The role of intraplacental vascular smooth muscle in the dynamic placenta: a conceptual framework for understanding uteroplacental disease.

Although non-innervated, the placenta must continually accommodate changes in uteroplacental pressure (due to spiral artery failure, maternal position, maternal emotional state, etc.), which might otherwise be expected to result in rapid feto-maternal water fluxes in the highly water-permeable human hemochorial placenta. Uteroplacental flow must also be under the same influences, producing temporary, or permanent, regions of poor intervillous flow, yet the reduction of umbilical vein oxygen content that would be expected to be produced by such shunts of feto-placental blood do not occur in the normal fetus. We suggest that there is a local villus tree mechanism matching intravillus flow of fetal blood to local uteroplacental oxygen content. By analogy to ventilation/perfusion (V/Q) matching in the postnatal lung we suggest the term U/Q matching for this mechanism in the placenta. We further suggest that such disturbances in flow matching are compensated for by the fetus, via complementary adjustment of umbilico-chorionic artery and umbilical venous flow resistances, utilizing the differing sensitivities of vascular smooth muscle tissues of embryonic and extra-embryonic origin to vasoactive agents. Copyright 2002 Elsevier Science Ltd. All rights reserved.