How the body is put together

In Discover you took the tour: what is in there and roughly where. This layer is the same body seen with sharper eyes. We will go down a level — to the tissues organs are built from — and up a level — to the cavities that hold the organs in place — and we will be precise about the words.

Precision matters here for a practical reason. "Somewhere in the belly" is not good enough for a doctor, a physiotherapist, a sports coach or a vet. Anatomy has a vocabulary that lets you name any point on any body without ambiguity, and it is not hard. It is about thirty words, and by the end of this layer you will own most of them.

The plan: four levels of organisation, the four tissue types, the direction words and the standard position they are measured from, bone as a living material, the skeleton counted bone by bone to 206, the joints in detail, the three muscle types and how opposing pairs actually produce a movement, then the cavities and every major organ with its mass and position. We finish with skin, layer by layer.

The ladder from Discover holds up under close inspection, and each rung has a proper definition.

A cell is the smallest unit that is alive: it takes in materials, uses energy, responds to signals and, in most cases, can make copies of itself. Your roughly 37.2 trillion cells are not all alike. Estimates count more than two hundred different kinds, and each kind is specialised: its shape is its job.

Look at the shapes and you can read the function straight off them. A red blood cell is a flat disc with a dip in the middle, which gives it the most surface for its size and lets it bend through the narrowest vessels. A nerve cell has a long, thin fibre — in your leg, some run most of a metre — because its job is to carry a signal a long way. A muscle cell is a long spindle packed with sliding filaments, because its job is to get shorter. A cell lining your intestine has a fuzzy brush of tiny projections on its free surface, because its job is to absorb.

Here is a genuinely surprising fact. Every organ in your body — the brain, the liver, a kidney, a toenail bed, the wall of your bladder — is built from just 4 kinds of tissue, mixed in different proportions.

Four building materials, two hundred cell types, dozens of organs. It is a little like discovering that every building in a city is made from brick, timber, glass and steel.

One protein deserves a name check: collagen. It is the body's rope. It is the main protein in tendons, ligaments, cartilage, skin and the flexible part of bone, and it is the most abundant protein in your whole body — very roughly a third of all the protein you contain.

Collagen is what makes things tough but bendy. It resists being pulled apart without being brittle. Notice where that matters: a tendon must not snap when a muscle yanks it; a ligament must not tear when a joint twists; skin must not split when you stretch. All three are collagen doing the same job in three places.

Bone uses collagen too, which brings us to what bone actually is.

Every direction word in anatomy is defined from one standard pose called the anatomical position: standing upright, feet together and flat, arms by the sides, palms facing forward, head and eyes looking straight ahead.

The palms-forward part looks odd, and it is the whole reason the pose exists. Turn your palm to face backwards and the two bones of your forearm cross over each other; now "the bone on the thumb side" has moved. Fix the pose and every word means the same thing every time, for every body, whatever position that body happens to be in at the moment. A patient lying flat on a scanner is still described as if standing in anatomical position.

The skeletons in museums give a badly wrong impression. A dry museum bone is to a living bone roughly what a dried leaf is to a living plant.

Bone is living tissue. It contains cells that build it, cells that dissolve it, blood vessels that feed it and nerves that report on it. It heals when it breaks — no other structural material you know does that. It gets thicker where it is loaded and thinner where it is not, which is why the bones of a tennis player's racket arm are measurably denser than their other arm. It is being quietly rebuilt all the time: over several years, most of the material in your skeleton is replaced.

Bone is a composite: two materials combined so that the mixture beats either one alone.

• About 35% of dry bone is organic, chiefly collagen — flexible, tough, resists being pulled apart.
• About 65% is mineral, mostly a calcium phosphate crystal — hard, stiff, resists being squashed.

Take one away and you can see what each contributes. Soak a chicken bone in vinegar for a few days and the acid dissolves the mineral out: what remains is floppy enough to tie in a knot, because only the collagen is left. Bake a bone dry in an oven and the collagen is destroyed: what remains is hard but crumbles like chalk under a squeeze.

Together they make something both stiff and tough, which is a rare and valuable combination. Engineers copy the idea: reinforced concrete is a hard, brittle material (concrete) with flexible ropes (steel bars) running through it, for exactly the same reason.

Now back to the count. A newborn has roughly 300 separate bones and cartilages; an adult has 206. About 94 pieces have merged. Where did they go?

• The skull. A baby's braincase is several plates separated by soft membrane, including the well-known soft spot on the top. Separate plates let the head compress slightly during birth and then expand quickly as the brain grows. The plates knit together over the first couple of years, and the seams keep tightening into adulthood.
• The sacrum. 5 separate vertebrae at the base of the spine fuse into one solid triangular block, which is what the pelvis hangs on.
• The coccyx. About 4 tiny vertebrae at the very bottom fuse into one small piece.
• The hip bone. Each hip starts as three separate bones that fuse into one during the teenage years.
• Long bones. Each has separate end-caps that fuse to the shaft when the growth plate closes.

Counting a baby's vertebrae the original way gives 33; in an adult the fused groups count as one each, giving 26.

The spine repays a closer look, because it solves two opposite problems at once: it must be rigid enough to hold you up and flexible enough to let you bend and twist.

The solution is to make it out of many short blocks rather than one long pole. Each vertebra can only shift a small amount against its neighbour, but 24 small shifts add up to a spine that can curl right over. Between each pair is an intervertebral disc: a tough ring with a soft, water-rich centre that acts as a cushion and a pivot.

Now look at the sizes. The 7 cervical (neck) vertebrae are small and light, because they carry only a head. The 12 thoracic (chest) vertebrae are bigger and each has a pair of ribs attached. The 5 lumbar (lower back) vertebrae are the biggest and thickest of all, because they carry the weight of everything above them. Size follows load, all the way down the column.

The ribcage is a compromise of a different kind. A solid box of bone would protect the heart and lungs perfectly and make breathing impossible. Instead you get 12 pairs of springy curved bones, each joined to the spine behind by a small movable joint and to the breastbone in front by flexible cartilage. When you breathe in, the whole cage swings up and out; when you breathe out, it drops back. Protection and movement, both.

The top 7 pairs reach the breastbone through their own cartilage. The next 3 pairs join the cartilage of the rib above rather than the breastbone itself. The last 2 pairs — the floating ribs — attach only at the spine and end free at the side, which is precisely what allows your waist to twist and bend.

The pelvis is the opposite design: heavy, rigid and fused. It has to be. It is the junction where the weight of your entire upper body transfers from a single central column, the spine, into two separate legs. It also forms a bowl that supports the bladder and the lower intestines from below.

More than half of your bones — 106 of 206, or 51.5% — are in your hands and feet. Two hands together hold 54 bones, which is 32 more than your entire skull.

Why so many small bones? Because bones are only useful in combination with the joints between them, and more bones mean more joints mean finer control. A hand with one bone would be a club. A hand with 27 can hold a pen, a cricket ball, a chapati and a violin bow, each with a different grip.

Feet have almost as many (26) for a related but different reason: a foot must be a springy, adjustable platform that reshapes itself to uneven ground at every step, and it does that by having many small bones lashed together with ligaments into a pair of arches.

Most of the joints you actually move are built to the same plan, called a synovial joint, and it is a beautiful piece of design:

• The ends of both bones are capped with smooth, slippery cartilage.
• The whole joint is sealed inside a tough capsule.
• Inside the capsule is a small amount of synovial fluid, a slippery liquid roughly the consistency of egg white, which lubricates the surfaces and feeds the cartilage.
• Outside the capsule, ligaments run from bone to bone, limiting how far the joint can travel.
• Crossing the joint, tendons carry in the pull of muscles that live some distance away.

The friction inside a healthy synovial joint is astonishingly low — lower than ice sliding on ice. Your knee joints have been doing this, thousands of times a day, for as long as you have been walking.

"Voluntary" and "involuntary" are about who gives the order, not about how important the muscle is.

You can decide to clench your fist, and skeletal muscle obeys. You cannot decide to push food along your intestine, and it happens anyway. This division is a very good arrangement: imagine having to remember to keep your heart beating during a maths test.

The diaphragm is the interesting exception. It is skeletal muscle, it runs automatically while you sleep, and yet you can take over and breathe deliberately whenever you choose — hold your breath, blow out a candle, sing a long note. It sits on the boundary between the two worlds, which is why it is worth knowing about.

Smooth muscle deserves one more note: it is why your gut keeps working and why your blood vessels can widen and narrow. When you blush, smooth muscle in tiny vessels in your face has relaxed and let more blood through. Nobody chose that either.

Say you want to lift a glass of water to your mouth. What actually happens?

A muscle contracts by getting shorter and fatter. It has two ends: the origin, on the bone that stays still, and the insertion, on the bone that moves. Between them the muscle crosses at least one joint. When the muscle shortens, the two ends are pulled towards each other, and the bone that is free to move — the insertion end — moves.

Lifting the glass: the biceps has its origin up on the shoulder blade and its insertion on the radius, one of the forearm bones, just past the elbow. It crosses the elbow. When it contracts, the forearm swings up towards the shoulder, and the glass comes with it.

Now put it down again. The biceps cannot push; it can only stop pulling. So a second muscle does the opposite job.

The triceps runs down the back of the upper arm and inserts on the ulna just behind the elbow, on the other side of the joint. When it contracts, it pulls the forearm back down and straightens the elbow.

Two muscles pulling opposite ways across the same joint are called an antagonistic pair. At any moment one is the agonist — the one doing the work — and the other is the antagonist, relaxing in a controlled way so the movement is smooth instead of a jerk.

• Bending the elbow: biceps is the agonist, triceps the antagonist.
• Straightening the elbow: triceps is the agonist, biceps the antagonist.

Every joint you can move in two directions has at least one such pair. Front of the thigh (quadriceps, straightens the knee) against back of the thigh (hamstrings, bends it). Front of the shin against the calf. Chest against upper back.

Organs are not loose inside you. They sit in cavities: spaces with defined walls, lined by smooth slippery membranes so that neighbouring organs can slide against each other without sticking.

• The cranial cavity is inside the skull and holds the brain, floating in a thin layer of fluid.
• The spinal canal runs down through the vertebrae and holds the spinal cord.
• The thoracic cavity is inside the ribcage: the two lungs in their own sealed sacs, and the heart in a space between them.
• The abdominal cavity lies below the diaphragm: liver, stomach, spleen, pancreas, intestines, and the kidneys pressed against its back wall.
• The pelvic cavity is the bowl below that, holding the bladder and the lower intestine.

The diaphragm is the wall between the last two groups. Above it: heart and lungs. Below it: everything to do with food and waste. That single sheet of muscle is the most useful landmark in the whole trunk.

Skin is the largest organ, at 1.5 to 2.0 square metres in an adult, and it is built in three layers, each with a clear job.

The epidermis is the thin outer layer, made of epithelial tissue in flat sheets. New cells are made at its base and pushed steadily upwards, filling with a tough protein and flattening as they go, until the outermost ones are no longer alive at all — a dry, sealed shield of flat cells that rub off constantly and are replaced from below. This layer also holds the cells that make the pigment that gives skin its colour and helps protect it from the Sun's ultraviolet light.

The dermis below is the thick living layer, made of connective tissue and packed with equipment: touch and pressure and temperature sensors, tiny blood vessels, sweat glands, hair roots, and a mesh of collagen and stretchy fibres that lets skin be pulled and spring back.

Beneath that is the subcutaneous layer, mostly fat. It cushions knocks, insulates against cold and stores energy.

The sense organs are worth one careful paragraph each, because each is a piece of physics built out of tissue.

The eye is an optical instrument. Light passes through a clear window at the front, then through a hole (the pupil) whose size is adjusted automatically by smooth muscle, then through a lens that can change shape to focus, and lands on a light-sensitive layer at the back where it becomes a nerve signal. Lens, aperture, screen: a camera uses the same three parts in the same order.

The ear is a mechanical instrument in three sections. The visible outer part collects sound and funnels it down a tube to a stretched membrane, the eardrum. Behind that, three tiny bones — including the stapes, at about 3.0 mm the smallest bone you own — pass the vibration inwards and concentrate it. Deep inside, a fluid-filled spiral turns different frequencies into signals on different nerve fibres. A separate set of three fluid-filled loops in the same bony region handles balance.

The nose and tongue are both chemical detectors: patches of specialised cells that respond to particular molecules, in air for smell and dissolved in saliva for taste. Most of what you call the "taste" of food is actually smell, which is why food is dull when your nose is blocked.

The skin, as we have just seen, carries the touch senses in its dermis.