What is the name of the fluid that lubricates the body's movable joints?

Synovial fluid is the fluid that lubricates the body’s movable synovial joints. It fills the joint cavity and forms a thin, slippery layer between the opposing surfaces of articular cartilage.

The fluid is produced by the synovial membrane, the specialized tissue lining the inside of a joint capsule. Its lubricating properties come especially from hyaluronan and lubricin, also called proteoglycan 4. Together, these substances help cartilage surfaces move with remarkably little friction.

Synovial fluid does more than lubricate. It helps absorb mechanical shocks, carries nutrients and oxygen to cartilage cells, and transports carbon dioxide and metabolic waste away from them. It is a non-Newtonian fluid, meaning its flow behavior changes under pressure and movement; it can become more viscous during a sudden load.

“Synovial” refers specifically to joints such as the knee, hip, shoulder, and elbow, which permit substantial movement. Plasma contributes to the fluid’s composition, but plasma itself is not the joint lubricant. Cerebrospinal fluid protects the brain and spinal cord, while lymph belongs to the lymphatic system.