To study pulmonary transvascular filtration of fluid and the normal adaptive response of newborn animals to excessive water in the lungs, we measured lung lymph flow, pulmonary vascular pressures, and the concentration of protein in lymph and plasma of nine unanesthetized 1− to 3-wk-old lambs, before, during, and after a rapid iv infusion of isotonic saline, 130–250 ml/kg·hr for 3–4 hr. During infusions, lung vascular pressures increased, the transvascular gradient of protein osmotic pressure decreased, and there was a 2− to 5-fold increase of lung lymph flow. When infusions stopped, lymph flow decreased, as the concentration of protein in plasma increased and pulmonary vascular pressures decreased to new steady-state levels. The concentration of protein in lymph did not change for several hours after the infusions. Body weight increased by 28% and extravascular lung water content was 19% above normal after saline; these changes were associated with mild tachypnea, hypercarbia, and hypoxemia. Sections of lung from these lambs had prominent cuffs of fluid surrounding large blood vessels.
We attribute these findings to an efficient network of pulmonary lymphatic channels and a distensible pulmonary perivascular space, or watershed, that accommodates excess lung fluid post-natally until its subsequent drainage by the pulmonary lymphatics and microcirculation, without serious compromise of respiratory gas exchange. It is likely that these same mechanisms permit gradual removal of residual fetal lung liquid during postnatal pulmonary adaptation. Other developments that should hasten reabsorption of fluid from the lungs after birth are a reduction of pulmonary microvascular pressure, a high concentration of protein in the plasma, and expansion of the pulmonary microcirculation, providing greater surface area for absorption of protein-poor fetal lung liquid.