eli5:Why do bruises under fingernails grow out, but bruises elsewhere just fade and go away?
In: Biology
A bruise is caused by capillaries, very small blood vessels, bursting due to some kind of trauma, so blood pools in that area. If they burst under your nail, some of that blood could be trapped between the nailbed and the nail itself. Whatever blood attaches to the nail gets moved out as the nail grows.
When you get a bruise elsewhere, your lymphatic system gradually absords the blood, since at that point it’s dead, and it eventually gets filtered out in your kidneys and passed in your urine.
The ones you see grow with the fingernail are not bruises – bruises are blood under the skin. What you’re seeing under the fingernail is blood ABOVE the skin, trapped between it and the fingernail. The bleeding stops, the blood dries to the inside of the fingernail but can’t get washed off. So, it moves with the fingernail as it grows until it gets cut off or hits open air where washing your hands gets water under that portion of the fingernail.
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