The bags contain a specific amount of anticoagulant and require 450ml blood. There's about 90ml blood per kg body weight, so a 50kg person has 4500ml. It's safe to donate 10%. The cutoff in the US is actually 110lbs (50kg).
I'm not sure. Curiously, some types of donation (whole, and double-red apheresis) have a minimum of 110 pounds here, while others (apheresis platelets, plasma) have a minimum of 114 pounds. I'd think if they were worried about scale accuracy or clothing weight, the same safety margin would apply to all donation types -- or at least all apheresis donations.
