Asked by: Karen Stephenson, Tonbridge
Bruises are injuries that cause bleeding under the skin. Because this blood has leaked from the capillaries, it isn’t part of the circulatory system any more and lies stagnant in the spaces between your cells. Initially the haemoglobin in the red blood cells will still have some oxygen bound to it and so will appear deep red. Within a few minutes, that oxygen will be released to the surrounding tissues and the bruise will darken to more of a purple colour.
After that, white blood cells are mobilised to begin the clean up. Over the course of a week to 10 days, the dead blood cells are broken down, engulfed and metabolised. The haemoglobin pigment is progressively converted into biliverdin (a greenish colour) to bilirubin (yellow) to hemosiderin (golden brown) before finally being cleared away.
Subscribe to BBC Focus magazine for fascinating new Q&As every month and follow @sciencefocusQA on Twitter for your daily dose of fun science facts.