Measurement of cardiac output and its distribution in rats under various sodium intakes, using 15 and 10 micron spheres.

We estimated cardiac output and its distribution to organs using simultaneously injected 15 micrometers and 10 micrometers radioactive microspheres. This study was carried out in anaesthetised rats with high cardiac output induced by sodium loading and low cardiac output by sodium restriction. In neither group were the values of cardiac output and blood flow to kidneys, brain, heart and spleen affected by microsphere size. However, the ratio of blood flow estimated with 15 micrometer to that with 10 micrometer microspheres averaged 0.67: 0.60 for liver, 1.09: 1.10 for duodenum, 3.51: 3.52 for testes, and 1.19: 1.07 for adrenals in high and low sodium rats respectively. A significant escape of 10 micrometer microspheres from testicular, adrenal, and gastrointestinal vasculatures may explain such finding. With regard to the liver, this view was reinforced by the finding of comparable hepatic blood flow values with either bead size after clamping of the portal vein. Additional preferential losses of 10 micrometer microspheres from muscular vasculature seemed to contribute to the fractions of injected radioactivities recovered in lungs (that is 6 and 2% for 10 micrometer and 15 micrometer spheres respectively). These results suggest that in the rat blood flow data provided by 10 micrometer microspheres should be carefully analysed according to the organ(s) studied.