Renal admittance plethysmography

It is shown that over a wide flow range, phasic changes in renal volume and the maximum rate of volume change may be reasonably predicted from changes in renal conductance using a model which assumes homogeneous geometric changes. However, the relationship deteriorates at high pump rates, stroke volumes, and therefore mean flow values. It is suggested that the expansion of the arteriolar bed occurring at the higher flow rates and stroke volumes causes a decrease in arteriolar compliance, which in turn causes a greater portion of the volume pulsation to be transmitted to the venous system, where the geometric assumptions may no longer be valid. The independent effect of rate is more difficult to explain. It may be due to a frequency resonance effect in which, at rates greater than the resonant frequency, the vascular bed remains at a greater state of expansion, again reducing arteriolar compliance and transmitting a greater portion of the volume pulsation to the venous system