Synovial Fluid - Overview

Overview

The inner membrane of synovial joints is called the synovial membrane and secretes synovial fluid into the joint cavity. The fluid contains hyaluronic acid secreted by fibroblast-like cells in the synovial membrane and interstitial fluid filtered from the blood plasma. This fluid forms a thin layer (roughly 50 μm) at the surface of cartilage and also seeps into microcavities and irregularities in the articular cartilage surface, filling all empty space. The fluid in articular cartilage effectively serves as a synovial fluid reserve. During movement, the synovial fluid held in the cartilage is squeezed out mechanically to maintain a layer of fluid on the cartilage surface (so-called weeping lubrication). The functions of the synovial fluid include:

• reduction of friction - synovial fluid lubricates the articulating joints
• shock absorption - as a dilatant fluid, synovial fluid is characterized by the rare quality of becoming more viscous under applied pressure; the synovial fluid in diarthrotic joints becomes thick the moment shear is applied in order to protect the joint and subsequently thins to normal viscosity instantaneously to resume its lubricating function between shocks
• nutrient and waste transportation - the fluid supplies oxygen and nutrients and removes carbon dioxide and metabolic wastes from the chondrocytes within the surrounding cartilage