Collateral arterial blood supply to the pregnant uterus in the sheep.

The common internal iliac artery in the sheep gives rise to the arteries which are the primary source of blood to the uterus. We have used a vascular occluder placed on the common internal iliac artery of the pregnant sheep to induce and study the effects of chronic fetal hypoxaemia. We have noted that uterine blood flow and fetal oxygenation improve within 5 d of occlusion of the common internal iliac artery. To investigate the basis of this observation we performed serial angiograms and postmortem examinations of the uterine arterial circulation in pregnant sheep under normal conditions and after 7 d of blood flow occlusion. We identified distal anastomoses on the surface of the uterus between the middle uterine and internal pudendal arteries by way of the urogenital arteries and their branches. Furthermore, the internal pudendal artery filled retrogradely through other branches when the vessel was ligated at its origin. The development of a collateral circulation to the uterus in response to chronic common internal iliac artery occlusion is a physiological adaptation to maintain an adequate supply of blood to the uterus and thus to maintain fetal oxygen delivery.