulmonary vascular bed. In lambs, the postnatal increase in lung lymph flow is small and transient, accounting for only 10-15% of the total amount of liquid drained from the lungs after birth. Normally, the pulmonary microcirculation directly absorbs most of the fetal lung liquid that is present at birth.