Transplacental gradients in the guinea pig

The authors measured plasma concentations of Na⁺, K⁺, Mg²⁺, Cl^-, Ca²⁺, HCO₃^-, phosphate, lactate, glucose, total amino acids, and total protein, and also the total (freezing point depression) osmolality and the colloid osmotic pressures. Conversion of chemically measured concentrations to osmolalities showed that unrecognized solute(s) were present in maternal (7 mM) and fetal (12 mM) plasma. Statistically reliable transplacental gradients existed only for calcium ion, phosphate, and amino acids. Ionic Na, K, Mg, Cl, Ca, HCO₃ and lactate were in electrochemical equilibrium at potential differences of -4.2 to +1.3 mV. Total plasma osmolalities were not significantly different in maternal and fetal plasmas in preparations in good condition, but fetal plasma osmolalities rose due to lactate secretion in asphyxiated fetuses. Colloid osmotic pressures were about 5 cmH₂O higher in maternal plasmas before 45 days gestation and about 6 cmH₂O higher in fetal plasmas after 60 days gestation. In the guinea pig, colloid osmotic pressures are at least as important as intravascular pressures in the regulation of transplacental water flow.