What is the name of the fluid that lubricates the joints?

Synovial fluid is the viscous liquid that lubricates the joints. It fills the cavities of synovial joints, such as the knee, hip, shoulder, and fingers, reducing friction as smooth articular cartilage surfaces move against one another.

The fluid is produced by the synovial membrane, which lines the inside of the joint capsule. It contains hyaluronan, a long-chain molecule that contributes to its slippery, egg-white-like consistency, and lubricin, a protein that supports boundary lubrication between cartilage surfaces. Synovial fluid also helps nourish cartilage, which has limited direct blood supply, and carries away some metabolic waste.

Synovial fluid is not the same as plasma, lymph, or cerebrospinal fluid. It is partly an ultrafiltrate of blood plasma, but joint tissues add important molecules and proteins. Its viscosity can change with pressure and movement, helping it provide both lubrication and shock absorption. Inflammation or injury may alter its amount, clarity, or composition, as occurs in arthritis or joint infection.