|MadSci Network: General Biology|
Under normal conditions, a healthy person should have some yellow in their urine. The reason being that one of the purposes of producing and expelling urine is to remove urobilins, the waste products of red blood cells, from the blood. If someone's urine completely lacked urobilins, this could be a sign of liver or kidney problems. In fact, urine that may look clear to the human eye would contain detectable yellow color in a spectrophotometer (a device that measures the wavelengths of light absorbed by a substance). Drinking large amounts of water will dilute one's urine, just as drinking to little water can concentrate one's urine, since the body tries to maintain a constant amount of water in it. Likewise, diuretics, like caffeine and alcohol, cause the kidneys to release excess water which also dilutes one's urine, sometimes making it appear clear. In general, a complete lack of color in the urine is no more healthy than yellow urine, especially if it is the result of excessive diuresis which can lead to dehydration. On the other hand, it is true that dark yellow urine usually indicates dehydration and should prompt one to drink more water.
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