Figure 1.

Schematic presentation of the hemodynamics of the intervillous space. As in any capillary network, the functioning of the intervillous space is determined by a balanced intra- and extravascular exchange under flow conditions that are tightly regulated by arteriolar and venular activity in accordance with osmotic forces of the perivascular tissue. (A): Hemodynamic effects in the intervillous space caused by spiral artery dilatation. In this condition, the flow volume increases, but flow velocity and pressure decrease. This condition allows optimal maternal–fetal exchange. (B): Hemodynamic effects in the intervillous space when spiral artery dilatation occurs after inadequate adaptation of the venous outflow. There is stasis of blood that cannot be drained efficiently and causes congestion, responsible for increased pressure and reduced flow velocity in the intervillous space. (C): Impact of the incomplete dilatation of the spiral arteries on the conditions presented in Figure 1B. The reduced spiral artery blood supply reduces the degree of congestion and pressure in the intervillous space, at the cost of a reduced flow volume. This mechanism might improve the maternal–fetal exchange when there are unbalanced pressure/flow conditions in the intervillous space as in Figure 1B.