As primary excretory organs, the kidneys maintain homeostasis by removing waste substances from the bloodstream. They comprise over a million units called nephrons, which serve as the kidney's functional units.

A nephron consists of two primary structures: the renal corpuscle and the renal tubule. The renal corpuscle contains the glomerulus, a network of capillaries where the first step of renal excretion, glomerular filtration, occurs. Blood pressure forces water, ions, and small molecules out of the blood and into the Bowman's capsule, forming the filtrate.

The filtrate then enters the renal tubule, where the two steps of renal excretion occur: tubular reabsorption and tubular secretion. During tubular reabsorption, useful substances like water, glucose, ions, and nonionized drugs are reabsorbed into the bloodstream. The process of reabsorption occurs predominantly in the distal tubules. In contrast, tubular secretion involves the active transport of substances such as uric acid and certain drugs from the blood into the renal tubule. This process primarily takes place in the proximal tubule via ATP-powered transporter proteins. Through these three steps, the kidneys effectively filter the blood and excrete out excess waste and drugs, contributing to overall body health.