RELATION BETWEEN TRANS‐SYNOVIAL FLOW AND PLASMA OSMOTIC PRESSURE, WITH AN ESTIMATION OF THE ALBUMIN REFLECTION COEFFICIENT IN THE RABBIT KNEE

Abstract

The volume of synovial fluid in a joint correlates inversely with plasma colloid osmotic pressure (COP). The inferred influence of plasma osmotic forces on trans‐synovial flow was investigated directly here, in isolated perfused hindquarters of sixteen rabbits. Flow of intra‐articularly infused Krebs solution across the synovial lining of the cannulated knee was recorded at controlled intra‐articular pressure (18 cmH2O). Colloid osmotic pressure in the synovial microcirculation was varied by perfusion with oxygenated red cells resuspended in albumin solution or plasma from an extra‐corporeal system at constant perfusion pressure. Studies in vitro showed that the COP versus concentration curve for commercial bovine albumin samples was variable and not reliably described by a widely used polynomial. The rate of trans‐synovial absorption Qs was a positive linear function of intravascular COP πn (r = 0·936, P 〈 0·001, n = 83). The average slope dQs/dπp was 0·20 µl min−1 cmH2O−1 (S.E. ± 0·01 µl min−1 cmH2O−1), the slope depending on hydraulic conductance and osmotic reflection coefficient. Trans‐synovial flow was a negative linear function of synovial capillary pressure (Pc). Absolute slope dQs/dπp was on average only 78% of dQs/dPc in the same joint. The osmotic reflection coefficient of the blood‐joint barrier to serum albumin was estimated from these slopes as 0·78‐0·81 (S.E.M. ± 0·06). Vascular perfusion with a hyperosmolar solution of glucose, sucrose or NaCl generated a transient, rapidly decaying osmotic absorption from the joint cavity, with a half‐life of 17‐60 s. A reversed osmotic transient occurred on reperfusion with isotonic fluid. It was concluded that the blood‐joint barrier, which comprises fenestrated endothelium and synovial intima, approximates to an imperfect semipermeable membrane for albumin solutions, justifying the application of Starling's hypothesis to trans‐synovial flow. For small solutes the tissues form a highly permeable but nevertheless slightly osmotically reflective membrane. © 1988 The Physiological Society