Your skeleton does far more than hold you upright. It protects vital organs, enables movement, produces blood cells, and stores minerals — five distinct functions working as one integrated system. Understanding those functions, and how cartilage, tendons, and ligaments support every joint, gives you a proper systems view of the human body.
What are the five functions of the skeletal system?
A useful mnemonic is SPMMS — Support, Protection, Movement, Mineral storage, and blood cell-Making.
| Function | How the skeleton achieves it |
|---|---|
| Support | Acts as a rigid framework that maintains body shape and holds organs in position |
| Protection | Hard bones enclose delicate structures — skull → brain, ribcage → heart and lungs, vertebral column → spinal cord |
| Movement | Bones act as levers; muscles pull on them across joints to produce movement |
| Mineral storage | Compact bone stores calcium and phosphorus, releasing them into the blood when needed |
| Blood cell production | Red bone marrow inside certain bones manufactures red blood cells, white blood cells, and platelets |
Notice that the last two functions are hidden inside the bone itself — your skeleton is a living, metabolically active organ, not an inert framework.
How do different types of joints allow movement?
A joint is any point where two or more bones meet. The amount of movement a joint allows depends on its type.
Ball-and-socket joints (hip and shoulder) allow movement in all directions — flexion, extension, abduction, adduction, and rotation. The rounded head of one bone fits into a cup-shaped socket of another. This gives the greatest range of motion but is more prone to dislocation.
Hinge joints (knee, elbow, ankle) allow movement in only one plane — like opening and closing a door. The knee allows a small amount of rotation when bent, but primarily flexes and extends.
Pivot joints (top of the neck, between atlas and axis vertebrae) allow rotation only — for example, when you shake your head to say "no".
Gliding joints (between small bones of the wrist and ankle) allow small sliding movements in multiple directions.
Fixed joints (sutures of the skull) allow no movement at all; the bones are interlocked and fused.
What is cartilage and why does it matter?
Cartilage is a firm but flexible connective tissue that has no blood vessels or nerves of its own. At KS3 you need to know two key contexts:
Articular (hyaline) cartilage covers the ends of bones at synovial joints. It is smooth and slightly slippery, reducing friction and absorbing shock as bones glide past each other. Without it, bone would grind on bone — the painful result you see in advanced arthritis.
Intervertebral discs contain fibrocartilage between the vertebrae of the spine. These tough, slightly compressible pads absorb the shock of running and jumping and keep the vertebrae separated so the spinal nerves can exit safely.
Because cartilage has no blood supply, it heals very slowly after injury — which is why knee cartilage damage can be so troublesome for athletes.
How does the skeleton protect organs?
The skeleton has evolved specific protective structures for the most vulnerable organs:
- The cranium (a dome of interlocking flat bones) completely encloses the brain. The interlocking suture joints absorb impact by distributing force across a wider area.
- The ribcage (12 pairs of ribs, the sternum, and the thoracic vertebrae) forms a flexible cage around the heart and lungs. Ribs are joined to the sternum by costal cartilage, allowing the chest to expand during breathing while still protecting the organs.
- The vertebral column runs through a bony canal that fully encloses the spinal cord, the main nerve highway between brain and body.
- The pelvis protects the bladder, reproductive organs, and the lower digestive tract.
The key design principle is that bones can be rigid where rigidity is needed (cranium) and flexible where movement is essential (ribcage) — an elegant engineering trade-off.
How are blood cells made inside bones?
Red bone marrow is found mainly in flat bones (sternum, ribs, skull, pelvis, shoulder blades) and at the ends of long bones (femur, humerus). It contains stem cells called haematopoietic stem cells that divide and differentiate to produce:
- Red blood cells (erythrocytes) — carry oxygen via haemoglobin; produced at a rate of roughly two million per second in an adult
- White blood cells (leukocytes) — defend against infection
- Platelets (thrombocytes) — fragments that help blood clot after injury
Yellow bone marrow (found in the shaft of long bones in adults) is mostly fat and is a reserve energy store. In a medical emergency — such as severe blood loss — yellow marrow can revert to red marrow and ramp up blood cell production.
How does the skeletal system work with muscles?
Bones act as levers, muscles provide the force, and joints are the pivots. Three connective tissues link everything together:
Tendons are tough, inelastic cords that attach muscle to bone. They transmit the pulling force generated by muscle contraction directly to the bone. Because they are inelastic, no energy is wasted in stretching — all the force reaches the bone.
Ligaments connect bone to bone across a joint. They are slightly elastic, allowing a joint's full range of movement while preventing excessive movement that could cause dislocation. Tearing a ligament (a sprain) is a common sports injury.
Antagonistic muscle pairs work in opposition. In the arm, the biceps flexes the elbow by contracting; the triceps extends it. One muscle always relaxes as the other contracts because a muscle can only pull, never push.
How can you remember the skeletal system for your exam?
A simple framework for exam answers:
- Name the function clearly (e.g. "protection").
- Give the specific bone or structure (e.g. "the ribcage").
- Name the organ it protects (e.g. "the heart and lungs").
- Explain the mechanism if the question is worth more than one mark.
For joints, always state the joint type, give an example location in the body, and describe the range of movement it allows.
For tendons vs ligaments, remember: Tendon → To bone from muscle; Ligament → Links bone to bone.
Frequently asked questions
What is the difference between a tendon and a ligament?
A tendon connects a muscle to a bone and is made of tough, inelastic collagen fibres that transmit the full force of muscle contraction to the skeleton. A ligament connects one bone to another across a joint; it is made of slightly elastic fibres that hold the joint together and limit excessive movement. Spraining an ankle tears the ligaments around that joint.
Why do babies have more bones than adults?
Babies are born with around 270 to 300 bones, many of which are still partly cartilage. As the skeleton develops, a process called ossification converts cartilage into bone, and many smaller bones fuse together. By adulthood the skeleton has settled at 206 bones. The skull, for example, starts as several separate plates (allowing the head to pass through the birth canal) and slowly fuses over the first two years of life.
What type of joint is found in the elbow?
The elbow is a hinge joint. It allows flexion (bending) and extension (straightening) in one plane only, much like the hinge on a door. This restricted range of movement is ideal for the precise control needed when lifting, throwing, or writing. The joint is stabilised by the medial and lateral collateral ligaments and lubricated by synovial fluid.
How does cartilage differ from bone?
Cartilage is a flexible connective tissue with no blood vessels or nerve supply, made mainly of collagen fibres and a gel-like matrix. Bone is a rigid mineralised tissue (calcium phosphate crystals in a collagen matrix) with its own blood supply and the ability to repair itself. Because cartilage lacks a blood supply it heals very slowly, while bone can knit back together after a fracture in weeks to months.
What is red bone marrow and where is it found in adults?
Red bone marrow is the tissue inside certain bones that produces all the body's blood cells — red cells, white cells, and platelets. In adults it is concentrated in flat bones such as the sternum, ribs, skull, and pelvis, and at the ends of long bones like the femur. The shaft of long bones contains yellow marrow (mainly fat), which can revert to blood-cell-producing red marrow if the body urgently needs more blood cells.
Ready to test your understanding of the skeletal system? Ask Professor Darwin at aitutors.me to quiz you on the five functions and walk you through the tricky bits step by step.