The effects of hydropenia, volume expansion, interstitial pressures, excluded volumes, and venous pressures on the robustness of capillary filtration coefficient determination.
The measurement of peripheral capillary filtration coefficient (Kf) by venous occlusion methods presupposes that filtration equilibrium does not occur along the length of the filtering surface, since this would result in an unpredictably low estimate for Kf. Filtration equilibrium occurs if the ratio of plasma flow (Q) to Kf is less than a critical value previously estimated to be between 200 and 250 mmHg. The minimum Q/Kf was reevaluated, taking into account the nonlinear relationship between protein concentration and oncotic pressure, changes in interstitial fluid hydrostatic and oncotic pressure, interstitial excluded volume, whether hydropenia or volume expansion is present, and whether the induced venous pressure change is low (10 mmHg) or high (40 mmHg). In all conditions the minimum Q/Kf ranged from 10 to 20 mmHg, while experimental conditions commonly have Q/Kf ratios from 1200 to 1500 mmHg. Thus, filtration equilibrium is unlikely to occur during Kf determinations under any circumstances in a living organism. Kf methodology is considerably more robust than previously reported.
Renal Section, Veterans Administration Medical Center, Tucson, Arizona.
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Katz
D000818:Animals; D001810:Blood Volume; D001834:Body Water / Q000502:physiology; D002196:Capillaries / Q000502:physiology*; D005110:Extracellular Space / Q000502:physiology; D006874:Hydrostatic Pressure; D008955:Models, Cardiovascular; D008962:Models, Theoretical; D014690:Venous Pressure