A clear, viscid fluid in the synovial joints and secreted by the membrane that lines the joints
The synovial joints are the joints that line opposing bony surfaces. The general components of a synovial joint are the joint capsule, a layer of articular cartilage, and a synovial cavity. The synovial cavity is the space between the opposing bones. This cavity is filled with a transparent, viscid fluid called synovial fluid (also referred to as synovia). This fluid comes from the inner layer of the joint capsule called the synovial membrane.
The synovial fluid is composed largely of hyaluronic acid and lubricin. It also contains phagocytic cells. Hyaluronic acid is a linear mucopolysaccharide. It is a non-sulphated glycosaminoglycan that provides lubrication and increases viscosity of articular cartilages. It is produced and released by the fibroblast-like type B cells of the synovial membrane. The lubricin is another lubricating substance released by surface chondrocytes of the articular cartilage. It helps provide boundary-layer lubrication. The phagocytic cells in the synovial fluid carry out the role of removing cellular debris and microbes in the fluid.
The functions of synovial fluid are to lubricate the articulating joints, to supply nutrients and oxygen, and to remove metabolic wastes.