Once the vessels are ruptured, the area swells as scavenger cells flood in to cart off injured cells. The swelling cuts off oxygen, making the hemoglobin in the blood turn blue. Later, as the hemoglobin breaks down, the bruise turns yellowish green and then brown. If you lightly bump your forearm on a doorknob, he says, a minor number of blood vessels may be damaged and the resulting bruise is barely noticeable. You'll wind up with a nasty bruise the size of a hockey puck.
Women generally have skin that is more easily bruised than men.
One reason is that their skin is thinner, possibly because the female hormone estrogen softens the blood vessels and affects the supporting network of collagen beneath the skin. Aspirin interferes with platelet function for several days. This means that if you take one aspirin today, it can still interfere with clotting five to six days later. Bumping into a desk corner during that time can make you look like you got in the way of a champion kick-boxer. Aspirin may also contribute to bruising in men.