Transvascular fluid shift and thoracic duct lymph: analysis of lymph formation in the rat.

To analyze the effect of changes in interstitial fluid on lymph production, intravascular infusions of saline were given to splenectomized rats under pentobarbital anesthesia at 3 different rates (2, 3, and 4 ml/100 g of body weight) over 10 min. Change of blood volume was continuously monitored and, simultaneously, thoracic duct lymph was collected during and after the infusion. Equilibrium was attained approximately 40 min after the infusion; regardless of the infusion rate, 10% of the infused volume was incorporated into the vascular space and 90% was filtered into the extravascular space. Thus, the amount of transvascular fluid shift showed a linear relationship with the infused volume. However, the drainage from thoracic duct lymph amounted to 5.9%, 11.4%, and 17.8% of the infused saline volume when given at the rate of 2, 3, and 4 ml/100 g, respectively. The relation of lymph flow and infused volume could be regarded as a nonlinear system. By means of a simulation study, this relation was found to be attributed to the nonlinearity of conductance for fluid movement from tissue to lymph duct, which was only one sixth of that determined for the capillary membrane. The drainage of lymph protein following the infusion was only about 50% of that expected from the interstitial protein concentration in even distribution of the infused saline. These characteristics of interstitial fluid space play an important role in absorbing water and, hence, buffering changes in circulating blood volume after volume loading.