U(IV) does not enter circulation except by direct injection into the blood (Wilson et al., 1953).

U(IV) is oxidized to U(VI) (Wilson et al., 1953), which is absorbed, followed by formation of the soluble uranyl ion (UO₂²⁺). About 60% of the uranyl ion forms soluble complexes with anions such as bicarbonate/carbonate and to a lesser extent chloride, acetate, and phosphate, citrate, malate, lactate, and other ligands (Dounce, 1949; Dounce et al., 1949; Gindler, 1973; Stevens et al., 1980; Durbin, 1984). These complexes are filtered by the proximal renal tubule. At low pH, the carbonate-uranyl complex is hydrolyzed releasing the uranyl ion which can undergo tubular reabsorption or can complex with renal tubular proteins causing nephrotoxicity (Bassett et al., 1948) to interact with (or be resorbed across) the brush border membrane (Dounce and Lan, 1949; Durbin, 1984).

Some of the uranyl ion complexes weakly with transferrin. This complex is not filterable at the glomerulus. It enables distribution of U to soft tissue and bone (Durbin, 1984).