If you suppose HP has anything to do with blood volume (which it does not) then you can estimate that:
A commoner has 4hp, in the real world the average adult male has about 10 pints of blood.
When I worked in blood bank (I'm Medical Lab Tech by trade) if a person came in with a HGB <10 and needed a transfussion we would give them 1 unit/~pint of blood. A normal HGB is ~12-15. Pts with a HGB of less thatn 8 were generally considered "critical". So you can only lose abotu 1/3 of your blood before you are considered to be in a life threatening situation.
So by this you could lose ~3-3.5 pints of blood and still be alive, but in critical condition.
If HP is assumed to be blood volume (which, again is a poor idea) then a human commoner should be incapacitated when they take 2hp of damge and be bleeding a lot when only taking 1. Anyway, after taking even one point of damage would leave a character with a wound bleeding bad enough to kill the PC without treament.
This would make for a really bad game.