What Are Bruises? What Causes Them?

Medically Reviewed by Sanjay Ponkshe on May 13, 2023
3 min read

Bruises are so common that you probably don’t think twice when you see a small black-and-blue mark on your shin or forearm. A bruise is a sign that you’ve been injured, but it’s usually not the sign of a serious injury.

To get a bruise, you have to be hit by something or you have to run into something. When this happens, small blood vessels are broken and bleed. Because there’s no cut in the skin above them, your red blood cells are trapped under the skin after your tiny blood vessels carrying them have burst open. So the blood pools at the site of the injury, leaving a mark for a while.

A bruise doesn’t just discolor the skin. It can be painful, too. And there’s likely to be swelling at the site where you’ve been hurt.

The pooled blood beneath your skin looks different as time passes, from the moment you get hurt to when you’re fully healed.

At first, a bruise is a reddish color, just like the blood beneath the surface.

After a day or two, as the oxygen from the blood cells is resorbed, the color of the bruise changes to purple, blue or black. This is why many people call bruises “black-and-blue marks.” The sore spot may stay this shade for up to a week.

After the black-and-blue phase, the bruise begins to fade. It turns green or yellow from the breakdown of hemoglobin, which carries oxygen in your red blood cells. A few days later, it turns brownish yellow or light brown.

In about 2 weeks, the bruise should be fully healed, and the skin should appear normal again.

Go see your doctor if you start to have a lot of bruises and you don’t remember getting hurt.

Many things increase your chances of bruising:

Being active. Children who are learning how to run, ride a bike or play a sport may bruise if they fall down or crash into things.

If you play contact sports, you’re likely to get bruises. Boxers may get black eyes. Soccer players may get bruised shins. Football players may get bruises on their arms and legs. You can also get bruised if someone hits or kicks you.

As you get older, you may be more likely to fall when you lose your balance or trip over something.

Taking medicine. Certain types of drugs can make you more likely to bruise:

  • Blood thinners. If you start taking a blood thinner or another drug (like aspirin) that makes it harder for your blood to clot, you may get more bruises than you’ve had in the past. This is because every time you bump into something, if you happen to break any small blood vessels beneath your skin, it will take much longer for your blood to clot, so a fair amount of blood will pool at the site of your bump.
  • Corticosteroids (steroids). These drugs can cause your skin to thin, so there’s less buffer between anything that you bump into and the small blood vessels beneath your skin. They’re more likely to break and bleed when they have less protection.

Aging. Older people are more prone to bruising than younger people. This is because the blood vessels beneath the skin are more fragile and more likely to break as someone ages.

Also, your skin thins as you get older, so there isn’t as much fat beneath the skin. That layer of fat you had when you were younger helped cushion the blood vessels from blows. That’s why the same sort of contact that causes a bruise today didn’t leave a mark on you years ago.

Having a health condition. Some diseases can make you more likely to bruise:

  • Blood-clotting disorder. If you have a blood clotting disorder such as hemophilia, you’re more likely to bruise than someone without the condition. If your blood takes a long time to clot, then more blood will pool at the site of the injury.
  • Blood disease. A blood disease can also lead to more bruising than normal. People who have certain forms of leukemia bruise easily if their platelets are low, even when they barely bump into something. Low platelets are also seen in cirrhosis and can cause increased bruising. In severe renal disease, platelets don't function normally (called uremia) and you can also see increased bruising.