Blood donation. Nurses and blood donors at a blood donation session. The donors are lying down, with tubes inserted in their forearms to collect blood. The blood collects in a bag (not seen) that is suspended beneath the bed. 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.