The red pigment of blood – haemoglobin serves two purposes; it carries carbon dioxide from body tissues to the lungs where it gives out carbon dioxide and takes up oxygen. Secondly, it carries the oxygen to various body tissues. Veins carry deoxygenated blood which has given up the oxygen. Deoxygenated blood appears slightly blue-red or purplish in colour. Since veins are thin walled and are usually located near the surface, they are visible through the skin and the deoxygenated blood coursing through them gives them bluish colour.