Umbilical vascular compliance in sheep.

To explore the mechanical properties of umbilical vessels and to test for possible interactions between maternal and fetal circulations, we recorded pressure-volume curves from isolated in situ placentas of 17 sheep. We estimated static umbilical compliance by extrapolating to infinitely slow rates of volume change. We found that compliance averaged .231 +/- .014 SE ml/mmHg per kg fetal wt under control conditions, a value that increased 26.5% when maternal arterial pressure was lowered 85 mmHg (clamping the aorta), but did not change when venous pressure was raised 37 mmHg (clamping IVC). After replacing blood with kerosene which does not penetrate small vessels because of interfacial tension, we found arteries accounted for 22% and veins for 41% of total compliance, leaving 37% attributable to small placental vessels and surrounding tissue. We conclude that umbilical vessels are about one-half as compliant as adult vessels on a body-weight basis, but only slightly less compliant than vessels elsewhere in the fetal body. When maternal vessels expand they interact with surrounding placental tissue, displacing fetal blood and altering the apparent compliance of umbilical vessels.