Although most blood is donated as whole blood, it is also possible to donate only a portion of blood using a technique called apheresis. Blood is drawn from the vein of a donor into an apheresis instrument, which separates the blood into different portions by centrifugation. By appropriately adjusting the instrument, a selected portion of the blood, such as the platelets, can be recovered, while the rest of the blood is returned to the donor either into the same vein or into a vein in the other arm. This process takes more time than whole blood donation, but the yield of platelets is much greater. Platelets collected by apheresis are particularly useful for patients who require numerous platelet transfusions, for example cancer patients who have received chemotherapy.