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

¹ Department of Physiology, St George's Hospital Medical School, London SW17 0RE

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 Q_s was a positive linear function of intravascular COP _n (r = 0·936, P 0·001, n = 83). The average slope dQ_s/d_p 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 (P_c). Absolute slope dQ_s/d_p was on average only 78% of dQ_s/dP_c 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.