periodically pumped for return to the donor although a double needle system may be utilized it is much more typical for patient comfort to employ a single needle 30 feeding whole blood from the donor under the impetus of a peristaltic input pump 32 into the inlet port 20 anticoagulant is concurrently combined with the input whole blood the anticoagulant is taken from an anticoagulant source 36 at a rate controlled by a separate anticoagulant pump 37 plasma from the plasma outlet port 22 is fed directly to a plasma collection bag 40 usually simply under gravity flow to reinfuse packed cells via the single needle however a return pump 42 is coupled between the reservoir 26 and the needle 30 a level sensor 43 a mass sensor may alternatively be used in a control system 44 shown only generally signals a sequence controller 45 such as a microprocessor when the blood reservoir is full the full signal activates the return pump 42 during dwell intervals in the plasma extraction cycle concurrently the input pump 32 and anticoagulant pump 37 are stopped and the magnetic drive 16 and rotor 14 are shifted to a lower rotational rate such as 600 r p m details of the control system 44 have been omitted for simplicity and brevity inasmuch as they may take many forms including manual on off actuation of the various pumps the proportion of anticoagulant from the source 36 is determined in this system by the anticoagulant pump 37 which is operated to deliver anticoagulant at a selectably controllable rate relative to the whole blood input flow the.