This is not a bag of blood. Granted, it did begin as a blood donation, drawn from the arm of a volunteer donor in Massachusetts. Within hours of collection, though, that precursory pint of warm whole blood had been centrifuged, fractionated and decanted into a red blood cell concentrate laced with a cocktail of chemical buffers and nutrients. The ruddy yield, shown here, is one chilled unit of processed blood product, suitable for a patient desperately in need of red cells. Such units—screened, packaged and tracked through their life cycles in keeping with the dictates of the U.S. Food and Drug Administration—are manufactured with assembly-line efficiency to optimize the safety and utility of a precious, limited resource.

A half-liter unit of whole blood, when spun, separates into layers. The 275-milliliter top layer of lemon-yellow plasma is rich in platelet cells, which are principal to blood clotting. The 225-ml bottom layer of red cells (erythrocytes), which shuttle oxygen and carbon dioxide around the body, is skimmed with a slick of the immune system’s white cells (leukocytes). Because different patients need varying boosts for different blood functions, packaging these layers separately lets each whole blood donation help several people. And reducing unnecessary biological material, such as the leukocytes in the red cell concentrate, lowers the risk that a patient’s immune system will reject a transfusion.