Blood donation. Blood donor lying down, with a tube inserted in a vein in her forearm. The donor is squeezing a cylindrical object in her hand to improve the circulation in her arm. Blood is collected in a blood bag (centre right) on a weighing scale. The digital display of the balance shows that a volume of 10O 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.