A nephron (from Greek νεφρός (nephros) meaning "kidney") is the basic structural and functional unit of the kidney. Its chief function is to regulate water and soluble substances by filtering the blood, reabsorbing what is needed and excreting the rest as urine. Nephrons eliminate wastes from the body, regulate blood volume and pressure, control levels of electrolytes and metabolites, and regulate blood pH. Its functions are vital to life and are regulated by the endocrine system by hormones such as antidiuretic hormone, aldosterone, and parathyroid hormone.

Cystic kidney disease describes several conditions in which fluid-filled cysts form in the kidneys. Cysts generally develop in weak segments of the tubules that carry urine from the glomeruli. The cyst's growth displaces healthy kidney tissue. The kidneys expand to accommodate the cyst, which can weigh as much as 20 pounds.

Three factors determine cyst classification: its cause (acquired, inherited), its features (complicated, simple, multiple, single), and its location (outer [cortical] or inner [medullary] kidney tissue).