I'm currently reading this article by Ken Miller on a plausible account of how the blood clotting mechanism in vertebrates might have evolved. There's some things I don't understand but which I hope someone here will explain to me: Here's the article: http://www.millerandlevine.com/km/evol/DI/clot/Clotting.html All quotes are from the hyperlinked article. What does it mean for the gene to be "switched on" only in the pancreas? And more importantly, why does the fact that it being switched on in both the pancreas and liver causes the enzyme to be released into the bloodstream? If serine proteases are, as the article states able to self-activate (by mere presence of amino acids in the bloodstream), and if, by the first quoted paragraph, where the serine protease is released into the bloodstream, then wouldn't it be possible for blood to clot itself even when no blood vessels are broken? If so, wouldn't this be disruptive to normal functioning?