Urinary Excretion Patterns for Substances with Simultaneous Secretion and Reabsorption by Active Transport

Substances, which are actively secreted and reabsorbed by the renal tubules, show different excretion patterns. Except for potassium, they all follow a“secretion proximal” pattern, i. e. secretion takes place at a site proximal to reabsorption or through identical cells. Thiosulfate, creatinine and uric acid are typical examples. — For thiosulfate, secretion Tm = reabsorption Tm. At low plasma levels its clearance is low; at higher levels the clearance equals glomerular filtration rate. — For creatinine, secretion Tm > reabsorption Tm; its clearance is low at low plasma levels, but exceeds glomerular filtration rate at high levels. The dependence of“endogenous creatinine clearance” on glomerular filtration rate and renal blood flow is discussed. — A similar pattern is shown by uric acid in the rabbit. In man, uric acid reabsorption Tm > secretion Tm, the net result being reabsorption at all plasma levels.