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1. The trans-synovial flow (Qs) of Ringer solution from the cavity of immobile knee (stifle) joints was determined in anaesthetized rabbits when intra-articular hydrostatic pressure (PJ) was elevated in steps from 2 to 25 cm H2O. 2. It has been demonstrated previously (Levick, 1978) that slope DQs/dPJ shows an abrupt sixfold increase at a 'breaking point' (PB) around 9 . 5 cm H2O, rising from a mean of 0 . 49 microliter.min-1 cm H2O-1 (PJ less than PB) to 2 . 81 microliter.min-1 cm H2O-1 (PJ greater than PB). 3. Perforation of the synovial intima by an intra-articular cannula increased dQs/dPJ below breaking pressure and thus largely abolished the breaking point phenomenon, indicating that the phenomenon might be simulated by a break-down in synovial resistance to flow. 4. Ligation of the femoral lymph trunks draining the joint did not significantly alter the relationship between Qs and PJ. The slope dQs/dPJ was 0 . 60 +/- 0 . 17 microliter.min-1 cm H2O-1 (mean +/- S.E.) below a breaking pressure of 8 . 8--10.5 cm H2O, and 2 . 90 +/- 0 . 64 microliter.min-1 cm H2O-1 above breaking pressure. Thus changes in synovial lymph flow did not explain the breaking point phenomenon. 5. Interruption of synovial blood flow by vascular clamps or by killing the animal reduced, but did not abolish fluid absorption; nor was the breaking point phenomenon abolished. Slope dQs/dPJ increased from 0 . 37 +/- 0 . 06 microliter.min-1 cm H2O-1 below breaking point (10 . 5 +/- 1 . 0 cm H2O) to between 1 . 82 and 0 . 96 +/- 0 . 15 microliter.min-1 cm H2O-1 above breaking pressure. Fluid accumulated in extra-synovial interstitial spaces. 6. When the synovial intima was divested of its surrounding tissues, lymphatic and vascular supplies by extensive dissection, the denuded synovium still showed a marked increase in hydraulic conductivity at normal breaking pressures. The breaking point phenomenon was therefore not caused by changes in extra-synovial interstitial pressure or compliance. 7. It is concluded that fluid absorption from the joint cavity occurs by two parallel pathways viz. the synovial capillary bed and the extra-synovial interstitial spaces. A simple analysis of the system indicates that the breaking point phenomenon cannot be explained by an abrupt increase in synovial conductivity (cf. Edlund, 1949) but is explicable if synovial conductivity (and possibly interstitial conductivity) becomes a continuous function of PJ above breaking pressure. This hypothesis reconciles the non-linear pressure-flow relationship with Starling's hypothesis for fluid absorption from connective tissue spaces (1896).

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