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Your nails can do more than just look pretty. They can tell you some things about your health!
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Sources:
https://www.aad.org/dermatology-a-to-z/for-kids/about-nails/how-nails-grow
http://health.clevelandclinic.org/2015/05/6-things-your-nails-can-say-about-your-health/
http://www.ncbi.nlm.nih.gov/pmc/articles/PMC3726881/
https://books.google.com/books?id=ekr4Bt683c8C&pg=PT244&lpg=PT244&dq=nail+pitting+inflammation+of+matrix&source=bl&ots=Vp2EmhSure&sig=-rAZbLYOuhJB6Y-eJraVMod0_p4&hl=en&sa=X&ved=0CEwQ6AEwBmoVChMI-sOBs9PYxwIVxZSICh3Lfg81#v=onepage&q&f=false
http://www.nhlbi.nih.gov/health/health-topics/topics/raynaud
http://www.nhs.uk/conditions/Fungal-nail-infection/Pages/Introduction.aspx
http://www.aafp.org/afp/2004/0315/p1417.html#afp20040315p1417-f6
(SciShow intro plays)

Hank: Fingernails and toenails: they're not just for decoration. They protect your fingers and toes and help you pick up tiny things like splinters or that piece of spinach stuck between your teeth after lunch, and they also tell you a lot about your health.

Nails are collections of dead cells that grow from a root called a matrix hidden just beneath your cuticle, the lighter, crescent-shaped area at the base. The matrix constantly makes new cells called plate cells and each layer pushes the old, dead plate cells out of the cuticle. When nails start to grow differently or even change color, it usually means that there's something wrong with the matrix or with the surrounding tissue.

So by working backward, you can often figure out what's doing the interfering and in the process maybe learn something new about what's going on in your own body. Beau's lines, for example are horizontal ripples on the surface of the nail, they look like little waves and they form when they matrix stops producing new cells for a while. When the matrix starts making new cells again, they push the nail out as usual, but there's an indentation marking the spot where it stopped, kinda like a tree ring.

And there's a reason why the matrix would have hit the pause button, probably it wasn't getting enough nutrients from the bloodstream. usually that means that the person has an infection of some other kind of serious illness. The body shifts its nutrient flow away from low priority activities like growing nails towards high priority activities like... not dying. That's why people who have high fevers for a while often develop beau's lines a month or so afterwards.

Pitted nails are another potential matrix issue, where the nail surfaces have indentations that look like very small potholes. These pits are linked to skin disorders like psoriasis and eczema which can cause inflammation of the matrix. An inflamed matrix produces new plate cells unevenly, so you end up with depressions on the nail surface.

Nails can also change color, something you've probably noticed if you've studied your nails on a cold day and realized they were blue. Generally, that means your extremities aren't receiving enough oxygen. Blood with less oxygen is darker and reflects light differently through your skin, making your nails look bluish. It could just be your body reacting to cold by restricting your blood vessels, but a person whose blood isn't receiving enough oxygen could have a respiratory illness like asthma or emphysema. Blue nails can also be a sign of raynaud's disease, a disorder marked by spasms in a person's blood vessels that narrow them. The narrowing reduces blood flow to the extremities, so they get blue nails.

Now, with blue nails it's not actually the nail changing color, but the bed underneath it, but nails can also turn yellow, and that's actually the nail changing color, this can happen for a lot of different reasons. In most cases, it's caused by a fungal infection called onychomycosis. Yeast or mold sets up shop within the actual nail plate turning it yellow. It doesn't smell to great either.

Other times, yellow nails mean something more serious like yellow nail syndrome, which doesn't sound super serious, but it happens when the matrix does produce new plate cells, but very slowly, so they pile up and create a thicker, yellowish nail plate. But like beau's lines, yellow nail syndrome is caused by something else. It could be a chronic respiratory disease, which would reduce the nail's oxygen supply and slow growth, or it could be a sign of an issue with the lymphatic system which distributes protein-rich fluids throughout the body, usually that issue is cancer or aids.

A black or brown streak in a nail can also be super serious, or nothing at all. In some people, that streak can signal subungual melanoma, a form of skin cancer that affects the nail bed, which is the skin underneath the nail plate. Melanoma often changes the color of the skin, including the skin under nails, but a streak might also be harmless. If you have a darker complexion, it's completely normal. In one study, 77% of black people over the age of 20 reported having a darker streak in their nail.

So it could be something, or it could be nothing, which is why if you're worried about the color or look of your nails, here's a tip: don't get all of your medical advice from the internet! These changes can mean more than one thing, so talk to a real doctor before jumping to any conclusions. We are not a real doctor.

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