Abstract: Apparatus is disclosed for collecting plasma. The blood collected at from a blood access is temporarily stored in a blood reservoir and subsequently allowed to pass through a plasma separator at increased speed per unit time by the action of circulating pump located downstream of the reservoir. The plasma separator is connected at its outlet to a circuit communicating with the blood reservoir, whereby the blood cells substantially free of plasma are combined with a fresh feed of blood from the blood access. Blood cells of higher Ht values, while in blood return, come predominantly out of the plasma separator so that the latter involves no sudden increase in pressure.