Why do bruises change colour?

Whether you’ve taken a tumble on your bike or whacked a knee on the coffee table, you’re probably familiar with the feeling: “ouch, that’s going to bruise.” In that moment of throbbing pain, you know your injury will leave a mark that will last a couple of weeks and cycle through a rainbow of nasty-looking colors. But what exactly goes on under the surface of your skin to make that happen?

A bruise occurs when some sort of trauma — like bumping your knee or stubbing your toe — damages your blood vessels but doesn’t break your skin. Tiny blood vessels called capillaries are sandwiched between your skin and other tissues in your body, so when they burst, their contents get trapped there. Your body’s job is to reabsorb the leaked blood, leading the bruise to eventually heal. And the healing process is a colorful one.

Bruised capillaries release fresh blood that’s colored red by hemoglobin, a protein that transports oxygen throughout your body. The oxygen-rich blood causes the injury to appear reddish at first. But after one or two days without circulation, the hemoglobin begins to break down, coloring the bruise blue, purple, or even black.

As the trapped hemoglobin breaks down, your body essentially repurposes it for spare parts. The iron in hemoglobin can be used to make new red blood cells, for example. With that iron gone, the decomposed hemoglobin turns into biliverdin, a green pigment responsible for the olive hue of a week-old bruise.

Finally, your bruise will turn yellow or light brown to mark the end of the healing process. That color comes from bilirubin, a byproduct of hemoglobin breakdown that’s also responsible for jaundice. Once your liver filters out the bilirubin, your bruise should fade away.

What about bruise shape? If you’ve ever gotten a direct hit from a paintball or felt the impact of a baseball batter’s line drive, you’ve probably seen a bruise heal in a donut shape, with a ring of darkened bruising around a lighter center area. That’s called central clearing, and it happens when the trauma is severe enough to force blood away from the point of impact.

Most bruises will disappear within two weeks on their own, but in certain situations, you might need some help from a doctor. One such problem is a hematoma, which looks like a bruise but can be much more dangerous. While a bruise comes from damage to smaller capillaries, hematomas usually result from more severe trauma to a larger blood vessel.

When a hematoma occurs, blood can actually get trapped in the tissue surrounding the damaged blood vessel. A surface-level hematoma may look like a red, fluid-filled lump, or in the case of a subungual hematoma, a dark spot under your nail. A deeper hematoma, while less visible, can pose a threat to internal organs and may require surgery to drain the blood.

However, the majority of bruises and hematomas are relatively harmless in the long term. For short-term relief, the best treatment is simple: RICE. No, not comfort food — rest, ice, compression, and elevation will slow the bleeding and reduce swelling. After a couple of days, switching from ice to heat will help clear away trapped blood by stimulating circulation. Or you could do absolutely nothing, sit back, and watch your body work its magic.

For me, one thing I do use to treat bruises is a bag of frozen vegetables. It doesn’t matter what kind — carrots, peas, lima beans, whatever — as long as they’re frozen. A bag of frozen vegetables is easy to apply to the bruise because it can form to the shape of the injured area. Also, like a cold pack, it can be used and refrozen again and again (just pick your least-favorite vegetables and label the bag — you don’t want to keep thawing and freezing veggies that you plan to eat!).

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