Abstract: The present invention relates generally to medical measurement systems for evaluation of organ function and understanding symptom and pain mechanisms. This model takes into account a number of factors such as volume and properties of the fluid and the surrounding tissue. Particular emphasis is on a multifunctional probe that can provide a number of measurements including volume of refluxate in the esophagus and to what level it extents. The preferred embodiments of the invention relate to methods and apparatus for measuring luminal cross-sectional areas of internal organs such as blood vessels, the gastrointestinal tract, the urogenital tract and other hollow visceral organs and the volume of the flow through the organ. It can also be used to determine conductivity of the fluid in the lumen and thereby it can determine the parallel conductance of the wall and geometric and mechanical properties of the organ wall.