There are approximately eight million volunteer blood donors. About four million patients receive transfusions of blood units each year. Each unit of blood is broken down into components, such as red blood cells, plasma and platelets. One unit of whole blood, once its separated, may be transfused to several patients, each with different needs. Annually, more than 23 million units of blood components are transfused.
Most blood donors are volunteers.
However, sometimes a patient may want to donate blood a couple of weeks before undergoing surgery, so that his or her blood is available in case of a blood transfusion. Donating blood for yourself is called an autologous donation.