Can anyone corroborate this? Somehow having 25% of my entire body's blood supply in my head seems a bit much.
About 20% of blood pumped from the heart goes to the brain itself, so if you count all the other tissues of the head - skin, muscle, glands, etc., 25% total doesn't seem too far off. But since the brain is encased in the skull and uses separate branches of vessels off the carotid artery, lacerations in those other tissues aren't accessing or siphoning off that brain blood, unless of course the injury is to one of those major vessels in the neck before they branch off.
I've seen a shitload of facial and scalp lacerations, and although they do bleed more than other parts of the body given their dense vasculature, unless you clip a good-sized artery they are relatively easy to stop from bleeding using direct pressure for a few minutes and maybe an icepack. Treatment after that would depend on the size and extent of the wound, which could require anything from a simple dry dressing to steri-strips, sutures, surgical glue, or surgical repair.