Articular cartilage covers the ends of long bones in synovial joints, providing smooth articulation and cushioning of the underlying bone during joint movement. The tissue can be viewed as a viscoelastic, composite material composed of collagen type II (and smaller amounts of other collagens) entrapping compressed (underhydrated) proteoglycan aggregates which generate a high osmotic/swelling pressure. This abundant extracellular matrix (ECM) is synthesized and turned over by relatively few cells, the chondrocytes. These cells produce a compartmentalized ECM, the components of which are heterogeneous and vary with anatomical location. They also undergo changes with age and altered functional requirements. Articular cartilage contains no separating basement membranes, nerves, lymphatics, or blood vessels. Access to nutrients and elimination of waste products occur via diffusion through the extracellular matrix. The turnover of collagen is much slower than that of proteoglycans. Products of the metabolic turnover of the matrix macromolecules are released continuously into the synovial cavity and ultimately reach the blood circulation where they can be measured as "markers" of metabolic changes.