Osmotic flow through the placental barrier of chronically prepared sheep.

An electromagnetic flow sensor was placed on the distal aorta of sheep fetuses in utero, and catheters were placed in a femoral artery and the common umbilical vein. Catheters were also placed in a carotid artery and a uterine vein of the pregnant ewe. Three days postoperatively maternal plasma was hyperosmotic with respect to fetal plasma by all methods: +5.8 +/- 1.4 SE by vapor-pressure osmometry, +2.2 +/- 0.7 SE by freezing-point depression osmometry corrected for bicarbonate loss; and +3.26 mosmol/liter by chemical measurement of plasma constituents. Maternal or fetal plasma was made hypertonic in vivo by infusion of concentrated solutions of mannitol, sucrose, or NaCl. Transplacental water flux was calculated from placental blood flows and arteriovenous differences in water content of the blood. The apparent osmotic conductivity of the placenta was 61 ml2-mosmol-1-kg-1, but this value should be divided by an unknown reflection coefficient to yield the true osmotic conductivity. Separate measurements were made of the placental diffusional permeability of Na+ and Cl- in five chronically prepared sheep fetuses: PSNa+ =0.20 +/- 0.04, PSCl- = 0.27 +/- 0.04 ml/(min-kg fetus). There was a highly significant positive regression between (total) placental permeability and fetal weight.