In an extracorporeal blood circuit system or a machine configurable in such a circuit, a bag blood reservoir has an infusion port at the upper end of the bag, an outlet port at the lower end of the bag, an inlet port at the lower end of the bag, and an IV port at the lower end of the bag. A split line is connected to the infusion port, having a first branch line connected to a source makeup fluid into the reservoir and a second branch line that evacuates from the reservoir. An air detector and a vent controller automatically configure a venting path in which the inlet lines to the pump and to the blood reservoir are open such that air rises to the upper end of the blood reservoir from which it can be vented through the infusion port into the air evacuation line.
