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.