Seven teams, one body

Tear a piece of roti, chew it, swallow. Ten seconds later it has gone somewhere you cannot see, and you stop thinking about it.

But that piece of roti has a long journey ahead. In about a day it will have been ground, soaked in acid, taken apart into pieces far too small to see, carried away in your blood, delivered to a muscle in your calf, and burnt there to let you run for a bus. Nothing about that journey is magic. It is a relay race, and the runners hand the baton to each other in a very particular order.

This lesson is about the teams that run that relay. Your body has around seven of them, and each one is brilliant at a single job and completely useless at every other job. The stomach cannot carry anything anywhere. The blood cannot dissolve a roti. The lungs cannot think. What keeps you alive is not any one team — it is the hand-overs between them.

A school is not one thing. It is a set of groups, each with its own job: teachers who teach, a kitchen that cooks, cleaners who clean, an office that keeps records. Every group depends on the others. If the kitchen stops, lessons carry on for a while — then everyone gets hungry and nothing works.

Your body is built the same way. A cell is the smallest living unit. Cells of the same kind, working together, make a tissue (muscle tissue, nerve tissue). Several tissues arranged into one working part make an organ (the heart, the stomach, a kidney). And organs that share one big job make an organ system.

That last step is the one worth slowing down for. The stomach on its own does not "digest your food". It softens and part-digests one mouthful. The finishing, the absorbing and the delivering are done by other organs. Only the whole team together turns a roti into energy in your leg.

Two more teams matter to this story, and you should meet them now even though they are quieter.

The endocrine system is a set of glands that send chemical messages through the blood instead of electrical ones through nerves. Nerve messages are like a shout: instant, aimed at one place, over in a moment. Hormone messages are like a letter posted to every house in the city: slow to arrive, but read by everyone, and their effects can last hours or years.

The immune system is the body's defence. It is not one organ but a travelling force — white blood cells, the lymph vessels they patrol in, and the bone marrow that makes them. It learns: meet a germ once, and the body usually recognises it faster the next time. That is exactly what a vaccine takes advantage of, safely and in advance.

Here is a strange and true way to think about your gut. It is a tube that runs right through you, from mouth to the far end, open at both ends. Anything in that tube has not really got inside you yet. A swallowed marble travels the whole length and comes out again without ever entering your body properly.

Food only truly gets in at one place: the wall of the small intestine, where the useful pieces cross into your blood. Everything before that point is preparation — grinding, soaking, cutting up — and everything after it is tidying up.

So the digestive system has two problems to solve. First, a roti is far too big and far too complicated to pass through a cell wall, so it must be taken apart. Second, once the pieces are small enough, somebody has to collect them. The digestive system solves the first problem itself. For the second, it hands over to the blood.

You have been breathing all the way through this lesson without deciding to, about 20 times a minute, and you will do it roughly 21,600 times before this time tomorrow.

Air comes in through your nose, where it is warmed to body temperature, moistened, and filtered by hairs and sticky mucus. (This is why breathing through the nose beats breathing through the mouth — the mouth does none of that.) It goes down the trachea, a tube held permanently open by rings of cartilage you can feel at the front of your throat, then splits into two bronchi, one for each lung, then splits again and again into ever smaller bronchioles, like a tree turned upside down.

At the end of the smallest twigs are the alveoli: microscopic air sacs, about 300 million of them in a pair of adult lungs. This is where the air finally meets the blood.

Every cell in you — bone cell, brain cell, toe cell — needs a delivery of oxygen and food and a collection of waste, and it needs both continuously. A cell that waits four minutes for oxygen can die. So the body runs a delivery service that never closes.

The heart is the pump, and it is really two pumps stuck together, side by side, beating at the same instant.

• The right pump takes tired blood arriving from the body and sends it a short distance to the lungs.
• The left pump takes freshly loaded blood back from the lungs and sends it to everywhere else — brain, fingers, kidneys, toes.

Each pump has a small upper chamber that collects (an atrium) and a big lower chamber that pushes (a ventricle): four chambers in all. Valves between them snap shut after each squeeze so blood cannot slide backwards — and that snapping is the "lub-dub" a doctor listens for.

The left ventricle has by far the hardest job, because it must push blood all the way to your feet and back. Its wall is roughly three times thicker than the right one's.

Every cell that uses food and oxygen makes waste, and the blood picks all of it up. If nothing removed it, blood would turn into a poisonous soup within a day. Cleaning it is the job of the excretory system, and its stars are your two kidneys — each about the size of your own clenched fist's inner part, sitting at the back of your waist, one on either side of the spine.

A kidney does not simply strain the blood the way a tea strainer catches leaves. It does something far cleverer and, at first, far stupider-sounding.

Step one: throw nearly everything out. Blood is pushed through a million microscopic filters in each kidney, and water, salt, sugar, vitamins and waste all pour out together. About 180 litres of fluid is filtered out of your blood in a day.

Step two: take back everything worth keeping. As the filtered fluid flows along a long looping tube, the body reclaims the water, all the sugar, most of the salt — about 178.5 litres of it. What is left is 1.5 litres of urine, carrying the waste the body actually wanted rid of.

Deliveries are no use without orders. The nervous system is the body's messaging network, and its whole advantage over the hormone system is speed.

It has three parts. The brain decides, remembers and imagines. The spinal cord, about 45 cm of nerve tissue running down the protected tunnel inside your backbone, is the main cable — and, as you will see, a small decision-maker in its own right. Nerves branch out from it to every square centimetre of you, some carrying messages in from your senses, others carrying orders out to muscles and glands.

Signals travel along the thickest, best-insulated nerve fibres at up to about 120 metres a second — that is 432 km/h, faster than any train in India. A message from your big toe to your spinal cord, about a metre, takes roughly 8 milliseconds. Thin, uninsulated fibres are far slower, around 1 metre a second, which is why a sharp jab registers instantly and a dull ache seems to arrive a moment later.

Muscles can do exactly one thing: pull. A muscle cannot push. It shortens, and that is its entire vocabulary.

So how do you straighten your arm? With a second muscle pulling the other way. Muscles work in opposing pairs: the biceps on the front of your upper arm bends the elbow, the triceps behind it straightens it. When one shortens, the other relaxes and is stretched. Feel it with your other hand while you bend and straighten — you can find both.

Bones give the pulls something to work on. A bone is a lever and a joint is its pivot, and that is why you can throw a ball much faster than your muscle actually shortens: a small movement at one end of a lever becomes a big movement at the other.

There are about 600 skeletal muscles in your body and 206 bones in an adult. A baby starts with roughly 300 separate pieces; about 94 of them fuse together as you grow, which is how a soft, foldable newborn skull becomes a hard helmet.

Now for the heart of this whole topic.

Every system you have met ends at a boundary it cannot cross. The lungs fill with oxygen and stop. The gut breaks down a roti and stops. Nerves carry an order and stop. At each of those full stops there is a hand-over point, a place where two systems touch closely enough for something to pass across.

Every hand-over in the body has the same three features, and once you can spot them you can predict where a hand-over must be even in a system you have never studied:

1. A very thin barrier. Usually one cell thick, sometimes two. Anything thicker is too slow.
2. A huge surface area, achieved by folding, branching or ballooning. Millions of alveoli; millions of villi; kilometres of capillary.
3. A steep difference across the barrier — more of something on one side than the other — so material moves by itself, with nothing to pump it.

Look at the table below and you will see the same three features again and again. Nature only really knows one trick for a hand-over, and it uses it everywhere.

Run up three flights of stairs and every system you have met changes at the same time, within seconds, without you instructing any of them.

Muscles demand far more glucose and oxygen, and start producing far more carbon dioxide and heat. Breathing goes from about 20 breaths a minute to 45 or more, and each breath gets deeper — so far more air moves per minute. The heart speeds up and squeezes harder, so the litres pumped per minute can rise from about 5 to around 20 — roughly 4 times as much. Blood is redistributed. Vessels to the working muscles widen and vessels to the gut narrow, so the share of blood going to muscle rises from roughly 20% to about 80%. (That redistribution is exactly why running hard straight after a big meal feels awful.) Skin flushes and sweat starts, to dump the extra heat.

What triggered all of it? Mostly the carbon dioxide rising in your blood — the same signal you met when holding your breath. One chemical change, sensed in the brain, and five systems change gear together.