The initial lymphatics as sensors of interstitial fluid volume

Lymphatic luminal pressure and interstitial fluid pressure (IFP) were measured simultaneously in the bat's wing with two servo-null micropressure systems during interstitial fluid volume expansion induced by direct interstitial infusion. Lymphatic baseline pressure, pulsatile pressure, and IFP were elevated by the interstitial infusion. Transmural pressure gradients occurred only transiently. The increased lymphatic pulsatile pressure was not dependent upon the transmural pressure gradient but remained elevated as long as the increased interstitial fluid volume (IFV) was maintained. The results suggest that lymphatic pumping capacity during edema is modulated by IFV rather than IFP and that this mechanism may play an important role in the regulation of interstitial hydration. A mechanism whereby the initial lymphatics may directly sense and control IFV is proposed based upon physiological and anatomical data.