skeletal system

Meaning: The skeletal system is the bones, cartilage, and ligaments that join them. It supports the body, protects organs (skull, rib cage), provides rigid levers that muscles pull on to produce movement, stores calcium and phosphate, and houses marrow. Red marrow makes blood cells (hematopoiesis); yellow marrow stores fat. Bone is living tissue: a collagen framework hardened by hydroxyapatite, a calcium phosphate mineral. Osteoblasts build matrix, osteoclasts resorb it, and osteocytes maintain it. Compact bone is organized into osteons; spongy bone has trabeculae and holds marrow. The axial skeleton (skull, vertebral column, ribs) differs from the appendicular skeleton (limbs and girdles).

On the MCAT: Expect questions linking bone to calcium homeostasis: parathyroid hormone raises blood calcium by stimulating osteoclast activity, while calcitonin from the thyroid lowers it. Also expect muscle–bone interaction, endochondral versus intramembranous ossification, and which tissue makes blood cells.

Common confusion: Tendons connect muscle to bone; ligaments connect bone to bone. Likewise, osteoblasts build bone and osteoclasts break it down. Bone is not inert: it is constantly remodeled, so reduced mechanical loading tends to lower bone density.