Resistor Circuit Analysis & Wheatstone's Bridge (FRCA Topic)

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WHEATSTONE'S BRIDGE - Used in arterial line circuitry to measure IABP.
The Wheatstone bridge is an electrical circuit for the precise comparison of resistances. It consists of a common source of electrical current and a galvanometer that connects two parallel branches, containing four resistors, three of which are known. One parallel branch contains one known resistance and an unknown (Rx). The other parallel branch contains resistors of known resistances. In order to determine the resistance of the unknown resistor, the resistances of the other three are adjusted and balanced until the current passing through the galvanometer decreases to zero - meaning there is no potential difference across the wire connecting the galvanometer.

The Wheatstone bridge is well suited also for the measurement of small changes in resistance and, therefore, is also suitable to measure the resistance change in a strain gauge. The strain gauge transforms strain applied to it into a proportional change in resistance. Thus, changes in resistance and current are measured, then electronically converted and displayed as systolic, diastolic and mean arterial pressures.

It knows these values by calibration in so far as if you know voltage and can measure the current, the resistance can be calculated by V=IR --> resistance is then benchmarked to values that can be displayed.

Newer pressure transducers contain four strain gauges, which form the four resistances in the Wheatstone bridge. The diaphragm is attached in such a way that when pressure is applied to it, gauges on one side of the Wheatstone bridge become compressed, reducing their resistance, whilst the gauges on the other side are stretched, increasing their resistance. The bridge then becomes unbalanced and the potential difference generated is proportional to the pressure applied. This setup of four strain gauges has the advantage that it is four times more sensitive than a single gauge Wheatstone bridge. It also compensates for any temperature change as all of the strain gauges are affected equally (temperature will affect the resistance of a strain gauge so in the single gauge setup, a change in temperature will skew readings). 
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See the Tutorial of the Week for a full IABP analysis and basics here.