Fractionation of the cardiac output of rats with isotopic potassium.

A FOREIGN substance, after a single intravenous administration, will be distributed initially to the organs in proportion to their blood flow. The substance will then be carried away from the organs by their venous drainage. For a certain period of time, however, the venous drainage will be negligibly small compared to the arterial delivery. During this time the fractional distribution of the substance among the organs will correspond to the fractional distribution of the cardiac output among them. In the case of a substance which is incompletely transferred from the blood to the tissues, or one which has a small volume of distribution within the tissues, the time during which the venous drainage is small will be short. The time will be greatest for substances which are completely transferred from the vascular system to the tissues with least hindrance and which have a large volume of distribution within the tissue. The radioactive isotope of potassium, K, is such a substance. The studies described here indicate that the venous drainage of Iv" is negligible compared with its initial deposition in the organs of the rat for at least 1 min. after a single intravenous administration of the isotope. I t is therefore possible to estimate the fractionation of the