The Chemistry of Bodily Fluid Colours
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A particular bodily fluid featured in the political news earlier this week, which got me thinking about the chemical causes behind the colours of the waste products we expel from our bodies. What makes urine golden, faeces brown, and bile green? The answers to all of these questions have a common chemical link.


We’re probably most familiar with the origin of blood’s red colour. This is result of the presence of haemoglobin, the oxygen-carrying protein found in red blood cells. Haemoglobin is made up of smaller subunits called haems; these iron-containing structures give blood its crimson hue. This is due to the alternating double and single bonds (conjugation) in the structure absorbing a specific wavelength of light.

Humans might have red blood, but other blood colours are possible in other animals. It all comes down to differences in chemical structure and conjugation. This also has a role to play in the colours of other bodily fluids – and the haem structures in blood are the point of origin for all of them.


The red blood cells in your bloodstream aren’t your lifelong companions. In fact, they only live for around 120 days, and your body is constantly creating new red blood cells to replace them – around two million new cells every second. You also have a recycling system for the red blood cells which are old or damaged.

The graveyard for red blood cells is the spleen. This isn’t a place they go to rest in peace, however; instead, a Frankinstein-esque repurposing of parts takes place. The haem constituents of haemoglobin are broken down, with the iron released to be reincorporated into newly manufactured haemoglobin. The remaining haem is converted to biliverdin.

Biliverdin is a green pigment found in bile, which is converted to bilirubin, a yellow pigment also found in bile. These bile pigments were originally thought to be insignificant breakdown products of haem, but more recently it’s been suggested that they might have some beneficial effects in the body. As well as being present in bile, bilirubin is also the reason for the yellow colouration the skin takes on when people are suffering from jaundice. Jaundice is due to high bilirubin levels, which can arise from a number of reasons. Both bilirubin and biliverdin can also contribute to the colour of bruises, as the blood that has leaked into tissues breaks down.


After production in the liver, bile is stored in the gall bladder. As well as containing bilirubin as a waste product from haem breakdown, it also contains bile acids, which help us absorb fats and fat-soluble vitamins during digestion. Bile is secreted into the intestines in order to accomplish this.

In the intestines, bilirubin is broken down further by bacteria into urobilinogen. This compound is colourless, but a significant amount of it is absorbed in the intestines and reenters the bloodstream. There, it is oxidised to urobilin, a straw-coloured compound which is then excreted by the blood-filtering kidneys. Urobilin is the compound that gives urine its typical golden colour.


Not all urobilinogen is reabsorbed into the bloodstream; some remains in the intestines, and continues its journey. On its way, it is converted into stercobilin, the compound that is responsible for the brown colour of human faeces. If stercobilin wasn’t present, your faeces would be more of a pale clay-coloured hue. This can sometimes happen if a condition leads to a blocked bile duct, preventing bilirubin from reaching the intestines. 

In babies, faeces can be a much darker colour (bordering on black) when the baby is first born. This might seem concerning, but it’s actually just the result of the waste present in the baby’s bowel at birth being expelled – a combination of amniotic fluid, bile and mucus. Lovely. The baby’s faeces eventually progress to a normal colour!

It’s fascinating (and sure, a little disgusting) to think that many of the substances that can come out of your body are related to each other by chemical breakdown. Though maybe don’t try to dazzle your family with your newfound knowledge at the dinner table!

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