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.
Is it true that shaving a baby's head (or cutting his hair very short) makes the hair grow in thicker and stronger? No. That has no effect on how thickly the hair grows in. Hair grows from a follicle beneath the scalp, and what you do to the hair on the surface doesn't affect the hair developing in the follicle.