Each human kidney contains about 1 million functioning units called nephrons which are primarily involved in urine formation. Urine formation ensures that the body gets rid of the final products of metabolism and excess water in an attempt to maintain homeostasis (a constant internal environment).
The majority of the substances dissolved in the urine (solute load) consists of nitrogenous wastes, mostly the end product of protein metabolism. The amount of urine depends in part on how much protein is in the diet. If normal waste products (uric acid, creatinine, and ammonia) are not removed appropriately, they collect in abnormal quantities in the blood – known as azotemia (an elevation of blood urea nitrogen and serum creatinine levels. The ability of the kidneys to adequately eliminate nitrogenous waste products is defined as renal function, and its inability to excrete the daily load of waste is known as renal failure.