This disclosure describes systems, methods, and apparatuses for forming concentrates of platelet-rich plasma or bone marrow cells having user-defined concentrations, concentration ranges, and/or volumes. Whole blood or bone marrow samples can be passed through one or two separation operations in which platelets or bone marrow cells are separated from red blood cells and concentrated in a plasma. During this separation and concentrating, a total number of platelets or bone marrow cells or a concentration of either is determined and then used to ascertain what volumes and concentrations need be mixed in order to produce a platelet-rich plasma concentrate or a bone marrow-rich plasma concentrate having a target concentration and/or volume.
