Bruises happen when connective tissues and muscle fibres under the skin are compressed but the skin doesn’t break. With a graze or a cut, the blood can easily escape but with a bruise, blood from your ruptured capillaries can’t. And with nowhere to go, the blood is trapped.
Bruises generally go through a whole range of colours. If you have a really nasty whack, it will start off red then within the first couple of days turn a purply-bluey colour. Within 5 to 10 days, it develops into a greeny-yellow and finally, before it fades away after about 10-14 days, it’ll go yellowy-brown. This obviously depends on the severity of a bruise, your age, if you’re on any medication or have an illness which means you’ll bruise more easily.
When the red blood cells break open after you hurt yourself, they release substances which signal the body to send white blood cells to the healing site and cause all these weird colour changes. The white blood cells consume the hemoglobin and then release chemicals which cause inflammation.
Your bruise will start off red because the tissues surrounding it fill with blood. The most significant substance of red blood cells is hemoglobin – the protein which makes blood red. The next colour, purple, is caused by the hemoglobin being broken down and losing its oxygen. This happens immediately after the injury and persists until all of the hemoglobin is broken down. The first breakdown product of hemoglobin is called a chemical called biliverdin, which appears within a few days. Biliverdin is green which is why after the bluey-purply phase, your bruise will go green.
The green biliverdin is then converted to a yellow molecule called bilirubin. And finally the brown colour, just as your bruise vanishes, is caused by hemosiderin – another chemical converted from bilirubin. And there we go. From red to blue to green to yellow to brown. Bruises really are fastinating.