Why does a bruise change colour as it fades?
Think of a time you bumped your leg on the edge of a table. A few hours later, a purple spot showed up on your skin. When you pressed it very gently, it felt a little sore.
Over the next days, that spot did something surprising. It turned green, then yellow, and then it was gone. So why does a bruise change colour as it fades?
Under your skin run blood vessels, which means the thin tubes that carry blood all around your body. A hard bump squeezes the skin so fast that some of the smallest ones break open.
Blood leaks out of the broken tubes into the soft space under your skin. Your skin stays whole on top, so the blood is trapped there. You see it shining through as a dark patch, and that patch is the bruise.
The trapped blood can no longer pick up fresh air from your breathing, so it turns darker. That is why a new bruise looks deep purple, almost blue.
Your body does not leave spilled blood lying around. White blood cells, which means little helpers in your blood that clean up messes, travel to the bruise to clear the old blood away. How do you think such small helpers could clear away a whole patch of blood?
They break it into smaller pieces, one bit at a time, the way you might eat a big cake slice by slice. When the white blood cells break the red colouring in blood the first time, it turns green. So the purple bruise starts to show green edges.
Next, the white blood cells change the green colouring once more. This time it turns yellow, so the bruise turns yellow too.
Your blood vessels nearby carry the yellow pieces away, and each day less is left under your skin. The colour grows paler and paler, until your skin looks just like it did before the bump.
Blood vessels and white blood cells. A bruise changes colour because a bump breaks your blood vessels, and white blood cells break the trapped blood down from purple to green to yellow until your blood vessels carry it all away.
Next time you spot a green or yellow bruise, you will know your body is busy cleaning up inside. Rest well, and let your helpers do their quiet work.
