Question: Why Is Urine Yellow? :)

Keith Brain answered on 21 Jun 2010:

The yellow colour comes from bilirubin, which is one of the main breakdown products of red blood cells. Red blood cells are packed with haemoglobin, which gives bloods its red colour and is necessary for carrying oxygen in the blood. However, red blood cells can only survive in the circulation for about 120 days – so, at around 4 months of age the red cells are broken down in the spleen and the liver, producing (ultimately) bilirubin. This is a yellow colour, and you can also see this colour in old bruises (for the same reason – it’s the same pigment/colour).

Most of the bilirubin doesn’t leave the body through the urine – it leaves through the liver and into the gut, where it is turned into other compounds. However, the colour is so bright, even a little makes a big difference.

Why does the intensity of this colour change? It’s mainly because the amount of bilirubin produced stays the same, but the amount of water your kidneys produce changes a lot (depending on how much you drink and how much water you lose in other ways such as sweating). So, darker urine doesn’t usually mean that you’re breaking down more red cells – just that you’re a bit drier than usual.