On the regulation of the renal response blood volume expansion by vascular parameters in the rat.

The dynamic patterns of body fluid volume distribution, of cardiovascular variables and of renal water, sodium and potassium excretion were studied in the anaesthetized rat following acute, iso-oncotic blood volume expansion. The increased renal excretion following expansion was soon reversed and its pattern to termination was not correlated with changes in pulse and mean arterial pressure, in plasma electrolyte concentration or in haematocrit. At the end of the renal response the change in extracellular fluid volume was not usually corrected, the blood volume was always well above its control value and the interstitial fluid was below its control value. In all cases the temporal pattern and the termination of the renal response corresponded closely with the temporal pattern and the return to the control value of the central venous mean and pulse pressure. The findings of thse experiments are not consistent with the view that any of the measured body fluid volumes directly and always determine renal excretion. It is proposed that in response to an acute blood volume expansion an animal may initially regulate neither its blood volume nor its extracellular fluid volume but rather a factor which is reflected in or related to the central venous pressure.