Oxygen diffusive conductances of human placentae from term pregnancies at low and high altitudes.

A morphometric model of oxygen diffusion is employed to calculate the partial, total and specific conductances of human placentae from low- and high-altitude term pregnancies. Placentae were obtained from indigenous and non-indigenous populations and the diffusion pathway dissected into six tissue compartments. Birthweights were reduced at high altitude but were greater in natives versus non-natives. The altitudinal differences were associated with changes in placental diffusive conductances. The partial conductance of the villous trophoblast was conserved but the conductance on the maternal side was increased as was the conductance of the villous stroma. Fetal conductances were conserved (plasma) or diminished (erythrocytes). We conclude that birthweights are reduced at high altitude despite the attempts to increase the total placental diffusive conductance for oxygen. The mechanisms underlying these adaptations are discussed.