Importance of valves and lymphangion contractions in determining pressure gradients in isolated lymphatics exposed to elevations in outflow pressure.

Lymphatic pressures were measured at several locations along an isolated lymphatic system exposed to elevations in outflow pressure. The objective of this study was to determine the contributions of lymphangion contractions and valve function to the observed pressure gradients. In each experiment, five bovine mesenteric lymphatic vessels (each with four to nine lymphangions) were joined in series by t-pieces connected to pressure transducers. The vessels were placed in an organ bath with input provided by a reservoir filled with Krebs solution. With a constant inflow pressure of 4 cm H2O, outflow pressures were elevated in 2- or 5-cm H2O increments. Except for inflow and outflow pressures which were fixed, the pressures measured at four other locations along the system were pulsatile due to lymphatic contractions. The mean pressures increased as outflow pressures were raised. While mean pressures were highest at the outflow end, estimates of the net pressure generated by each segment suggested that all segments, including those at the most upstream locations, increased their contractile activity. In addition, diastolic pressure gradients formed across the system. These did not appear to be due to valve failure (endurance limit of valves was 168 +/- 32 cm H2O) but rather, appeared to relate to the progressive inability of lymphangions to empty which, for a given lymphangion, began to occur at a mean outflow pressure of 9.8 +/- 1.1 cm H2O.