- The substance responsible for lubricating synovial joints is synovial fluid.
- Synovial joints are characterized by the presence of a joint cavity filled with this fluid.
- Synovial fluid is a viscous, egg-white-like substance produced by the synovial membrane, which lines the inner surface of the joint capsule.
- Its primary functions include reducing friction between the articular cartilages during movement and providing nutrients to the avascular articular cartilage.

- While the articular cartilage covers the ends of bones within the joint, it serves mainly as a smooth surface to facilitate movement but does not itself produce lubrication.
- Bursa fluid is found within bursae, which are fluid-filled sacs that reduce friction between tissues such as tendons and bones, but this fluid is not the main lubricant for synovial joints.
- The term synovial membrane secretions broadly refers to the production of synovial fluid, but the correct and specific answer is synovial fluid as the substance mediating lubrication.

Therefore, synovial fluid is essential for joint health and efficient, pain-free movement.

Reference: Gray's Anatomy, 41st Edition, Volume 1, Chapter 8 / Page 648