The genitourinary system encompasses either a female or a male reproductive system and the urinary system, which consists of two kidneys, two ureters, a urinary bladder, and a urethra.

The kidneys are the main organs of the urinary system. They perform several functions essential for the preservation of life. The kidneys filter the blood to maintain internal homeostasis; they also excrete into the urine what is not reabsorbed by the tubules as well as metabolic wastes such as urea (an end product of protein metabolism formed from ammonia), creatinine (an end product formed when phosphocreatine is used for muscle contraction) and uric acid (an end product of purine metabolism). Urine is produced by the kidneys through an extremely complex process, involving glomerular filtration, tubular reabsorption, and tubular secretion. The filtration process requires adequate blood volume and cardiac output.

The kidneys filter the blood through glomeruli, which are semipermeable membranes. Normally, the pores in the glomerular membranes are small enough to keep blood cells, platelets, and large plasma proteins from being filtered out. When capillary permeability is increased, as occurs in many diseases, plasma proteins and blood cells may pass into and be found in the urine. Glomerular filtration rate (GFR) refers to the amount of blood filtered by the glomeruli in a minute (or in a given time). GFR may serve as an indicator of the kidney's filtering capacity. (GFR is commonly measured by a creatinine clearance test, optimally based on a 24-hour urine collection.)

The kidneys also function to help regulate the body's acid‒base balance through processes such as bicarbonate ­reabsorption and acid secretion. With the right amount of ­aldosterone and antidiuretic hormone (ADH), healthy kidneys can ­maintain fluid and electrolyte balance through excretion of the right amounts of ingested electrolytes, and regulate blood pressure. ADH, also known as vasopressin, and aldosterone play important roles in the renal reabsorption of water and sodium.

In concert with the liver, the kidneys can turn inactive forms of vitamin D into an active form to facilitate the absorption of calcium from the gastrointestinal (GI) tract. Inactive vitamin D is usually obtained in the diet or produced via the action of sunlight on cholesterol found in the skin.

The kidneys also produce erythropoietin, a hormone that stimulates the production of red blood cells (RBCs) in the bone marrow. In renal failure, a deficiency of erythropoietin may lead to anemia.

Renin, an enzyme produced from the renal cells, can convert angiotensinogen (a serum globulin fraction) to angiotensin I. This inactive form of angiotensin is then converted to angiotensin II, an active form of angiotensin, which can stimulate vasoconstriction and aldosterone secretion, causing the blood pressure to increase.