Why do bruises change colour

A. Bruises change colour over time, because of the degradation of haemoglobin, the red oxygen carrying pigment red blood corpuscles, in the blood.

Q. So it is possible to tell how old a bruise is from its colour

A. Only roughly as the exact timescale of haemoglobin degradation isn't absolute.

Generally speaking a fresh bruise starts off dark blue or purple and goes from brown to green and ends up yellow. In a normal healthy person a bruise can take as little as 72 hours or as much as one week to pass through this colour spectrum. One thing to bear in mind is that the deeper the bruise, the longer it'll stick around for.

Q. What causes bruises

A. Bruises appear after the skin tissue has been injured in some way, usually by a blunt blow.

If, as a result, the capillaries, tiny blood vessels, rupture, the blood leaks out, technically known as 'extravasation', from the damaged capillaries into the surrounding tissue.

Q. I often get bruises away from places where I actually knocked myself, why

A. Because blood can spread out quite a far way from the site of original injury. It's not unusual for a bump on the top of the head to result in a black eye.

Q. Why do my bruises take days to materialise

A. For the same reason as the above answer. Leaked blood spreads and it can often take some time doing this.

Q. Why don't my hands bruise

A. Different parts of the body are more sensitive than others. For example the eyelids are very prone to bruising while the palms of the hands and soles of the feet hardly ever bruise.

Q. Why do some people bruise more easily than others

A. There are various factors involved. Obviously if someone is involved in a job or sport that leads to more bumps and knocks than average then they're more likely to end up with bruises.

People with skinny arms and legs don't have as much fat to protect their capillaries from damage.

Alternatively there may be some problem with a person's liver, which manufactures important blood clotting substances.

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by Lisa Cardy