A pressure measuring device has tubing with an inline “pillow” portion that is more flexible than the tubing portion. The pillow portion may be created by heating a portion of a length of medical tubing and forcing air into the heated portion to expand, and thin, its walls. The tubing and pillow portion may be used in a negatively pressurized portion of a blood circuit such as an arterial return line. An assembly of interleaved support members adhesively bonded to the pillow portion holds the pillow portion in an expanded state even under negative pressure. Further, when the pillow portion tries to collapse, the interleaved support members compress a strain gauge to provide a pressure signal corresponding to the negative pressure exerted by the fluid therewithin.
