Watching blood being cleared up
Why Does a Bruise Change Colour?
A bruise is blood spilled under unbroken skin, and its colours are the stages of that blood being taken apart.
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Step 1 of 5
A bruise is a leak with the lid on.
A knock tears small vessels but leaves the skin whole, so the blood has nowhere to go. It spreads through the tissue and sits there.
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Step 2 of 5
Fresh, it looks red, then blue-purple.
Just-spilled blood is still carrying oxygen and looks red. Within a day it has given that up, and dark blood seen through skin looks blue-purple.
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Step 3 of 5
Cleaning cells arrive and take it apart.
White cells move in and start dismantling the haemoglobin. It cannot simply be reabsorbed whole — it has to be broken into pieces first.
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Step 4 of 5
The pieces are coloured, and they change.
The green stage and the yellow-brown stage that follow are two different breakdown products. Seeing them means the bruise is being cleared, not getting worse.
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Step 5 of 5
It sinks and spreads before it goes.
Trapped blood drifts downward through the tissue, which is why a bruised shin ends up coloured near the ankle and looks bigger than the knock that caused it.
A newborn's jaundice is the same yellow chemical from the same process — a baby's liver simply cannot clear it fast enough at first.
The short version
A bruise changes colour because trapped blood is dismantled in stages, and each breakdown product has a colour of its own.
Try it yourself
Photograph a bruise once a day. The sequence — purple, green, yellow — is the same every time, because the chemistry is.
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