Mechanical characteristics of the canine thoracic duct: what are the driving forces of the lymph flow?

This study is designed to better understand the mode of lymph transport, pa rticularly through the extrinsic pumping by external compression of the lym ph vessel. The pressure-diameter relationship of lymphatic segments isolate d from the canine thoracic duct was examined using a laser optical micromet er measurement system. Results revealed that the thoracic duct displayed a high extensibility or compliance in the physiological pressure range, yet b ecame progressively less so with increasing internal pressure. The calculat ed incremental circumferential modulus of the thoracic duct under physiolog ical pressure (range of 2 to 6 cm H2O) showed values ranging from 1.2 x 10( 4) to 3.61 x 10(5) dyn/cm(2). At a pressure of 35 cm H2O, the modulus reach ed a limiting value of approximately 6.0 x 10(6) dyn/cm(2). In the physiolo gical pressure range, the relative wall thickness (h/R-o) of the canine tho racic duct was approximately 3.5%, which was much lower than that: reported for canine arterial segments and similar in value to that of the canine ju gular vein. In conclusion, the pressure-diameter curve of the canine thorac ic duct was shown to resemble that of venous vessels. However, the circumfe rential elastic modulus of the thoracic duct wall was lower than the moduli of veins, proving that lymphatics are more compliant than veins. This sugg ests lymph Row in the thoracic duct may be better promoted by external comp ression of the lymphatic vessel.