ABSTRACT
The renal system has evolved to ensure that the body eliminates various waste products of metabolism, in the form of nitrogen-containing compounds like urea, acids like phosphoric acid, and potentially toxic metabolites of many exogenous chemicals, including drugs. At the same time, the renal system must control the volume and concentration of urine to balance the variable daily intake of fluids and electrolytes. Although we tend to think that the main product of the kidneys is urine, it is in fact more accurate to say it is extracellular fluid, with urine being merely the waste output. Measurement of urinary composition can be useful in determining the underlying kidney function, especially when combined with simultaneous measurements in plasma. Failure of renal function is associated with severe metabolic derangement and the various forms of renal failure comprise a major cause of ill health and death.
