Peripheral lymphatic responses to outflow pressure in anaesthetized sheep.

1. Lymph flow and pressure fluctuations were measured by cannulating popliteal efferent and distal hind-limb afferent lymphatic vessels in anaesthetized sheep. The cannula outflow height was raised above the vessels to increase lymphatic outflow pressure. 2. Lymph flow decreased non-linearly as the outflow was raised. The rate of decrease increased with increasing outflow height. 3. Lymphatic contraction frequency rose and stroke volume fell with increasing outflow height. 4. The calculated power necessary to move lymph along the cannula initially increased with outflow height but it reached a peak and was reduced again by raising the outflow further. Calculated lymphatic stroke work followed a very similar pattern. 5. Lymph flow was maintained up to a greater outflow height in afferent than in efferent vessels. Curves relating frequency, power and stroke work to outflow height were shifted to the right in the afferent lymphatics. 6. These results are consistent with an intrinsic lymphatic pump which can be stimulated by increasing pressure. At high pressures, however, the pump fails.