What is synovial fluid?

Synovial fluid fills the cavities of synovial joints such as the knee, shoulder, and hip. It is produced by the synovial membrane and contains molecules like hyaluronic acid and lubricating proteins that help the joint surfaces glide. The fluid also delivers nutrients to cartilage, which has no direct blood supply. Changes in synovial fluid can be associated with inflammatory joint diseases. Its lubricating role is one reason healthy joints can move thousands of times a day with limited wear.