Fetal whole-body permeability--surface area product and reflection coefficient for plasma proteins

The transcapillary distribution of protein between the vascular and interstitial spaces largely depends on the protein permeability characteristics of the capillary membranes, i.e., the reflection coefficient and permeability - surface area product for plasma proteins. Our purpose was to estimate whole-body values for these parameters in chronically catheterized, near-term fetal sheep. Average experimental changes in plasma protein concentration, blood volume and vascular pressures following fetal hemorrhage or intravascular saline infusion were analyzed using mathematical modeling and parameter optimization techniques. The mathematical model consisted of three interacting compartments: (1) fetal blood, (2) fetal interstitium, and (3) maternal blood. We found that the mean fetal whole-body protein reflection coefficient averaged 0.61 following hemorrhage and 0.80 after intravascular saline infusions. The protein permeability - surface area product averaged 0.36 ml/min/kg for the hemorrhages and 0.70 ml/min/kg following saline infusions. The major findings are that the fetus has (1) a permeability - surface area product averaging 15 times that of the adult, (2) a protein reflection coefficient approximately 70 to 90% of adult values, and (3) the ability to decrease its capillary surface area following hemorrhage.