Produce red blood cells in bone marrow (hematopoiesis) Reservoir for storage of essential minerals (i. e. calcium and phosphorus) Bone is hard, rigid, and very dense. Joint: place of union of two or more bones: functional units of the musculoskeletal system, enable the mobility needed for activities of daily living. Immovable: bones are united by fibrous tissue or cartilage. Synovial joints: freely movable, bones that are separated from each other and are enclosed in a joint cavity. Cavity is filled with synovial fluid (a lubricant: a layer of resilient, avascular cartilage covers the surface of opposing bones, surrounded by a fibrous capsule and is supported by ligaments. Ligament: fibrous bands running directly from one bone to another that strengthen the joint and help prevent movement in undesirable directions. Bursa: enclosed sac filled with viscous synovial fluid; located in areas of potential friction (e. g. shoulder, knee) and help muscles and tendons glide smoothly over bone. Account for 40%-50% of the body"s weight.