Microvascular architecture and exchange in synovial joints.

The microcirculation of the synovial lining of joints displays many fascinating adaptations to function. One primary function is to supply nutrients to the avascular cartilage, whose chondrocytes are metabolically active but are relatively vast distances from the nearest capillary (> 1 cm in the center of a human knee). Exchange is facilitated by a high density of fenestrated capillaries situated very close to the synovial surface (an arrangement disrupted in rheumatoid synovium) with fenestrations preferentially oriented toward the joint cavity. Even so, diffusion alone is too slow to supply central chondrocytes with glucose. The problem is solved by the synovial microcirculation generating intra-articular fluid (synovial fluid) that transports glucose by convection during joint movement. Synovial fluid is a plasma ultrafiltrate into which hyaluronan has been secreted, and it also serves to lubricate the joint. The joint cavity offers unusually easy access to the interstitial side of the microcirculation because the synovial cell layer is discontinuous, with an interstitial matrix between capillary and joint cavity. This allows the experimental study of the effect of the extravascular Starling "forces" (hydraulic and colloid osmotic pressure) on fluid exchange. One unexpected outcome has been the finding that a substantial part of the hydraulic resistance to fluid transport between blood and joint cavity resides in the series interstitial layer rather than in the capillary wall. Another is that, under appropriate boundary conditions (joint angle, etc.) fluid can simultaneously filter into the cavity in some regions (synovium directly overlying capillaries) and out of the cavity into subsynovium in other regions (synovium in the mesh space between capillaries) to produce a turnover of synovial fluid.