Blood sampling. Nurse taking a sample of blood from that donated by a volunteer. Blood has collected in the pouch (lower centre) from a tube (bottom) inserted in a vein in the donor's arm. A sample of this blood is being transferred to a vacutainer (upper left). This blood sample will be screened for blood diseases, such as hepatitis and HIV, and typed to determine the blood group. Donor blood is given to patients in transfusions to replace lost blood, for example during surgery or after accidents. It is essential that this blood is clean and that it is of the correct blood type.