What’s happening? Blood vessels under the skin have been broken and are leaking blood into the surrounding tissues. Bruises usually start as purple. As we all know from middle-school biology, blood that has not been oxygenated by your lungs is a dark color, which is why bruises first appear with that telltale eggplant tone. They then evolve through a series of colors, finally ending with a faint yellow/brown as the body begins to break down the pooled blood into a series of waste products and clear them away.
Why is your body doing it? A bruise is simply an indication of some kind of bodily trauma—minor or major. “It doesn’t serve a purpose,” says Cain. “It’s just a sign of your body’s ongoing healing attempt.”
What should you do? “The best treatment for a bruise is rest and ‘tincture of time,’ ” says Cain. Elevating the injured area and applying compression to it—as you raise your arm and then get it wrapped with a dressing after giving blood—may decrease the size of the bruise if you do it soon after the injury. If you’re really sore, Cain recommends trying over-the-counter painkillers such as naproxen or ibuprofen. You should see a doctor if you bruise very easily (from the most minor injuries), your bruises do not go away, or if you have a very large bruise (for instance, it covers a large part of your arm or leg), which can be a sign that you have lost a lot of blood.