The influence of blood pressure on trans‐synovial flow in the rabbit.

Fluid exchange across the synovium lining the cavity of extended knees was studied in twenty rabbit hindquarters. The isolated hindquarters were perfused with blood from an extracorporeal pump‐oxygenator system at controlled arterial pressure (PA) and venous pressure (PV). Intra‐articular pressure (PJ) was set at 18 cmH2O in most experiments. The rate of trans‐synovial absorption of Krebs solution (QS), measured by a drop‐counter, was a negative linear function of PA (70‐160 mmHg) and of PV (2‐42 cmH2O). The sensitivity of exchange to venous pressure ( dQS / dPV , mean ‐0.25 +/‐ 0.03 microliter min‐1 mmHg‐1) was greater than its sensitivity to arterial pressure ( dQS /dPA, mean ‐0.17 +/‐ 0.02 microliter min‐1 mmHg‐1). The ratio of the slopes, ( dQS / dPV )/( dQS /dPA), indicated the pre‐ to post‐capillary vascular resistance ratio. From this ratio and PA, PV, synovial capillary pressures PC were calculated by the equation of Pappenheimer & Soto ‐ Rivera (1948). Trans‐synovial absorption rate was a negative linear function of PC. This observation supports the application to the synovial cavity of Starling's hypothesis for fluid exchange between interstitial spaces and plasma (Starling, 1896). The slope dQS /dPC equalled the net hydraulic conductance of the blood‐joint barrier (synovium plus capillary endothelium), and averaged ‐0.425 +/‐ 0.025 microliter min‐1 mmHg‐1 at PJ = 18 cmH2O. When PJ was subatmospheric, as is common in extended normal joints, fluid exchange (measured indirectly as dPJ /dt) was only approximately 1/4 as sensitive to blood pressure as at PJ = 18 cmH2O. This observation supported the view that the conductance of the blood‐joint barrier depends on PJ.