Donated blood. Bag of donated blood awaiting processing. To collect blood a tube is inserted into the forearm of a donor, through which blood drains into a bag. A bag holds about 475 millilitres. When collection is complete the tube is sealed with a clamp (green) and the blood is taken for storage or separation into the different blood components. Blood bags are carefully labelled so that they can be matched to samples that are taken for screening for blood diseases, and typed for blood group. Such samples are sent to laboratories in vacutainers (centre left).