Abstract: An isolating and blood pressure transmitting apparatus for use in an extracorporeal blood treatment system or the like comprises a generally cylindrical, transparent plastic chamber with a housing having axially opposing inlet and outlet portions. The inlet portion is connected to a blood pumping and purifying portion of the system for receiving pressurized blood therefrom; the outlet portion is connected to a conventional pressure transducer which, in response to blood pressure transmitted thereto, controls blood pumping valving to maintain blood pressure in the system within predetermined limits. The chamber housing has a diameter and an axial length substantially greater than the diameter of the inlet and outlet portions, the chamber having an appreciable volume to absorb pressure surges. A thin flexible membrane, formed in a frustoconical shape is disposed across the housing to physically isolate the pressure transducer from the blood pumping and purifying portion of the system.