The Skeletal System

Your skeletal system is the framework of 206 bones that holds your body up, gives it shape, and protects your most important organs. Without your skeleton you would be a soft blob unable to stand or move. Your bones do more than just support you: they store minerals, make blood cells, anchor your muscles and act as levers to power every movement you make. Babies are born with about 270 bones, but some fuse together during childhood to give the 206 you will end up with.

  • Bones in adult206Plus a few small sesamoid bones in joints
  • Bones in newbornapprox. 270Some fuse during childhood
  • Longest boneFemur (thigh)approx. 45 cm in adults
  • Smallest boneStapes (in ear)approx. 3 mm
  • % of bones in hands and feetapprox. 50%106 out of 206
  • Bone strengthStronger than steelFor its weight

What bones do

  • Support: bones hold up your body and give it its shape.
  • Protection: the skull protects your brain; ribs protect your heart and lungs; vertebrae protect your spinal cord.
  • Movement: muscles pull on bones to move you around. Without bones you could not stand, walk, run or pick anything up.
  • Blood cell production: red bone marrow inside many bones makes new red and white blood cells.
  • Mineral storage: bones store calcium and phosphorus, releasing them when needed.
  • Energy storage: yellow bone marrow stores fat as a reserve.

What bones are made of

Bones are amazingly strong despite being light. They have two main parts:

  • Compact bone: the hard, dense outer layer. Strong and rigid.
  • Spongy bone: a honeycomb-like structure inside many bones. Lighter than compact bone, but very strong for its weight. Often contains marrow.

The chemistry of bone is roughly 70% mineral (mainly calcium phosphate, which provides hardness) and 30% organic material (mostly a protein called collagen, which provides flexibility). The combination makes bone stronger than steel for its weight, and flexible enough not to shatter under stress.

The major bones of your skeleton

  • Skull: 22 bones, mostly fused. Protects the brain and forms the face.
  • Spine (vertebral column): 33 vertebrae (some fused in adults), forming an S-shaped column.
  • Ribs: 12 pairs of curved bones forming a protective cage around the heart and lungs.
  • Pelvis: the bony hip ring that supports your weight and protects your abdominal organs.
  • Arms: humerus (upper arm), radius and ulna (forearm), 27 bones in each hand.
  • Legs: femur (thigh, the largest bone), patella (kneecap), tibia and fibula (lower leg), 26 bones in each foot.

Around 106 of your 206 bones (more than half) are in your hands and feet. Both have over 50 small bones each: 27 in each hand, 26 in each foot.

Joints: where bones meet

Bones meet at joints. There are several types:

  • Ball-and-socket joints (shoulder, hip): allow movement in any direction.
  • Hinge joints (elbow, knee): allow back-and-forth movement only.
  • Pivot joints (in the neck): allow rotation.
  • Gliding joints (between wrist bones): allow small sliding movements.
  • Saddle joints (base of thumb): allow movement in two planes.
  • Fixed joints (skull): no movement at all.

Most movable joints are lined with a smooth tissue called cartilage, with a slippery fluid in between to reduce friction. Joints are held together by tough bands of tissue called ligaments.

Fact Your femur (thighbone) is the longest, strongest bone in your body. It can carry a vertical load of around 30 times your body weight before breaking. That is roughly the same strength as a piece of concrete the same size. The femur has to be that strong because every step you take loads it with several times your body weight.

How bones heal

Unlike most other tissues, bone can completely heal a break. The process takes weeks to months and goes through several stages:

  1. Bleeding: a clot forms around the break.
  2. Soft callus: cartilage-like tissue forms around the break in the first 1 to 2 weeks.
  3. Hard callus: the soft callus is gradually replaced by new bone over 2 to 8 weeks.
  4. Remodelling: the new bone is slowly reshaped over months or years to match the original.

A well-set adult bone usually heals as strong as it was before, sometimes stronger. Children's bones heal much faster than adults' (often in half the time) because they are still actively growing.

Did you know? The smallest bones in your body are the three tiny bones of the middle ear: the malleus, incus and stapes (the "hammer, anvil and stirrup"). The stapes, at just 3 millimetres long, is the smallest bone in your body. All three together transfer sound vibrations from your eardrum to your inner ear. Without them, you would not be able to hear properly.
Deeper dive: how bone constantly rebuilds itself

One of the most remarkable things about bones is that they are not static. Every bone in your body is being constantly broken down and rebuilt, in a process called bone remodelling. Two kinds of specialised cells work together:

  • Osteoclasts: dissolve and remove old bone tissue, releasing the minerals into the bloodstream.
  • Osteoblasts: build new bone tissue, depositing fresh calcium and protein in the empty spaces.

This system means your skeleton is essentially being recycled all the time. Estimates suggest that about 10% of your skeleton is replaced every year, and you have an entirely new skeleton roughly every 10 years. Your bones today are not the bones you had as a child.

Bone remodelling responds to how you use your bones. Exercise (especially weight-bearing activities like running, walking and lifting) stimulates osteoblasts to add bone. Long periods of inactivity (or weightlessness in space) cause osteoclasts to remove bone faster than osteoblasts can replace it. Astronauts on long ISS missions lose 1 to 2% of their bone mass per month and have to exercise for 2 hours every day to limit the damage.

As people age, the balance between bone removal and bone building gradually shifts towards more removal. This is why older people often develop osteoporosis: bones that have become thin, weak and brittle. Diet (especially calcium and vitamin D) and exercise are the best protection. Many doctors recommend weight-bearing exercise throughout life to keep your bones strong well into old age.

For the muscles that move your bones, see the muscular system. For the system that protects you, see the immune system.