The Skeletal System

What Is the Skeletal System?

The skeletal system is the body's framework of bones, cartilage, ligaments, and tendons. In adults it contains 206 bones (newborns start with roughly 270, some of which fuse during growth, with ossification typically complete around age 25). Far from being a static scaffold, bone is living, metabolically active tissue that constantly rebuilds itself throughout life.

The skeletal system performs five core jobs:

• Support — gives the body its shape and holds up soft tissues.
• Movement — bones act as levers that muscles pull on; joints allow motion.
• Protection — the skull shields the brain, the rib cage guards the heart and lungs, and the vertebrae protect the spinal cord.
• Mineral storage — bone is the body's main reservoir of calcium and phosphate.
• Blood cell production — red bone marrow makes red cells, white cells, and platelets through a process called hematopoiesis.

How the Skeleton Is Organized

Anatomists divide the skeleton into two parts: the axial skeleton and the appendicular skeleton.

The axial skeleton (about 80 bones) forms the central axis: the skull (cranium), the hyoid bone, the vertebral column, and the thoracic cage (sternum and ribs). The spine has 33 vertebrae — 7 cervical, 12 thoracic, 5 lumbar, plus the fused sacrum (5 fused vertebrae) and coccyx (4 fused vertebrae). The thoracic cage includes the sternum and 12 pairs of ribs.

The appendicular skeleton (126 bones) includes the upper limbs, lower limbs, and the shoulder and pelvic girdles that anchor them to the axial skeleton. Six tiny auditory ossicles in the middle ears are counted separately.

How Bone Tissue Works

Bone is built from two architectures. Compact (cortical) bone is the dense outer shell that resists bending and provides strength. Spongy (trabecular) bone is the lighter, honeycomb-like interior found at the ends of long bones and inside flat bones; it houses marrow and helps absorb stress.

The bone matrix — the extracellular material that makes up roughly 90% of bone volume — has two key ingredients:

• Hydroxyapatite — calcium-phosphate mineral crystals (the inorganic matrix) that give bone its hardness and resistance to compression.
• Type I collagen — protein fibers (the organic matrix) that provide flexibility and tensile strength so bone bends slightly instead of shattering.

Four cell types maintain this matrix:

• Osteoblasts build new bone by secreting matrix (osteoid).
• Osteocytes are mature cells embedded in bone (about 90% of all bone cells) that sense mechanical load and direct remodeling.
• Osteoclasts dissolve and reabsorb old bone.
• Osteoprogenitor cells are stem cells that become new osteoblasts.

Remodeling and Calcium Balance

Bone remodeling is a tightly coupled, lifelong cycle in which osteoclasts break down old bone and osteoblasts rebuild it. This repairs micro-damage, adapts bone to mechanical stress, and releases or stores minerals. Bone mass typically peaks in the 20s (somewhat earlier in women, into the late 20s in men), then gradually declines.

Bone also helps regulate blood calcium, which is essential for nerves, muscles, and the heart. Parathyroid hormone (PTH) raises blood calcium partly by stimulating bone breakdown, while vitamin D boosts calcium absorption from food. Calcitonin can lower blood calcium. This hormonal interplay keeps calcium within a narrow, life-sustaining range.

Key Structures of the Skeletal System

• Bones — rigid organs providing support, protection, mineral storage, and marrow.
• Bone marrow — soft tissue inside bones; red marrow produces blood cells, while yellow marrow stores fat and can convert back to red marrow when needed.
• Cartilage — smooth, flexible tissue cushioning joints and forming structures like the nose and ears.
• Joints, ligaments, and tendons — joints allow movement; ligaments connect bone to bone; tendons connect muscle to bone.

Common Skeletal Conditions

The information below is general and educational; it describes conditions but is not a diagnosis or treatment plan.

• Osteoporosis — bones become porous and fragile, raising fracture risk. Per CDC NCHS data (2017–2018), about 12.6% of U.S. adults aged 50 and older had osteoporosis, with a much higher rate in women (19.6%) than men (4.4%). An additional 43.1% had low bone mass (osteopenia), a warning stage.
• Osteoarthritis — the most common joint disease, in which protective cartilage wears down. According to the CDC, it affects roughly 1 in 7 U.S. adults (more than 32.5 million people).
• Fractures — breaks from trauma or weakened bone; hip and spine fractures are especially serious in older adults.
• Scoliosis — abnormal sideways curvature of the spine, often appearing in adolescence.

> This article is educational and is not a substitute for professional medical advice, diagnosis, or treatment. Talk to a qualified clinician about screening (such as a DXA bone-density scan) and about any bone or joint symptoms before making changes to your care.

How to Keep Your Skeletal System Healthy

Several evidence-based habits help build and protect bone across the lifespan:

• Get enough calcium. Adults generally need about 1,000–1,200 mg/day, and teens ages 9–18 need 1,300 mg/day. The tolerable upper intake limit is 2,500 mg/day for adults up to age 50 and 2,000 mg/day after 50.
• Get enough vitamin D. Bone-health organizations often advise older adults to aim for 800–1,000 IU/day to support calcium absorption (the general RDA is 600 IU for ages 1–70 and 800 IU after 70).
• Do weight-bearing and strength exercise. Walking, jogging, dancing, and resistance training stimulate bone formation and improve balance, which can reduce fall and fracture risk.
• Don't smoke and limit alcohol, both of which are associated with weaker bone.
• Prevent falls — good lighting, secure rugs, and balance training matter, especially with age.

Building strong bones early and protecting them through adulthood gives the skeletal system the best chance of supporting you for life. Individual needs vary, so discuss supplements, screening, and exercise with your healthcare provider.

Sources

• [Physiology, Bone — StatPearls, NCBI/NIH](www.ncbi.nlm.nih.gov/books/NBK441968)
• [Anatomy, Appendicular Skeleton — StatPearls, NCBI/NIH](www.ncbi.nlm.nih.gov/books/NBK535397)
• [Osteoporosis or Low Bone Mass in Older Adults: United States, 2017–2018 — CDC NCHS Data Brief No. 405](www.cdc.gov/nchs/products/databriefs/db405.htm)
• [Osteoarthritis — CDC Arthritis Program](www.cdc.gov/arthritis/osteoarthritis/index.html)
• [Calcium — Health Professional Fact Sheet, NIH Office of Dietary Supplements](ods.od.nih.gov/factsheets/Calcium-HealthProfessional)
• [Vitamin D — Health Professional Fact Sheet, NIH Office of Dietary Supplements](ods.od.nih.gov/factsheets/VitaminD-HealthProfessional)
• [Lifestyle Approaches to Promote Bone Health — Bone Health and Osteoporosis, NCBI/NIH](www.ncbi.nlm.nih.gov/sites/books/NBK45523)