But Malcolm ' s left hand was the hand that was cut off , and Prometheus got cut on his right <| sep |> I actually had to look up ' gill ', because I ' ve never heard that used as a unit of measurement before and I ' m English , they still use ' stones ' for weight over there ffs . Assuming that it ' s not a convinient typo , a gill would be 142 ml imperial or 112 ml American , because fuck even having a consistent number with the measurements that already make no fucking sense , right ? . Your volume of blood is dictated by the amount of oxygen required by your body to function . The bigger you are , the more tissue you have to supply oxygen to . The more muscle you have , the higher the O 2 requirements per % of your total body volume muscle needs far more oxygen to functionsurvive than fat etc . Of course you can ' t drain someone of 23 of their total blood volume and expect them to function without issue , but you could happily take 6 ' gil ' from someone 852 ml by maximum value and they should be pretty much fine , though probably pretty uncomfortable if they were on the smaller side . This is why they have heightweight cutoffs for donating blood you take ~ 450 ml with a standard blood donation , as that ' s the really conservative amount at which most people are going to feel little to no ill effects from .