Mass-balance approach for estimating transcapillary fluid and protein movement

This study assesses the applicability of a procedure based on mass conservation to estimate transcapillary protein transport (PT) in patients. In a tissue that exhibits net transcapillary protein transport, total transcapillary fluid movement (FMT) is defined according to the law of mass conservation as: FMT = FA (CV - CA)/CV + PT/CV where FA is arterial plasma flow and CV and CA are respectively venous and arterial protein concentration. The first term above (FMO) was estimated from changes in venous plasma oncotic pressure. The second term was estimated both from the rate of tissue accumulation of 125I-albumin and from the mass-balance relationship as (FMT - FMO). FMT was determined in a jejunal segment as the sum of the changes in tissue weight, fluid secretion, and lymph flow. Increasing venous pressure by 10 and 20 mmHg produced changes in FMT and PT which, as estimated by the two methods, were not significantly different. Thus PT can be estimated from the mass-balance relationship without employing either radioactive labels or noxious tissue manipulation; such an approach should be suitable for clinical application.