Methods, systems, and kits for extracorporeally circulating and processing blood are described. The systems include a pump, a processing unit, and blood drawn return lines for accessing a patient's vasculature. Blood flow through the return line is measured and pump speed controlled to maintain a desired blood flow rate. Alarm conditions can be initiated when expected pump performance differs from that needed to maintain the control point flow rate. By using a ultrasonic flow detector, gas bubbles in the blood flow can be detected.