Lymph flow in sheep limbs during local exposure to subatmospheric pressure

1. Lymph flow and pressure were measured via cannulae inserted into afferent lymphatics draining the feet of anaesthetized sheep. 2. When the cannula outlet was at limb level, local exposure of the limb to graded decreases in ambient pressure caused graded increases in lymph flow with pressure values down to -50 mmHg. 3. When the cannula outlet was lowered below limb level to offset the negative pressure gradient imposed on the lymphatic vessels by suction, lymph flow rose progressively with decreasing ambient pressure values down to -70 mmHg. 4. When negative pressure gradients were imposed on the lymphatic vessels by raising the lymphatic cannula outlet in progressive steps above limb level, the vessels were able to expel lymph against gradients of up to 50 mmHg but lymph flow was greatly reduced against a gradient of 70 mmHg. 5. The results suggest that subatmospheric pressure may affect local lymph flow in two ways. By increasing blood capillary transmural pressure it may increase lymph flow by increasing tissue fluid formation. By imposing a negative pressure gradient along the lymphatics it may decrease lymph flow, especially at the most negative pressures, and the lymph flow response to subatmospheric pressure may be the algebraic sum of both effects.