Developmental Changes in Pleural Liquid Protein Concentration in Sheep

The source of normal pleural liquid is thought to be the systemic circulation of the pleural membranes rather than the pulmonary circulation of the nearby lung. Evidence for a systemic origin comes from the low protein concentration of pleural liquid in adult sheep, which is consistent with protein sieving from a high-pressure circulation. During normal development from fetal to adult life, systemic vascular pressure increases. We therefore reasoned that if pleural liquid comes from the systemic circulation, pleural liquid protein concentration relative to plasma protein concentration should decrease during normal development. To test this hypothesis we did thoracotomies on 14 fetal, 9 newborn, and 15 adult sheep and collected pleural liquid and plasma for measurement of total protein and albumin concentrations. In separate experiments we measured steady-state systemic and pulmonary vascular pressures in age-matched chronically instrumented fetal, newborn, and adult sheep. The protein concentration in ple...