[Estimation of regional circulation in rats: results obtained by using 15 and 50 micron diameter microspheres and...

Radioactive microspheres, 15 or 50 micron in diameter, were used to estimate the distrubtion of cardiac output and the degree of shunting of microspheres through the systemic and pulmonary circulations in anaesthetized rats. Extraction of 15 micron spheres by the pulmonary capillaries was nearly 100% and the amounts of microspheres per gram of lung tissue were not significantly different in the various lobes of lung. After injection into the left ventricle, the proportion of microspheres shunted to the lungs was almost identical using 15 or 50 micron spheres. Similar results were observed after injection into the internal of external carotid artery. The distribution of cardiac output showed a significant difference between 15 and 50 micron spheres, the proportion of 50 micron spheres found in the stomach being higher, which suggests the existence in this organ of arteriovenous shunts larger than 15 micron. The rubidium method yielded higher fractions of cardiac output in the liver (hepatic artery), lung and skin whereas the microspheres distribution to the heart, spleen and digestive tract exceeded that of rubidium. The origins of these differences are discussed.