MODEL RELEASED. Blood donation bag held by a laboratory assistant. The bag contains about half a litre of blood. An anti- coagulant chemical is added prior to donation to prevent the blood from clotting. Following donation, a sample of the blood will be screened for pathogens. The blood can be used whole or can be split into its constituents of white and red blood cells and plasma. The red blood cells can be concentrated and given to someone who has lost a lot of blood. Plasma can be given to someone who has suffered a large fluid loss, such as a burns victim.