Blood flow of the reproductive system in renal hypertensive rats during pregnancy

An experimental study on pregnant rats was undertaken to explore whether renal hypertension interferes with uteroplacental blood supply and fetal weight. Renal hypertension was induced by standardized clamping of the left renal artery. Two days before expected delivery, blood flow to the reproductive organs was determined by microsphere technique in normal control rats, rats with short-standing renal hypertension induced early in pregnancy, and rats with established renal hypertension induced 4 weeks before pregnancy. Myometrial and placental blood supply was considerably reduced in renal hypertensive rats compared to that in normotensive pregnant rats, with the reduction in placental blood flow being as much as 68% in rats with established renal hypertension. Nevertheless, there was no reduction in fetal weights, placental weights, or number of fetuses in the litters. These findings suggest that the nutritional blood supply of the placenta normally has a considerable overcapacity, perhaps a necessary safety margin so that the fetus can manage the circulatory demands associated with delivery. If hypertension causes intrauterine growth retardation only by means of reduced placental blood flow, this reduction in flow obviously must be considerable.