I've tried reading around, but all I find is all sorts of whacky names for various substances in the human body and can't make head or tail of any of it! Two parts, really:

Firstly, does blood always clot on contact with air? I was of the impression it did, but the things I have looked through have made no mention of what part of the atmosphere would cause clotting. If I left a blood sample out, having extracted it "non violently" (i.e. by not cutting the skin or whatnot), would it clot?

Secondly, whatever the answer to the above is, what is the mechanism for clotting? If via the atmosphere, what triggers clotting but is prevented from doing it inside our body (as Nitrogen/Oxygen are both quite plentiful in our blood), and if otherwise, what's the process?