The Skeleton & Bones

Your skeleton is the rigid framework that gives your body its shape — like the steel frame inside a building. Without it, you would be a soft blob unable to stand, walk, or even breathe properly. The adult human skeleton has 206 bones. At birth you actually have around 270 — many small bones gradually fuse together as you grow. By your mid-twenties, this process is complete and you have your final 206. The skeleton does five important jobs: 1. Support — holds your body upright and gives muscles something to pull on 2. Protection — the skull protects the brain; the ribcage shields the heart and lungs; the vertebral column protects the spinal cord 3. Movement — bones act as levers that muscles pull to create movement 4. Blood cell production — the soft tissue inside bones (bone marrow) makes all your red blood cells, white blood cells, and platelets 5. Mineral storage — bones store calcium and phosphorus, releasing them into the blood when needed The skeleton is divided into two parts: - Axial skeleton (80 bones) — the central "axis" of the body: skull, vertebral column (spine), and ribcage - Appendicular skeleton (126 bones) — the "appendages": arms, legs, shoulder girdle, and pelvis

Bone feels hard and dead — but it is actually a living tissue that is constantly being broken down and rebuilt. Right now, your bones are changing. Bone is made of two components working together: - Collagen fibres — a protein that gives bone flexibility. Without collagen, bone would be brittle like chalk and snap easily. - Calcium phosphate (hydroxyapatite) — a hard mineral that gives bone its strength and rigidity. Without calcium, bone would be bendy like rubber. Together they create a material that is both strong AND slightly flexible — much better than either component alone. Two types of bone tissue: - Compact bone — the dense, solid outer layer you see on the outside of most bones. Like a thick wall. Very strong. - Spongy bone — the inside of many bones. Not actually spongy like a bath sponge — it is a network of thin struts (trabeculae) arranged along lines of stress, like the inside of a cathedral arch. Lighter than compact bone but still strong. Bone marrow: Inside many bones is bone marrow — a soft, fatty tissue. There are two types: - Red bone marrow — found in flat bones (pelvis, sternum, ribs, skull). This is where all your blood cells are made — a process called haematopoiesis (from Greek: haema = blood, poiesis = making). Every second your bone marrow produces about 2 million new red blood cells. - Yellow bone marrow — found in the long bones of adults (femur, humerus). Mostly fat — a stored energy reserve. Why bones need vitamins: - Vitamin D — helps the body absorb calcium from food. Without enough Vitamin D, calcium cannot be deposited into bone properly → bones become soft (rickets in children, osteomalacia in adults). - Vitamin C — essential for making collagen. Without it, bone collagen is weak → this contributed to scurvy in sailors.

Have you ever wondered how bones get longer as you grow? The answer is fascinating — and important for understanding some sports injuries in teenagers. Long bones grow from special zones called growth plates (epiphyseal plates): A growth plate is a thin layer of cartilage near each end of a long bone (like the femur or tibia). Cells in this cartilage divide constantly, pushing the bone longer from the inside. New bone tissue is then laid down to replace the cartilage. This process continues through childhood and adolescence. In girls, growth plates typically close (turn to solid bone) around age 14–16. In boys, around 16–18. Once the growth plates close, you stop getting taller. Why growth plate injuries matter: If a teenager fractures through a growth plate — a Salter-Harris fracture — it can damage the cells that make the bone grow. If severe, one part of the bone may grow faster than the other, causing a limb length discrepancy or angular deformity later. This is why growth plate injuries in children are taken seriously and followed up carefully. Bone remodelling — bones constantly renew themselves: Even in adults, bone is never static. Specialised cells are always at work: - Osteoblasts — bone-building cells. They lay down new bone matrix (collagen + minerals). Think: "blast" = build. - Osteoclasts — bone-dissolving cells. They break down old or damaged bone and release minerals back into the bloodstream. Think: "clast" = crush. - Osteocytes — mature bone cells embedded in the matrix that detect mechanical stress and signal which areas need more bone. In a healthy young adult, old bone is broken down and replaced completely about every 10 years. Exercise (especially weight-bearing exercise like running and jumping) stimulates osteoblasts to build denser, stronger bone. This is why exercise during adolescence is so important for lifelong bone health.

A joint (also called an articulation) is wherever two or more bones meet. The type of joint determines how much movement is possible. Three types of joints by movement: 1. Fibrous joints — no movement Bones are held tightly together by fibrous connective tissue — no space between them. The joints between the bones of your skull (called sutures) are fibrous joints. Once you are an adult, the skull bones are completely fused and cannot move at all. 2. Cartilaginous joints — slight movement Bones are connected by cartilage, allowing a small degree of movement. The joints between your vertebrae (the intervertebral discs) are cartilaginous joints. The disc acts as a shock absorber. You can bend and twist your spine because each disc gives a little — but no single joint moves much. 3. Synovial joints — free movement The most common and most mobile type — found in the hip, knee, shoulder, elbow, and fingers. Features: - Articular cartilage — smooth, slippery cartilage covers the ends of the bones so they glide over each other without friction - Synovial fluid — a thick, slippery fluid (like egg white) inside the joint cavity that lubricates movement - Joint capsule — a fibrous sleeve that surrounds the joint and keeps it together - Ligaments — tough bands of connective tissue that cross joints and prevent excessive movement (they hold bone to bone) Common joint problems: - Sprained ankle — ligaments around the ankle are stretched or torn when the ankle rolls over. The lateral ligaments are most commonly sprained. - Osteoarthritis — the articular cartilage gradually wears away over decades. Without its smooth coating, bone rubs on bone → pain, stiffness, and reduced movement. One of the most common conditions in people over 60. - Rheumatoid arthritis — an autoimmune condition where the immune system attacks the synovial lining of joints → inflammation, swelling, and eventually joint destruction.

Understanding bones helps you understand one of the most common reasons people go to hospital — fractures. What is a fracture? A fracture simply means a broken bone. The word "fracture" sounds complicated, but it just means a crack or complete break. There are different types: - Closed fracture — the broken bone stays under the skin. The most common type. - Open (compound) fracture — the broken bone pierces through the skin. Much higher risk of infection because bacteria can get directly to the bone. - Stress fracture — a tiny crack from repeated low-level stress over time, rather than a single big impact. Common in runners (feet, shin), gymnasts, and military recruits (march fracture). The bone cannot repair itself as fast as it is being damaged. - Greenstick fracture — only in children. Young bone is more flexible, so instead of snapping all the way through, it bends and cracks on one side — like a fresh green stick. - Compression fracture — most common in the spine (vertebrae), especially in elderly people with weak bones (osteoporosis). The vertebra is crushed downward by the weight above it. How bones heal: 1. Immediately after a fracture, a blood clot (haematoma) forms between the broken ends 2. Within days, cells invade the clot and begin laying down a soft repair tissue (callus) 3. Over weeks, the callus is gradually replaced with proper bone 4. Over months to years, the bone is remodelled back to its original shape Osteoporosis — when bones become dangerously thin: Osteoporosis (from Greek: osteon = bone, poros = pore/passage) means the bones have lost so much density that they become fragile and fracture easily. It is extremely common — particularly in women after the menopause (because oestrogen protects bone density) and in elderly men. A fragility fracture (a fracture from a minor fall or sneeze) is the main danger — hip fractures in the elderly have a 20–30% mortality rate within one year. Prevention: adequate calcium and Vitamin D, weight-bearing exercise, and medications that slow bone loss.