Control of vascular volume in sheep umbilical circulation

Vascular and extravascular volumes were measured using a single-injection, double-indicator technique, in a perfused umbilical circulation in which umbilical artery (Pfa) and umbilical vein (Pfv) pressures could be independently varied. At constant Pfv, when Pfa was raised from 25 to 35 mmHg, vascular volume increased 16.1 plus or minus 6 ml (mean plus or minus SD) from a control value of 113 plus or minus 38.3 ml. Extravascular volume increased 25.2 plus or minus 7.7 ml from 128 plus or minus 53.3 ml. During further increases in Pfa to 65 mmHg, vascular volume increased approximately 10 ml for each 10 mm Hg increment, but no change in extravascular volume occurred. At a constant Pfa when Pfv was raised to 15, 20, and 25 mmHg, vascular volume increased 12.2 plus or minus 3.6, 22.2 plus or minus 5.4, and 28 plus or minus 12.5 ml from the measurements at low Pfv. There was no increase in extravascular volume during elevation of Pfv. At the lower values of Pfa (25-35 mmHg) recruitment of previously unperfused channels and distension are seen. Over the higher ranges of Pfa change and during Pfv elevation there is distension of vascular bed.