A bruise occurs when small capillary blood vessels break under the skin.
The hemoglobin in this leaked blood gives the bruise its classic red-purplish hue.
The body then calls in white blood cells to repair the damage at the site of the injury, causing the red cells to break down. This causing produces the substances that are responsible for the color changes.
The breakdown products of hemoglobin are biliverdin, which is green; and then bilirubin, which is yellow. Later, the debris at the bruise site clears and the color fades.
It is the same process that disposes of red cells past their use-by date. White cells called macrophages break down defunct red cells in the spleen, liver, bone marrow, and other tissues. Bilirubin is taken up by the liver, where it is converted to bile and used in the digestion of food. It is bilirubin that helps to give feces their characteristic color.
The breakdown product of hemoglobin, bilirubin, is yellowish in color and is normally excreted from the body as a component of bile. Bile itself is secreted to help digest fat. This is very efficient recycling.
An accumulation of excess bilirubin in our body can occur in medical conditions such as hepatitis, giving the skin a yellow tinge also known as jaundice. One can sometimes observe this in some newborn babies.
Jaundiced skin will itch because bilirubin is an irritant, while bruises are tender to touch. Ultraviolet light helps in breaking down bilirubin and is also the treatment for jaundiced babies.
Bruises sometimes take a long time to appear because the damage can occur deep in the body tissues.
The body under the skin is not of course an amorphous mass, it has discrete muscles and organs, separated by planes of fibrous tissue, these can be seen clearly when we look at joints of meat from the butcher.
When blood leaks from damaged vessels it is often prevented from reaching the skin’s surface quickly by these planes of tissue, or it may simply take a while to diffuse through subcutaneous tissue.
The fibrous tissue sheaths also explain why a bruise occasionally appears some distance from the original impact, the leaking blood has tracked under the sheath and surfaces only where the fibrous tissue ends.