Abstract: A blood pump for implant or paracorporeal use having a rigid case with inlet and outlet valves defining a pumping cavity; an integral thin walled flexible sac freely confined within a pumping cavity; and a flexible diaphragm within the cavity conforming to the shape of the adjacent side of the sac. The case includes a control ring projecting into the cavity between the diaphragm and sac. The diaphragm is moved between diastolic and systolic positions to pump blood through the valves. During pumping, the ring distributes flexing of the diaphragm over an increased area while also preventing the interior walls of the sac from contacting each other. The outlet port and the portion of the sac immediately surrounding the outlet port are less flexible than the remainder of the sac away from the port and the portion of the diaphragm overlying the outlet port is less flexible than the remainder of the diaphragm.