Levick J R
Department of Physiology, St. George's Hospital Medical School, London, United Kingdom.
Microcirculation. 1995 Sep;2(3):217-33. doi: 10.3109/10739689509146768.
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.
关节滑膜衬里的微循环表现出许多令人着迷的功能适应性。其一项主要功能是为无血管的软骨提供营养，软骨细胞代谢活跃，但距离最近的毛细血管相对较远（在人类膝关节中心超过1厘米）。靠近滑膜表面的高密度有孔毛细血管促进了物质交换（这种排列在类风湿性滑膜中会被破坏），这些有孔毛细血管的孔优先朝向关节腔。即便如此，仅靠扩散向中央软骨细胞供应葡萄糖的速度还是太慢。滑膜微循环产生关节内液（滑液），在关节运动时通过对流运输葡萄糖，从而解决了这个问题。滑液是一种已分泌透明质酸的血浆超滤液，它还起到润滑关节的作用。由于滑膜细胞层不连续，在毛细血管和关节腔之间存在间质基质，所以关节腔能够异常方便地接触到微循环的间质侧。这使得我们能够通过实验研究血管外的斯塔林“力”（流体静力压和胶体渗透压）对液体交换的影响。一个意想不到的结果是发现，血液与关节腔之间液体运输的大部分水力阻力存在于串联的间质层而非毛细血管壁。另一个结果是，在适当的边界条件（关节角度等）下，液体可以同时在一些区域（直接覆盖毛细血管的滑膜）滤入关节腔，而在其他区域（毛细血管之间网眼空间的滑膜）从关节腔滤出进入滑膜下层，从而产生滑液的周转。
