Ask a Scientist: Why Is Urine Yellow?

Answer: Of the three types of cone cells in your retina, the medium and long ones respond the most and tell your brain "yellow" in reaction to the 570 nm reflected light which reaches your eye because the urobilins in urine does not absorb that wavelength of light.

The yellow color in urine is due to chemicals called urobilins. Urobilin is a chemical formed, eventually, from the breakdown of the hemoglobin in your blood.

Hemoglobin in the blood is what transports oxygen from the lungs to the rest of the body (i.e. the tissues) where it releases the oxygen for cell use.

You start with a heme group (left), then remove the iron (Fe) and you end up with urobilin (right).  When your urine contains many urobilins, it's more yellow, and when it's low on urobilins, it is more clear.

There are a few steps in between.

[ Bilirubin is the breakdown product of the heme part of hemoglobin from worn-out red blood cells.  Urobilinogen is a colourless product of bilirubin reduction. It is formed in the intestines by bacterial action. Urobilin is produced when Urobilinogen is oxidized by intestinal bacteria; it can also be produced when urobilinogen is exposed to the environment upon urination, resulting in its oxidation to urobilin. Most bilirubin is partly broken down in the liver, stored in the gall bladder, broken down some more in the intestines, [some of] its metabolites remain in the bloodstream to be extracted by the kidneys where, converted to urobilins, it gives urine that familiar yellow tint. ... 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.   - Science In Action: Why Is Urine Yellow? ]

Urobilin is produced when Urobilinogen is oxidized by intestinal bacteria; it can also be produced when urobilinogen is exposed to the environment upon urination, resulting in its oxidation to urobilin. - Wikipedia

Our blood is amazing. Ethan Siegel on science blogs says, "there are about 30 trillion red blood cells in the human body, meaning you are both destroying (and making) new red blood cells at a rate of around 2.7 million cells per second. What's more, is that every red blood cell has about 270 million hemoglobin molecules in it, with each one capable of carrying four oxygen molecules (and having four heme groups)."

If your urine is dark yellow, you probably are not drinking enough water.  See the hydration scale. Um, I think you are supposed to squint at it, because if my urine had those little dots I'd freak out.

The colors we see are the result of our eyes and brains responding to different wavelengths of light.

Isaac Newton discovered that white light splits into its component colors when passed through a prism.  You see a rainbow of colors, but, you can put those colors back together and get white light if you pass them through another prism that bends the light back together.

Light is electromagnetic radiation of a wavelength that is visible to the human eye (in a range from about 380 or 400 nanometres to about 760 or 780 nm). Visible yellow light has a wavelength of about 570 nanometers.  Urine is yellow not because urobilin emits 570 nm light, but that it absorbs light better that is not in the yellow range.

You see yellow because your retina's color opponent mechanism sends a signal up your optic nerve that, in the battle between yellow and blue, yellow has won.