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Why is poo brown?

We eat foods of many different colours, but they always come out the other end in shades of brown and yellow. Dr Karl travels through the human body to find out why.

How come food enters your body in so many different and attractive colours, but always comes out brown? And why does water enter your body colourless, but always come out yellow?

The cause is the same for both brown poo and yellow wee — it's chemicals from dead red blood cells.

The story begins in the bone marrow. It takes about seven days to make a red blood cell, and your bone marrow manufactures around 2.4 million of them each second.

Red blood cells take about 20 seconds to do a complete loop of the body. They get pumped out from the heart to the periphery, then flow back via veins into the heart and into the lungs to be oxygenated, and back into the heart ready to be pumped out again.

Your blood is about 45 per cent cells (over 95 per cent of them red blood cells), and 55 per cent salty water.

Red blood cells are much smaller than most of the other cells in the human body — about seven microns across. (A micron is one millionth of a metre. For comparison, a human hair is about 70 microns across.)

Red blood cells make up about one quarter of the 100 trillion cells in your body — but they don't make up one quarter of your weight, because they are so small.

Each red blood cell carries about 270 million haemoglobin molecules. Between all of them, they carry 2.5 grams of iron, which is about 65 per cent of your body's total iron stores.

Haemoglobin has two parts — 'haem' and 'globin'.

I'll talk about the 'globin' part first, just to get it out of the way. There are four 'globin' molecules in each haemoglobin molecule. (And yes, they are kind of globular). Each 'globin' is quite big, and is made from various amino acids.

Now for the 'haem' part, which ends up making poo brown and urine yellow. Haem is quite a small molecule — about 100 times smaller than a globin molecule. Right in the middle is a single atom of iron.

Getting back to the red blood cells — they are very flexible. They have to be, because on each circuit of the body, they have to squeeze through capillaries that are about 20 per cent smaller than the red blood cells themselves.

The red blood cells get old after about 100 to 120 days. They get more stiff, and find it more difficult to squeeze through the capillaries. Then they get recycled in the spleen.

Have you ever wondered just what the spleen does? Well, it does immune system stuff, it stores spare blood for use in an emergency, and it also breaks down red blood cells.

The spleen is an organ on the left side of your tummy, high up and under the ribs. It's about 11 centimetres long, and weighs between 150 and 200 grams. It 'catches' the old and inflexible red blood cells with mechanical filtration, and breaks them down. It takes a lot of energy to make haemoglobin, so your body doesn't break it all the way down to its individual atoms.

The 'globin' part of the haemoglobin molecule is broken down into individual amino acids. They are recycled to make proteins, including haemoglobin.

The 'haem' part of the haemoglobin is broken down into iron and a chemical called 'biliverdin'.

From here on, the biochemistry gets very complicated, so I'll ignore the fancy stuff, and cut straight to the summary.

Poo is brown and urine is yellow because of a chemical that gets released when red blood cells are broken down; This chemical goes through seven major 'transformations' before it ends up as the brown chemical that gives the brown colour to faeces; During the process of being 'transformed', it gets shunted all over the body. It travels between the spleen, the bloodstream, the liver, the gall bladder and the duodenum before it finally exits your body into your toilet bowl — giving your faeces that familiar brown colour; During one of the intermediate stages, this chemical is yellow in colour. It 'escapes' from the gut into the bloodstream, but is picked up by the kidneys and ends up in your bladder. This time, it leaves your body via your urethra, and again it enters the toilet bowl — giving your urine that familiar yellow colour.

It seems strange to have so many complicated steps in making your poo brown — something seeming so simple on the faeces of it.

© Karl S. Kruszelnicki Pty Ltd 2011

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