Pulmonary vascular responses during advancing gestation in fetal lambs in utero.

bed in which we deduce that hydrostatic pressure is elevated. In the newborn lamb, the results suggest a lower capillary pressure and larger capillary bed than in the fetus. The pulmonary vascular bed of the fetus, we conclude, may contain large unperfused areas; and vasodilation at birth may take place mainly by recruitment of additional areas. There is support for these ideas in the very nonlinear pressure-flow curves characteristic of the fetal and neonatal circulation:1.6 they suggest the presence of unperfused areas which are recruited when inflow pressure is increased. The fluid in the fetal acini is not a plasma filtrate. Its formation depends on active C1 transport across the acinar epithelium. The epithelium also forms a barrier to the passive penetration of most plasma solutes and therefore to the passive accumulation of liquid. Absorption of lung liquid at birth requires changes in the pulmonary epithelium, only some of which have yet been identified.