Why Do We Get Nosebleeds?

One moment, you're fine. The next, moment it seems like your nose is recreating a scene from The Shining. Why do we get nosebleeds?

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Sources:

http://www.uofmhealth.org/health-libr...
https://www.ncbi.nlm.nih.gov/books/NB...
http://www.aafp.org/afp/2005/0115/p30...
http://care.american-rhinologic.org/e...