Transcapillary Starling pressures in the fetus, newborn, adult, and pregnant adult.

We used the method of membrane osmometry to determine capillary pressure (Pc), plasma protein osmotic pressure (IIp), interstitial protein osmotic pressure (IIif), and interstitial free fluid pressure (Pif) in subcutaneous tissue. These pressures were measured in the sheep and guinea pig fetus, newborn, adult, and pregnant adult. Although there were some similarities, we found major species differences. 1) IIp was a minimum in the fetus and gradually increased to a maximum with adulthood in the guinea pig, whereas IIp attained adult values in the newborn lamb. 2) IIp and Pc did not change with pregnancy in sheep, but, in the guinea pig, IIp and Pc decreased by approximately 30% during pregnancy. 3) The maximum and minimum IIif occurred in the fetal and adult guinea pig, respectively, whereas this was reversed in the sheep. 4) IIif decreased with pregnancy in the guinea pig but not in the sheep. 5) The minimum Pif was found in the newborn of both species. These data suggest that the forces that determine the intravascular-interstitial fluid distribution are different in the fetus, newborn, and adult and also vary among species.