Weight is a good guideline for blood volume in people of average build. This is why we set a minimum weight limit of 7st 12lbs (50 kg) because if more than 13% of blood volume is taken there is more risk of the donor feeling faint after donating.
Not surprisingly, the one factlet most often offered up as revealed truth is wholly false: Your hair and nails do not continue to grow after you die; it's all optical illusion. The sorry fact of the matter is that we dehydrate after dying. Our flesh dries and, in so doing, pulls away from nails and hair.
So here's The Straight Dope. Hair and fingernails aren't actually growing. Dead is dead. The only things growing after you're dead are worms, bacteria, and flowers. What is happening, however, is that the skin around the hair and fingernails will desiccate (i.e., lose water) and thereby shrink. When the skin shrinks, it retracts, making hair and fingernails look longer, as if they'd grown. Think of it this way: a fifty foot tree grows in ten feet of swamp water. The visible part of the tree is 40' tall. A few months later, a drought causes the water level to drop five feet.