A three-compartment model for the transport and distribution of Hippuran.

It is generally believed that after an intravenous injection of Hippuran its concentration in the plasma can be described as the sum of two exponentials. However, by collecting samples during the first minute after injection of the tracer, a third exponential term was found with a half-life of less than one minute. To explain its presence it was assumed that the plasma shares its Hippuran with two peripheral compartments, the rate of transfer from one compartment to another being proportional to the amount in the first of them. The proportionality factors have been determined for 20 adults and averaged. Substituting these averages in the differential equations for the distribution process, the only variable remaining is the renal excretion rate a (the fraction of plasma cleared per minute by the kidneys). If this procedure, which appears to be justified by the experimental results, is correct the shape of the plasma curve will be determined exclusively by a. It is shown that the (small) third compartment has a minor influence on the shape of the renogram curves.