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

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Abstract
Fluid exchange across the synovium lining the cavity of extended knees was studied in 20 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 (.ovrhdot.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 (d.ovrhdot.QS/dPV, mean -0.25 .+-. 0.03 .mu.l min-1 mmHg-1) was greater than its sensitivity to arterial pressure (d.ovrhdot.QS/dPA, mean -0.17 .+-. 0.02 .mu.l min-1 mmHg-1). The ratio of the slopes, (d.ovrhdot.QS/dPV)/(d.ovrhdot.QS/dPA), indicated the pre- to postcapillary vascular resistance ratio. From this ratio and PA, PV, synovial capillary pressures PC were calculated by the equation of Pappenheimer and Soto-Rivera. 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. The slope of d.ovrhdot.QS/dPC equalled the net hydraulic conductance of the blood-joint barrier (synovium plus capillary endothelium) and averaged -0.425 .+-. 0.025 .mu.l 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 .apprx. 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.