If you were to make a surface cut in a bruise, would the blood come out green?
The colour of a bruise is dependent on several factors. The amount of blood released into the tissues, the age of the bruise, the colour of the skin and the position on the body. The changing colouration is due to the blood infiltrating the tissues after the initial blow has broken small blood vessels. The blood, specifically the red blood cells, are then broken down in the tissues as part of the healing process. The red colouration, haemoglobin, in the red blood cells is composed of various pigments, as they are broken down they alter colour and give the well known mosaic green/blue/yellow effect. Blood does not oxygenate on exposure to air, it starts to dry and darken as you see in a scab formation. Veins appear blue because you are looking at the blood through the skin and the walls of the vein. Arteries are usually much deeper in the tissues/muscles and not normally visible, but if they were they would look the same as veins.
