A blood chamber for an arterial line of an extracorporeal blood system. The chamber is a flow through device for blood with an air cushion over the blood and access structure for sampling and treating the blood. A pressure tap in the chamber communicates the air cushion with a pressure transducer to facilitate measurement of blood pressure within the chamber. Undesirable obstruction of the pressure tap by blood foam is reduced by introducing blood into the chamber through a submerged inlet. The blood chamber may be situated upstream from a blood pump in a single needle extracorporeal dialysis system so as to avoid pump starvation.