Apheresis is a special blood donation procedure in which plasma or selected cellular elements, such as platelets or white cells, are separated from the other parts of the blood and retained. Blood is drawn and processed through a cell separator, and the other cells and plasma are returned to the body. Apheresis takes approximately two hours compared with a whole blood donation of about eight to 10 minutes. During apheresis, the patient or donor is connected to the apheresis machine. A needle is inserted into a vein in each arm. Blood is withdrawn from one arm, run through the machine to extract the stem cells, and all the remaining components of the blood are returned to the donor through the needle in the other arm. In the case of a patient donating their own stem cells, this process is performed using a central venous catheter, which has two lumens.