Urine represents the biofluid end-product of the renal filtration process. Normally it is a transparent, sterile, pale-yellow liquid (although clearly color varies with the person's hydration status).
Urine is one of the most easily accessible biofluids in the human body and has been intensively researched for many years with the earliest studies from antiquity.
Urine production
Urine is produced by the kidneys via a process of filtration and absorption of, and secretion into, the renal blood flow. The substances removed from the blood comprise many waste metabolites, but also excess water, sugars, and a wide spectrum of other chemicals.
The average healthy adult produces approximately 1.5 to 2.0 liters of urine per day.
Physiological composition
The knowledge of the composition of normal human urine was markedly advanced in 2013 by the publication of the human urine metabolome 1. This was the first systematic study, using state-of-the-art analytic techniques, to define the chemical constituents of normal urine. As explicitly stated by the authors, this list of constituents is not expected to be 'complete', since as analytic techniques improve it is likely additional, chemical species will be discovered.
Currently ~3100 chemicals have been characterized in urine. This list does not include renally-excreted drugs or plant-derived food chemicals, the inclusion of which could considerably elongate the tally of known substances.
The main substances, other than water and simple sugars, include urea (via the breakdown of amino acids), salts (sodium, potassium and chloride), creatinine, ammonia, organic acids, pigments from the degradation of hemoglobin (e.g. urobilin which gives urine its characteristic color), and hydrophilic toxins.
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