[Reg]

Reg asked the Naked Scientists:
   
When I get a bruise on my toe between the knuckle and the nail, it grows out as a scab under the nail. But when I get a bruise anywhere else, it just disappears. What happens to the bruise? (I know it must come out as waste, but how does it get there?)

What do you think?

[chris]

There are several aspects to this.

A bruise is blood within tissue but outside a blood vessel. The usual cause is trauma, which damages blood vessels and allows red blood cells to escape. These are initially well-charged with oxygen, so the bruise starts off a red colour. But quite quickly the oxygen is consumed, which makes the haemoglobin in the red cells alter its colour to become bluey-purple. This is the colour of the mature acute bruise.

The next thing that happens is that immune cells move in to repair the damage and clear up the debris. Phagocytes (white cells) consume the blood cells, releasing the haemoglobin that they contain. This is dismantled metabolically inside the cells into its component parts. The globin protein component is broken down to amino acids whilst the iron-containing "haem" part, known as a porphyrin ring, is released.

Further enzymes, including one called microsomal haem oxygenase, then break open the haem molecule to release the iron atom and also produce an organic substance called biliverdin, which is a green colour. This biliverdin is subsequently reduced to its chemical relative bilirubin, which is yellow. The iron, meanwhile, is grabbed by a molecule called haemosiderin, which produces a browny-orange pigment.

As these breakdown products are often free within the tissue, they do their own chromoatography experiment, spreading away from the site of the original injury, often with help from gravity (so a bruise to the upper arm can often cause discolouration that tracks down to the elbow). This is why bruises spread out and also display a range of colours (the purplish haemoglobin, the greeny biliverdin, the yellow bilirubin and the rusty haemosiderin) as they evolve and organise (are repaired).

Eventually these chemicals are absorbed back into the bloodstream or carted off by white cells, and the bruise disappears.

With subungal (under the nail) haemorrhages - as occur when the tin can falls off the shelf onto the big toe) - the process is slightly different because the blood is trapped between and within the nail and the nailbed, so it cannot so easily be broken down as it can within tissue. This is why these "black" nails need to grow out, taking the bruise with them.

Chris