The Chemicals Responsible for the Color of Urine and Feces

Have you ever wondered what chemical makes urine yellow? It's because urine contains a pigment called urochrome or urobilin. Depending on your hydration level, urochrome can make urine appear straw-colored, yellow, or amber.

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What Is the Chemical Composition of Urine?

By Anne Marie Helmenstine, Ph.D.

Pigments in Blood to Urine and Feces

You have a lot of red blood cells, but each cell has a relatively short lifespan of about 120 days. When red blood cells die, they are filtered out of the blood by the spleen and liver and the iron-containing heme molecule is degraded into biliverdin and then bilirubin. Bilirubin is excreted as bile, which makes its way into the large intestine, where microbes change it into the molecule urobilinogen. This molecule, in turn, is converted by other microbes into stercobilin. Stercobilin is excreted through feces and is what gives them their characteristic brown color.

Some of the stercobilin molecules are reabsorbed into the bloodstream, where they are oxidized to become urochrome (urobilin). Your kidneys filter out this molecule and it exits your body in urine.

In addition to having a characteristic color, urine glows under a black light, but this is due to high levels of phosphorus.