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1. In order to study the acute effects of blood volume changes on the vascular resistance of portal-systemic collaterals (collateral vascular resistance), a model of total portal vein occlusion with 100% portal-systemic shunts was developed in the rat. In this model, we determined the haemodynamic effects of haemorrhage (1.8 ml/100 g body weight) or intravenous infusion of a volume expander (1.8 ml/100 g body weight). Cardiac output and regional blood flows were measured by the radioactive microsphere method. 2. Haemorrhage significantly reduced arterial pressure from 108 +/- 4 to 92 +/- 4 mmHg (mean +/- SEM), cardiac output from 56 +/- 4 to 24 +/- 2 ml min-1 100 g-1 body weight, portal pressure from 15.1 +/- 1.5 to 10.0 +/- 1.4 mmHg and portal tributary blood flow from 19.9 +/- 2.3 to 8.3 +/- 1.4 ml/min. Consequently, collateral vascular resistance significantly increased from 6.6 +/- 0.9 x 10(3) to 11.1 +/- 2.0 x 10(3) kPa 1(-1) s. 3. Volume expansion reduced arterial pressure from 98 +/- 3 to 90 +/- 3 mmHg, and significantly increased cardiac output from 43 +/- 3 to 55 +/- 3 ml min-1 100 g-1 body weight, portal pressure from 13.9 +/- 0.7 to 16.5 +/- 0.8 mmHg and portal tributary blood flow from 16.4 +/- 1.3 to 28.2 +/- 3.2 ml/min. Consequently, collateral vascular resistance significantly decreased from 7.0 +/- 0.5 x 10(3) to 4.9 +/- 0.4 x 10(3) kPa l-1 s. 4. This study shows that in rats with portal hypertension, portal-systemic collateral vascular resistance is modified by alterations in blood volume.

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