As far as I can tell, blood loss can result in fainting, which, of course, is bad news when something is trying to kill you, and I assume you can bleed out if the bleeding continues without being stopped (and I've never seen bleeding stopping spontaneously on the PC, even though it's common in animals). I've lost dogs and companions to blood loss after a fight because I simply didn't have enough time to treat the companions in time (saving 2 and losing 2, or something), as well as before treatment of others was possible.
I've had a case where my character at the time ran away from a fight, fainted, woke up, managed to stop the blood flow, and then survived (because the the threat had presumably been unable to find my character).
I don't treat wounds that risk opening when my character has lost a lot of blood, but do when it should be possible to treat the wound before it becomes dangerous in case the wound would reopen.
I expect the UrW blood recovery is based on time alone, but I haven't been in a situation with a blood loss and a lack of food (and never any serious lack of water).
Given how UrW behaves, I would guess the blood loss meter indicates not total amount of blood, but when you would have a 50% chance of fainting, or something like that, before taking resistance/susceptibility to fainting into consideration.