Blood donation. Blood bag on a weighing scale. As blood is collected from a vein in the forearm of a donor (not seen), it is weighed on this balance. The digital display of the balance shows that a volume of 215 millilitres (ml) of blood has been collected so far. Usually a total of about 475ml is collected. This blood will be given to patients in transfusions to replace lost blood, for example during surgery or after accidents.