Clearance

One of the crucial functions of the kidneys is to remove metabolic wastes and excess amounts of ingested substances from the blood. Nitrogenous wastes such as urea and creatinine are generated by metabolism and removed from the body by the kidneys. The average excretion rate normally reflects the rate at which these substances are added to the blood by metabolic processes, thus keeping the body in balance for these substances. Other substances, those that are crucial for normal function, including sodium and potassium, enter the body by ingestion and are also removed from the body by the kidneys. These excretion rates parallel ingestion rate in the long term but are altered transiently to reflect other needs, such as the regulation of blood pressure or plasma osmolality.
Ridding the body of a substance is often called clearance. This term in a biomedical context has both a general meaning and a specific renal meaning. The general meaning of clearance is simply that a substance is removed from the blood by any of several mechanisms. For instance, a drug may be cleared by excretion in the urine or the feces, or it may be transformed by the liver and other peripheral tissues to an inactive form. Renal clearance, on the other hand, means that the substance is removed from the blood and excreted in the urine.