The Skeletal System

What Is the Skeletal System?

The skeletal system is the body's internal framework. It gives you shape, protects your vital organs, provides anchor points for muscles, and carries out a range of functions that go far beyond simply holding you upright.

When most people picture a skeleton, they think of something dry, hard and lifeless. In reality, living bone is a dynamic tissue, threaded with blood vessels and nerves, constantly remodelling itself in response to the forces placed upon it. Your skeleton today is genuinely different from the one you had seven to ten years ago, because bone is continuously being broken down and rebuilt in a process called remodelling.

The adult human skeleton contains approximately 206 bones, though this figure can vary slightly from person to person. A baby is born with around 270 to 300 bones; many of these fuse together during childhood and adolescence until the adult number is reached, typically in the mid-twenties.

Axial and Appendicular: Two Halves of the Skeleton

Anatomists divide the skeleton into two broad divisions to make it easier to study and describe.

The axial skeleton forms the central axis of the body and consists of 80 bones:

• The skull (including the cranium and face bones)
• The hyoid (the U-shaped bone in the throat that anchors the tongue)
• The vertebral column (spine), made up of 33 vertebrae in childhood that eventually fuse to produce 26 in adults
• The thoracic cage (ribcage), consisting of 12 pairs of ribs and the sternum (breastbone)

The appendicular skeleton consists of the remaining 126 bones and includes everything attached to the axial skeleton: the shoulder girdles, arms, hands, pelvic girdle, legs and feet. "Appendicular" simply means "relating to the limbs."

Types of Bone by Shape

Not all bones are the same shape. Bones are classified into four main categories based on their form, and the shape of each bone is directly related to its function.

Long bones are longer than they are wide and act as levers for movement. The femur (thigh bone), tibia, humerus, radius and ulna are all long bones. They have a shaft called the diaphysis and two wider ends called epiphyses. The hollow centre of the shaft contains bone marrow.

Short bones are roughly cube-shaped and are found where strength and a small range of movement are needed. The carpal bones of the wrist and tarsal bones of the ankle are short bones.

Flat bones are thin, plate-like and often curved. They protect organs and provide large surfaces for muscle attachment. The skull bones, sternum, scapulae (shoulder blades) and most of the ribs are flat bones.

Irregular bones have complex shapes that do not fit neatly into other categories. The vertebrae, sacrum and many facial bones are irregular bones.

A fifth type, sesamoid bones, develop within tendons where they cross joints — the patella (kneecap) is the largest example. They protect tendons from wear and can improve the mechanical advantage of certain muscle actions.

The Six Functions of the Skeleton

The skeleton is far more than a passive frame. It carries out at least six important functions:

1. Support — bones bear the weight of the body and the force of gravity. Without the skeleton, the body would collapse like a deflated tent.
2. Movement — bones serve as rigid levers on which muscles act. When a muscle contracts, it pulls on a bone via a tendon, rotating it around a joint. The design of each bone and joint determines the range and type of movement possible.
3. Protection — the skull protects the brain; the vertebral column protects the spinal cord; the ribcage protects the heart, lungs and great blood vessels; the pelvis protects the bladder, reproductive organs and lower digestive organs.
4. Mineral storage — bone tissue stores about 99% of the body's calcium and 85% of its phosphorus. These minerals can be released into the blood when other tissues need them, and deposited back when levels are adequate. The skeleton is in effect a mineral bank.
5. Blood cell production (haematopoiesis) — red bone marrow, found mainly in flat bones and the epiphyses of long bones in adults, produces red blood cells, white blood cells and platelets. The skeleton is responsible for manufacturing around two to three million red blood cells every second.
6. Energy storage — yellow bone marrow, which fills the shaft of long bones in adults, is mostly fat (adipose tissue). In times of energy shortage, the body can draw on this lipid store.

The Anatomy of a Long Bone

A long bone such as the femur gives a good picture of bone's internal architecture, which is more complex than it first appears.

The outer surface is covered by a thin membrane called the periosteum, which contains the blood vessels and nerves that supply the bone and also houses cells called osteoblasts that deposit new bone tissue. Beneath the periosteum is a dense layer called compact (cortical) bone — this is the hard, strong outer shell that gives bone its rigidity.

Deeper inside is spongy (cancellous or trabecular) bone. Despite its name, it is not soft — it is a lattice of bony struts arranged along the lines of mechanical stress, much like the internal bracing of an engineering structure. This arrangement provides great strength while keeping weight to a minimum.

The hollow shaft of a long bone contains the medullary cavity, lined by a membrane called the endosteum. In children and adolescents, this cavity and the spongy bone of the epiphyses contain red marrow. As we age, much of this red marrow is replaced by yellow marrow.

At the ends of long bones, a layer of smooth articular cartilage covers the joint surface. This cartilage reduces friction and absorbs shock during movement. For more detail on what happens at the junction between bones, see our guide on joints explained.

How Bones Grow

Before birth and in early childhood, much of the skeleton exists not as bone but as hyaline cartilage — a firm, flexible tissue. Bone gradually replaces this cartilage in a process called endochondral ossification. This is the main way long bones grow.

Growth in length occurs at the epiphyseal plate (growth plate), a layer of cartilage near each end of a long bone. Cartilage cells at the plate divide and push older cells towards the shaft, where they are replaced by bone tissue. This process continues until the late teens or early twenties, when the growth plates close (ossify) and height is fixed.

Bones also grow in width throughout life. Osteoblasts (bone-forming cells) in the periosteum deposit new bone on the outer surface, while osteoclasts (bone-resorbing cells) on the inner endosteum remove bone from the inside. Together, these processes widen the bone while maintaining an appropriate wall thickness.

A separate process called intramembranous ossification produces flat bones such as the skull bones directly from a fibrous membrane, without a cartilage template first.

Bone Remodelling: A Lifelong Process

Even after growth is complete, bone is continuously remodelled. Specialised cells work in coordinated teams called basic multicellular units (BMUs):

• Osteoclasts are large cells that dissolve and reabsorb old bone tissue, releasing the minerals into the blood.
• Osteoblasts are bone-forming cells that synthesise new bone tissue in the spaces left by osteoclasts.
• Osteocytes are mature osteoblasts that have become embedded in the bone matrix. They sense mechanical load and direct remodelling activity accordingly.

Remodelling serves two purposes: it repairs micro-damage accumulated from daily loading before it leads to stress fractures, and it adjusts bone architecture to match the mechanical demands being placed on it. Bones that experience regular loading — through weight-bearing exercise, for example — become denser and stronger. Bones that are not used, as during prolonged bed rest or space travel, lose density relatively quickly.

This responsiveness is the reason weight-bearing physical activity is routinely recommended for bone health throughout life. However, the specifics of any exercise programme for bone health are best discussed with a healthcare professional, particularly for those with existing bone conditions. You can use our skeleton explorer tool to see how different bones sit within the complete framework.

Notable Bones and What Makes Them Special

Some bones stand out for their size, their function or their interesting properties:

• Femur — the thigh bone is the longest and strongest bone in the body. It must withstand forces several times body weight during activities like running and jumping.
• Stapes — one of the three tiny bones of the middle ear (the ossicles), the stapes is the smallest bone in the body at about 3 mm long. It transmits sound vibrations from the eardrum to the inner ear.
• Hyoid — the only bone in the body that does not articulate (form a joint) with any other bone. It is held in place by muscles and ligaments and supports the tongue and throat.
• Sacrum — formed by the fusion of five vertebrae, the sacrum links the spine to the pelvis and transfers the weight of the upper body to the legs.
• Patella — the kneecap is a sesamoid bone embedded in the quadriceps tendon. It improves the mechanical advantage of the quad muscles and protects the front of the knee joint.

Keeping Your Skeleton Healthy

Several factors influence skeletal health across the lifespan. Understanding them helps put common recommendations into context.

Calcium is the main mineral in bone. Adequate intake across childhood and adolescence is important for building peak bone mass. Dairy products, leafy green vegetables, fortified plant milks and small fish with edible bones are calcium-containing foods. Vitamin D is essential because it enables the gut to absorb calcium. Most vitamin D is made in the skin on exposure to sunlight, with relatively little coming from food.

Physical activity, particularly weight-bearing and resistance exercise, stimulates bone formation through osteoblast activity. This is one reason children are encouraged to be active — building maximum bone mass early in life provides a buffer against loss in later decades.

Hormones play a major role. Oestrogen and testosterone both help maintain bone density. This is why bone loss accelerates in women after menopause, when oestrogen levels fall significantly. The condition of low bone density — called osteoporosis — affects millions of adults worldwide and greatly increases the risk of fractures from minor falls.

For a closer look at what bone is made of and how fractures heal, see our bones explained guide. To understand the full musculoskeletal picture, including how muscles attach to and move these bones, visit the muscular system guide. You can also try the anatomy quiz to test what you've learned about the skeleton.

This guide is educational. Any concerns about bone health, bone density or fractures should be discussed with a qualified healthcare professional.