Why do bruises go through a range of colours before they fade? I can see why they would be red or purple, but what accounts for the yellowish-green colour? And why do they often take a day or two to appear? Surely the damage and bleeding occurs at the time of injury.
鈥� A bruise occurs when small capillary blood vessels break under the skin. The haemoglobin in this leaked blood gives the bruise its classic red-purplish hue. The body then ropes in white blood cells to repair the damage at the site of the injury, which causes the red cells to break down. This produces the substances that are responsible for the colour changes.
The breakdown products of haemoglobin are biliverdin, which is green, and then bilirubin, which is yellow. Later, the debris at the bruise site clears and the colour fades.
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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 faeces their characteristic brown colour.
Claire Adams, Belmont, Western Australia
鈥� The breakdown product of haemoglobin, bilirubin, is yellowish in colour 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 know 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.
Frankie Wong, Sydney, Australia
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 鈥� it has discrete muscles and organs, separated by planes of fibrous tissue (these can be seen clearly when looking at joints of meat from the butcher). When blood leaks from damaged vessels it is often prevented from reaching the skin鈥檚 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 鈥� the leaking blood has tracked under the sheath and surfaces only where the fibrous tissue ends.
Stewart Lloyd, Consultant Occupational Physician, Brigg, North Lincolnshire, UK