Osmotic flows across the blood-joint barrier

SUMMARY The effective osmotic pressure across the blood-joint barrier is a key factor controlling synovial fluid volume and joint effusions. The effect of plasma colloid osmotic pressure (COP) on trans-synovial flow was studied directly in rabbit knees expanded by intra-articular fluid infusion. The synovial microcirculation was perfused with blood of varying COP. Absorption of fluid from the joint cavity was a linear function of COP; but albumin COP was only 78 % effective across the blood-joint interface. Hyperosmolar solutions of small solutes (e.g., glucose) generated transient osmotic flows across the blood-joint barrier, but far less effectively than albumin. The hydraulic permeability of synovium increased at pathological intra-articular pressures-a phenomenon of potential importance to effusion kinetics. Key words: osmotic pressure, synovial absorption, fluid exchange. Synovial fluid is derived primarily from plasma, and disturbances in the balance of forces between plasma and the joint cavity can create large clinical effusions. Fluid movement between blood and the joint cavity is considered to obey Starling'