Question

Why water and air is not present as lubricant between knee joints?

Answer 1

The inner membrane of synovial joints is called the synovial membrane and secretes synovial fluid into the joint cavity. Synovial fluid is an ultrafiltrate from plasma, and contains proteins derived from the blood plasma and proteins that are produced by cells within the joint tissues.[2]The fluid contains hyaluronan secreted by fibroblast-like cells in the synovial membrane, lubricin (proteoglycan 4; PRG4) secreted by the surface chondrocytes of the articular cartilageand interstitial fluid filtered from the bloodplasma.[3] 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.[4] 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