MODEL RELEASED. Blood donation. Blood donor lying down, with a tube inserted in her forearm to collect blood. She is squeezing a cylindrical object to encourage good circulation in her arm. The blood collects in a bag (bottom) that is attached to a weighing scale. Usually about 475 millilitres of blood is collected. Mobile blood units travel to prearranged areas, such as shopping centres and work places, so that potential blood donors can attend at their convenience. Donor blood is given to patients in transfusions to replace lost blood, for example during surgery or after accidents.