Cartilage covers the bones for frictionless movement and to dissipate forces on the joint. It is living tissue, but it has no blood supply or nerves.
Synovial fluid fills the joint cavity.
It lubricates the cartilage and, like a surrogate blood supply, nourishes it and keeps it clean. Normal synovial fluid is thick and viscous. Joint disease in the dog often starts with injury to the cartilage. This can happen any time he jumps, runs, or otherwise stresses his joints. It also can result when the joints fit too loosely, allowing the bones to bump together repeatedly.