Urine is a biological fluid secreted by the kidneys. Urine is the largest liquid waste product of the vertebrate body's metabolism. Urine is secreted by the kidneys by filtration from the blood, and then by secretion and recovery of certain molecules from the "primitive" urine to form the "final urine". The latter is expelled from the body through the urinary system. The elimination of urine by emptying the bladder is called micturition. Excretion and reabsorption are regulated by several hormones, including antidiuretic hormone (ADH for short), cortisol and renin-angiotensin (part of the renin-angiotensin-aldosterone system).

Many chemicals that are inhaled or ingested, or that pass through the skin into the body, can then be detected by urine analysis, as parent molecules and/or metabolites, as well as certain molecules that are indicative of a disease state (e.g. albumin). The main component of urine is water, with urea as the main nitrogenous waste product.