How does the skeletal system work?

Your skeleton does four jobs at once. It is a frame that holds you up, a shield for soft organs (the skull round the brain, the ribs round the heart and lungs), a set of levers that muscles pull on, and a factory: the soft marrow inside many bones makes your blood cells. It also stores most of your body's calcium. We are facing the person, so their right side is on your left.

A typical adult has 206 bones. The axial skeleton (80 bones) runs down the middle: the skull, three tiny ear bones on each side, the hyoid in your throat, the spine and the ribcage. The appendicular skeleton (126 bones) hangs off it: the shoulder and hip girdles, the arms and the legs.

The spine has 26 bones: 7 cervical vertebrae in the neck, 12 thoracic ones that carry the ribs, 5 big lumbar ones in the lower back, then the sacrum and the coccyx (tailbone). Look from the side: it is an S, not a straight rod. The curves work like a spring.

There are 12 pairs of ribs. The top 7 join the sternum (breastbone) through blue cartilage; the next 3 join the cartilage above; the last 2 are floating ribs. More than half of all your bones are in your hands and feet: 27 in each hand and 26 in each foot. The longest bone is the femur, about a quarter of your height. The smallest is the stapes in your ear, about 3 mm long (see EarClear).

A newborn baby has about 270 to 300 bony pieces, many of them still soft cartilage. They fuse as you grow: five vertebrae become one sacrum, and three bones become each hip bone. To see bones in a real person, doctors use X-rays: see XrayClear.

Bone is not dry chalk. It is living tissue, about two thirds mineral (hard calcium phosphate crystals) and one third collagen (tough, bendy protein fibres) and water. The mineral makes it hard; the collagen stops it snapping like glass.

Here a femur is cut open along its length. Outside is the periosteum, a thin skin full of nerves and blood vessels (it is why a broken bone hurts so much). Under it is compact bone, built from millions of tiny tubes called osteons, each about 0.2 mm across, with a blood vessel in its central Haversian canal. Compact bone is about 80% of your skeleton's weight.

The ends are spongy bone: a lattice of struts called trabeculae, lined up along the directions the load pushes. Between them is red marrow, the factory that makes your blood cells, about 2 million red cells every second (see CirculationClear). The ends are capped with smooth, slippery cartilage, shown in blue.

The shaft is hollow. In a child the space is full of red marrow; in adults most of it has turned to fatty yellow marrow. In a growing child, a disc of cartilage near each end, the growth plate, is where the bone gets longer.

Why hollow? When a bone bends, the outer edges do most of the work and the middle hardly helps. Move the same bone to the outside, as a tube, and it becomes stiffer and stronger for the same weight. Bicycle frames and bamboo use the same trick (see CycleClear).

A joint is where two bones meet. The shape of the ends decides how it can move, just like the hinges and ball joints in machines.

A hinge joint, like your knee or elbow, bends one way only: the knee to about 135°, the elbow to about 150°. A ball-and-socket joint, like your hip or shoulder, lets a round head swing in any direction and turn. The shoulder's socket is shallow, so it moves the most of any joint but dislocates more easily. The hip's socket is deep: less range, more stability.

A pivot joint turns round a peg. When you shake your head "no", the first neck bone, the atlas, spins round a peg on the second, the axis. Gliding joints between the small wrist and ankle bones just slide a little. And some joints hardly move at all: the wavy sutures of your skull lock the plates together. In a baby they are still open (the soft spots), so the head can squeeze through birth and the brain can grow.

Moving joints are synovial joints. The bone ends are capped with glassy cartilage 2–4 mm thick, sealed in a capsule and bathed in a few millilitres of slippery synovial fluid. The result is more slippery than ice on ice.

Ligaments tie bone to bone and stop a joint moving the wrong way; the knee's crossed cruciate ligaments are inside it. Tendons tie muscle to bone: the one below your kneecap lets your thigh muscle straighten your leg (see MuscleClear, coming soon).

Every bone that moves is a lever: a stiff bar turning round a pivot (a joint), pushed by an effort (a muscle) to move a load.

Your forearm is a class 3 lever: the effort is between the pivot and the load. The biceps pulls on the forearm only about 4.5 cm from the elbow, but the load in your hand is about 35 cm away. To hold a 5 kg bag with your forearm level, the biceps must pull about 7 to 8 times as hard as the load, around 400 newtons. That sounds like a bad deal, but it buys speed: a small shortening of the muscle swings your hand a long way, fast. Throwing, typing and playing the tabla all depend on it.

Bones take big forces too. Standing, the ground pushes up with your body weight. Walking, it pushes about 1.2 times your weight with every step; running, about 2.5 times. Inside the hip the force is even bigger, because muscles pull across the joint as well: about 2.4 times body weight walking and up to about 5 times running. For an 80 kg runner that is about 4,000 N through each femur, many times a stride.

Bone is up to it. Compact bone can take about 170 MPa of squeeze, more than four times good concrete, while weighing a quarter as much as steel. A femur's shaft could hold about 60,000 N in a straight squeeze; bending and twisting use up most of that, which still leaves a safety margin of about 2 to 4 times.

Your spine works like a crane. Lean forward and the weight of your upper body is far from the pivot, so the back muscles must pull hard and squeeze the discs. That is why you lift with bent knees, holding the load close, and why a school bag should weigh no more than about 10% of your body weight (India's School Bag Policy 2020), carried on both shoulders.

Your skeleton is being rebuilt all the time. Big cells called osteoclasts dissolve tiny pits in old bone, over about 2 to 4 weeks. Then osteoblasts move in and fill each pit with fresh bone, over about 4 to 6 months. Some osteoblasts get trapped and become osteocytes, the bone's sensors. About 10% of an adult's skeleton is replaced every year, so you get a new one roughly every 10 years.

Why bother? Remodelling repairs tiny cracks from everyday use, and it lets bone follow the load: bones you use hard get thicker, bones you rest get thinner. Astronauts in space and people on long bed rest lose bone fast. Running, skipping and dancing build it.

Bone is also your calcium bank: about 99% of the calcium in your body, around 1 kg, is in bones and teeth. Your nerves and muscles need blood calcium kept just right, so if it drops, a hormone from the parathyroid glands tells osteoclasts to release some from bone. To bank calcium, the gut needs vitamin D, which your skin makes in sunlight (see SkinClear), then the liver and kidneys switch it on. Milk, curd, ragi, sesame and green leafy vegetables are good sources of calcium.

Children grow taller at their growth plates, discs of cartilage near the ends of long bones. The cartilage keeps growing and turning into bone. At the end of puberty the plates turn completely to bone and close, usually around 13 to 15 in girls and 15 to 17 in boys, and then you stop getting taller.

This chapter explains common bone and joint problems. It cannot tell what is going on in your own body. If you are hurt or worried, see a doctor.

A fracture is a broken bone. Amazingly, bone heals itself with real bone, not a scar. First a blood clot forms round the break. Within a week or two, cells build a soft callus of cartilage that bridges the gap, then turn it into hard, rough new bone. Over the following months, remodelling smooths it back into shape. Most adult breaks take about 6 to 12 weeks to knit, children's faster. A cast or splint holds the pieces still so they join straight. Doctors check the break, and the healing, with X-rays (see XrayClear).

Osteoporosis means bones have lost so much mass that they break easily, often the hip, spine or wrist. Bone peaks around age 30, then slowly thins; women lose it fastest in the years after menopause. Worldwide, about 1 in 3 women and 1 in 5 men over 50 will break a bone because of it. In Indian studies, about 3 in 10 women past menopause had it in the spine. A DXA scan measures bone density. The best defence starts young: weight-bearing exercise (walking, running, skipping, dancing), enough calcium and vitamin D, and no smoking.

Osteoarthritis is the cartilage in a joint wearing thin, so bone rubs on bone. It is very common in knees, especially later in life. Staying active and keeping a healthy weight help. Scoliosis is a sideways curve of the spine, found in about 2–3% of teenagers. Most curves are small and are simply watched; a few need a brace or surgery.

A sprain is a stretched or torn ligament, usually in the ankle or wrist. A fracture is a broken bone. Both can swell and hurt, so you cannot always tell by looking. If you cannot put weight on it, it looks bent, or the pain is on the bone itself, get it checked.