BLOOD-VOLUME RESPONSE TO DRAINAGE OF LEFT THORACIC-DUCT LYMPH IN THE OVINE FETUS

Although it is known that left thoracic duct lymph flow rate in the ov ine fetus is several times that in the adult relative to body weight, the role of the lymphatic system in maintaining fetal blood volume has not been explored. In the present study, the left thoracic lymph duct was drained without return to the fetal circulation in 10 chronically catheterized animals for 7 h. Four experiments in which the lymph was returned to the circulation served as control. During the 7-h drainag e, the total volume drained was 174.4 +/- 19.6 ml. Thoracic duct lymph flow rate decreased gradually and averaged 22.2 +/- 3.4% (SE) below c ontrol at 6-7 h compared with a small increase in the control experime nts (analysis of variance: P = 0.0013 comparing 2 groups). Fetal blood volume decreased (P < 0.0001) by 8.2 +/- 1.1% at 3 h because of a dec rease in plasma volume with a further small decrease in blood volume t o 9.7 +/- 0.9% below normal at 7 h. The decreases in blood volume at 1 , 3, 5, and 7 h were 49.5, 40.2, 24.6, and 21.7%, respectively, of the volume of lymph that was drained. Fetal vascular pressures and heart rate did not change significantly. Although protein concentrations in plasma (-6.8 +/- 1.3%) and lymph (-7.5 +/- 2.1%) decreased, the lymph- to-plasma protein concentration ratio remained constant at 0.72 +/- 0. 01. These observations suggest that the ovine fetus has a remarkable a bility to compensate for the loss of large volumes of lymph, in that a rterial pressure, venous pressure, and heart rate are maintained and t he decrease in blood volume appears limited to a maximum of 10%.