Synovial fluid and Friction

These joints all have something in common ─ they are called synovial joints, which means that they have a cavity in the joints that contains synovial fluid. Joints are not just made of bone, because if they were, friction would keep them from moving smoothly. The body uses cartilage and the synovial fluid to reduce friction in joints. Osteoarthritis happens when cartilage wears away over time and exposes bone to bone and increases friction, which is very painful.
Synovial fluid is a fluid-like material that is present in many of the joints of the body. It serves the purpose of lubricating and nourishing certain parts of the joint. The joints in which synovial fluid is present are known as synovial joints, and these include the elbow, knee, shoulder, and hip joints, among others.
Synovial fluid has a thick consistency, somewhat like an egg. It is not like most other fluids present in the body and elsewhere, partly because it does not flow like a liquid. It may be more accurate to think of synovial fluid as a type of connective tissue, because of its composition and because of the work it performs.
Each synovial joint in the body is somewhat like its own organ, with needs and nutritional requirements that differ from other areas of bone. Synovial fluid performs certain mechanical functions, such as cushioning joints and making it easy for bones and cartilage to move past each other. It also has the job of bringing oxygen and other nutrients to the cartilage and other areas of the joint. In addition to providing nutrients, it also removes carbon dioxide and other waste products from the cartilage, and takes these back into the bloodstream to be removed from the body.