Extension of the isogravimetric method for the determination of the capillary filtration coefficient CFC

In 1948, J.R. Pappenheimer made the decision to allow blood pressure to act from the capillary toward the interstitium in CFC determination. With this decision, he was inevitably confronted with the fact that an interstitium with unknown properties stood in his way during CFC determination. Depending on how these properties turn out, the experimental conditions change drastically and the measurements give a wrong result. In the example of muscle tissue, contrary to expectations, the effective fitration pressure (EFP) across the capillary membrane does not remain constant in this method, but quickly drops to zero. As a result, the measurement result is calculated to be much smaller than the actual permeability of the capillary membrane. When this problem was recognized in the method of Pappenheimer and Soto Rivera, the simpler way was chosen by allowing the pressure to act in the opposite direction, coming from the interstitium and going outward. As a first step, the tissue was flooded with fluid and then squeezed outward in a pletysmograph. A tissue model was first used to demonstrate that the EFP remained constant in this case. The new method for CFC determination has shown that the CFC in the muscle tissue can be estimated to be many times larger than the CFC measured in the past using the method of Pappenheimer and Soto Rivera. For the muscle tissue in the rabbit experiment, a CFC= 0.130±0.012 ml/mmHg.100g was determined at that time, which was considerably larger than the data known from the literature around 0.008 ml/mmHg.100g. © FASEB.