Why do I always pee yellow?

The stuff that makes your pee go yellow is a chemical called urobilinogen. This is a breakdown product of the haemoglobin that makes your red blood cells red. When you have red blood cells, they're made in the bone marrow, you have millions and millions of them and you destroy something like 10^11 (100,000,000,000) of these cells every day.

When you break down the red blood cells, which have a lifespan of about 120 days, the haemoglobin molecule, responsible for binding up the oxygen and oxygenating your tissues, gets split open by chemicals called enzymes. There's a little atom of iron sat in the middle of a haemoglobin molecule. This iron is released and can be recycled in the body but the coloured molecule that used to wrap up the iron, the haemoglobin molecule, has to be broken down even further.

Once you've unwrapped the iron you have this molecule called biliverdin which is green. It then gets oxidised a bit and becomes something called bilirubin which is a brown or yellow colour. This floats around in the bloodstream and gets picked up by the liver which adds a couple more chemicals to it to make it dissolve in water. The liver then squirts it out in your bile. That's why bile is yellow [which you'll see] if you're sick: people who are having norovirus problems at the moment, 3,000,000 of them in the UK know about that only too well. The bile goes into your intestines, which are rich in bacteria. Bacteria in the intestines chop up the extra atoms and chemicals that were added to the sides of the bilirubin molecule and knock some of them off, turning it into something called urobilinogen. The urobilinogen is still yellow but it does dissolve in water. This gets reabsorbed into your intestines and some of it ends up in your bloodstream. Because it goes into water then when the blood goes through the kidneys it ends up in your urine and it comes out as a yellow colour. So that's why you have yellow urine!Some of it carries on in your intestines and bacteria change it a bit more and it turns into another molecule called stercobilin and that's brown. That's why you do brown poos!