Why Is Urine Yellow?

What true scientist has not asked, at some time or other, "Why is pee yellow?"

Some European alchemists in the middle ages apparently thought one possible reason was that there was gold in urine. This led to fruitless, and possibly quite disgusting, efforts to extract that gold.

The yellow color in urine is due to chemicals called urobilins. These are the breakdown products of the bile pigment bilirubin. Bilirubin is itself a breakdown product of the heme part of hemoglobin from worn-out red blood cells. Most bilirubin is partly broken down in the liver, stored in the gall bladder, broken down some more in the intestines, and excreted in the feces (its metabolites are what make feces brown), but some remains in the bloodstream to be extracted by the kidneys where, converted to urobilins, it gives urine that familiar yellow tint. (Here is a great diagram of some of these reactions, from the Boehringer Mannheim Biochemical Pathways at ExPASy.)

These same yellow chemicals also cause the yellow color of jaundice and of bruises, both of which result when more hemoglobin than usual is being broken down and/or the processing of its breakdown products by the liver is not able to keep up.

Why do we pee at all?

Urine is mostly water, which just has to be replaced. We excrete water not just to get rid of it if we have drunk too much, but primarily to carry away toxins that would otherwise build up in our systems. The important part of urine is urea (also known as carbamide), (NH₂)₂CO. The real waste product our bodies have to get rid of is ammonia (NH₄⁺, when in solution), which is formed by the breakdown of amino acids -- the building blocks of proteins. But ammonia is so toxic that only tiny concentrations can be tolerated. So any ammonia in the bloodstream is rapidly converted to urea in the liver. That urea is then removed from the bloodstream in the kidneys, and left in concentrated form in the urine (about 2% of urine is urea.) (More on the "urea cycle")

Urea was "discovered" by Hilaire Rouelle in 1773 (that is, he was the first chemist to isolate it in pure form and begin to understand its composition). It was the first organic compound to be artificially synthesized from inorganic starting materials when, in 1828, Friedrich Woehler prepared it by the reaction of potassium cyanate with ammonium sulfate. Woehler was really trying to make ammonium cyanate, but by synthesizing urea he disproved the theory that the chemicals of living organisms are fundamentally different from inanimate matter, thus inventing the field of organic chemistry.

Fish and amphibians lack the urea cycle for removing ammonia from the blood, since they can usually excrete ammonia directly via the gills or through the skin. This is one reason that ammonia in the environment is so highly toxic to aquatic animals. So do fish need to pee? Yes: not to excrete nitrogenous compounds, but for osmoregulatory purposes. Freshwater fish are always absorbing water from their environment by osmosis, and have to pump it out. Saltwater fish don't absorb water from the sea (blood and seawater have about the same saltiness and osmotic potential), but they do have some wastes to get rid of. More here.

(More on industrial uses of urea here.)

Where does the ammonia in our systems come from?

Ammonia is generated during the deamination (breakdown) of amino acids in the liver. Other sources of ammonia include bacterial hydrolysis of urea and other nitrogenous compounds in the intestine, the purine-nucleotide cycle and amino acid transamination in skeletal muscle, and other metabolic processes in the kidneys and liver. The normal physiological concentration in blood is less than 35 micromol/l. A five- to ten-fold increase in this concentration causes toxic effects, especially on the central nervous system.

Other urine facts

• Unusual-colored urine (black, dark orange, or brown, for example) can be a sign of serious medical problems.
• Some other colors can result from pigments in the diet, such as betacyanin found in red beets.
• Urea is apparently used as an additive in cigarettes, to enhance flavor.
• Urea is widely used as fertilizer, since plants can use it as a source of nitrogen.
• Although today urea is manufactured by the millions of tonnes through industrial processes, the urea in urine can be economically valuable if other sources of fixed nitrogen are scarce.
  
  • It can be used as plant fertilizer (when diluted). (It's organic, you know.)
  • The urea in urine can be broken down into ammonia again (generating the characteristic smell of stale urine) which be further oxidized by bacteria to nitrate, so useful in the production of gunpowder.

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David Wheat's Science In Action site has articles about science and math in the real world, weird science, science news, unexpected connections, and other cool science stuff. There is an index of the articles by topic here.

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