Sargia Harrison, who is homeschooled at Kinross, asks :-

How come bruises come up purple while blood is red?

David Heaton, a Haematologist (Blood Doctor) at Christchurch Hospital, responded.

Blood, as you say, is red and this is because of a coloured substance in the red cells called haemoglobin. Haemoglobin carries oxygen from the lungs through the body and while doing that, its colour is bright red.

After it loses its oxygen to the tissues however it has a more bluish colour. Blood flowing through the skin is generally carrying plenty of oxygen so a person's skin looks pink. However if the blood is in a bruise under the skin it will lose its oxygen to the tissues and change to a darker colour.

The brown pigment (melanin) in the overlying skin also influences the colour of the bruise so that it now looks purple. In medical language we call bruises purpura which comes from the Latin word meaning purple. You may have also noticed that after a few days a bruise usually turns yellow. This is because the body is breaking down the haemoglobin to bilirubin. Bilirubin in Latin means red bile. Bilirubin however is not red but yellow. Rather confusing really.