I skinned my knee ten days ago and the blood ceased to flow within a couple of hours. I'm still alive.

Are you sure you're not a robot? :) (Sorry.)

And, if it just needed to be Summers blood, the obvious thing to do was to track down Hank and keep him stashed somewhere in case they needed to throw him into the portal.

That would actually make a pretty good fic. "Um, Buffy, why are you blindfolding me? And why is it so windy, and why does it feel like I'm walking towards the edge of some rickety construction...?" "Don't worry about it, dad, just keep walking."