Bruises, also commonly called bruises or contusions, are usually caused by broken blood vessels. When the body is injured, the skin is not always broken, but the muscle and tissues under the skin can still be damaged. Small capillaries and veins can rupture, allowing blood to pool in the area. Some conditions can also lead to bleeding that causes bruising without any type of injury. Most types of light bleeding under the skin are those that cause the discoloration known as a bruise.
There are three general types of contusions: subcutaneous, intramuscular, and bony or periosteal. Subcutaneous contusions are bruises just under the skin and are usually caused by minor bumps or injuries. In the case of intramuscular hematomas, the bruises and the blood are within the muscle itself. With these bruises, the original injury is usually severe enough to bruise the skin, but those superficial bruises can heal much faster than the muscle injury. Muscle bruises can remain long after visible skin bruising has faded.
The third type of bruise, a bone or periosteal bruise, is usually the most painful. In this case, the bruises and blood are inside the bone itself. These are usually deep bruises that are sometimes misdiagnosed as bone fractures. Periodic bruising and blood collecting in bone tissue usually last much longer than bruising to the skin or muscles.
Most visible bruises start out pink or red and may darken after a day or two. The initial pink or red color is the red blood showing through the skin. As the blood ages under the skin, it begins to break down and becomes darker, changing the bruise from red to shades of purple or black. Aging bruises can take on a green or yellow hue as they fade, as the remnants of blood under the skin break down further. Eventually, the body tissues reabsorb the blood, and the bruise usually goes away within days.
In most cases, bruising and bleeding under the skin are not serious and will usually resolve on their own. People who bleed easily, such as hemophiliacs or people taking blood-thinning medications, may need to watch for bruising as a sign of internal bleeding that may be serious. If unexplained bruising and blood collecting under the skin appear, it could be a sign of a reaction to a medication or a medical condition that may require treatment. However, in most cases, a bruise is simply a sign of a minor bump or injury that will usually go away in a week or two as the blood is reabsorbed by the body.