The excretion of body wastes and toxic substances are the primary function of the kidneys. The functional excretion unit is the nephron. Each kidney contains about one million nephrons. The nephron has three primary regions that function in the renal excretion process, the glomerulus, proximal tubule, and the distal tubule.
The kidney clearance is a measurement of the renal excretion ability. The excretion of the radiopharmaceutical used for kidney clearance begins approx. four minutes after application.

Biliscopin® Infusion, an iodinated contrast medium for intravenous cholecystography is secreted actively by the hepatocytes. The biliary excretion rate of a biliary contrast medium is determined by the binding to plasma proteins and liver receptor proteins, and by its transport maximum for the liver. Because of its high hepatocellular transport rate, Biliscopin® Infusion leads rapidly to high grade contrast density in the intra-hepatic and extra-hepatic biliary ducts as well as in the gallbladder.
Computed tomography with biliary contrast agents provides maximum quality preoperative imaging of the biliary tract.

Cholografin® Meglumine is a contrast agent for intravenous cholangiography and cholecystography. Iodipamide's primary excretion through the hepato-biliary system and concentration in bile allows visualization of the gallbladder and biliary ducts.

A bioassay is a medical test to measure, and quantify radionuclides in the body by direct (in vivo) or indirect (in vitro) analysis of tissues or excretions from the body.

See also Radioimmunoassay.

A biokinetic model describes the time course of absorption, distribution, metabolism and excretion of material or radiation introduced into the body of an organism.