When you donate a unit of whole blood, it rarely goes to just one patient as-is. Blood banks routinely separate it into its components — red blood cells, plasma, and platelets — because different patients need different parts, and separating them lets one donation help more than one person.

The separation happens using a centrifuge: whole blood is spun at high speed, and because red cells, platelets, and plasma have different densities, they settle into distinct layers that can be carefully drawn off one at a time. Red cells are typically used for patients with anemia or blood loss from surgery or trauma; plasma, rich in clotting factors and proteins, goes to burn victims and patients with clotting disorders; platelets are used for cancer patients and others whose own platelet counts are dangerously low.

This is also why a single donation is often described as helping up to three patients — the same unit of blood can end up split three ways, each component going to someone with a completely different medical need, rather than sitting unused because it wasn't a perfect match for any one patient's exact requirement.