How the systems work, and how they hand over

In Discover you met the teams and saw that the hand-overs matter more than the teams. This layer asks the harder question: how does each hand-over actually work?

The answer turns out to be the same answer, six times over. Your body has solved "get this substance from here to there" once, properly, and then reused the solution everywhere it was needed. Learn the pattern and the six hand-overs stop being six things to memorise and become one idea seen from six angles.

Along the way you will find out why chewing changes how much food you get out of a meal, why your lungs contain a surface the size of a small classroom, why the left side of your heart is three times thicker than the right, and why a kidney throws away a hundred and eighty litres a day on purpose.

Substances move between systems by diffusion: the plain fact that if something is crowded in one place and scarce in another, and there is a way through, it will spread out until it is even. Nothing decides it. Nothing pushes it. Perfume spreads across a room by diffusion; so does oxygen across an alveolus wall.

Diffusion is wonderfully cheap — it costs the body no energy at all — but it has one crippling weakness: it is hopeless over distance. Diffusion across a hair's width is almost instant. Diffusion across a centimetre would take hours. A body the size of yours could never be run by diffusion alone.

So every hand-over point in your body is built to give diffusion exactly the conditions it needs, and the circulatory system is built to cover the distances diffusion cannot. Three design features appear every time.

Digestion happens in two ways at once, and separating them makes the whole system make sense.

Mechanical digestion makes the pieces smaller without changing what they are. Teeth grind, the stomach churns, bile breaks fat into droplets. Nothing is chemically altered — a crushed grain of rice is still rice.

Chemical digestion changes what the pieces are, by cutting the long molecules into short ones. Starch becomes glucose; proteins become amino acids; fats become fatty acids and glycerol. This is the work of enzymes.

Why do both? Because chemistry only happens at a surface. An enzyme can only attack the outside of a lump of food. Grind that lump into a thousand smaller lumps and you have created an enormous amount of new outside without adding a single gram. That is why chewing properly genuinely matters: the mechanical work makes the chemical work possible.

Two organs sit beside the gut rather than in it, and send in chemicals through tubes. They are the chemical works.

The liver is the largest internal organ and the busiest. For digestion it makes bile, a greenish fluid stored in the gall bladder and squirted into the small intestine when fat arrives. Bile contains no enzymes; it is a detergent. It breaks a blob of ghee into millions of droplets, creating the surface that fat-cutting enzymes need — mechanical digestion, done chemically.

But the liver's bigger role comes after absorption. All the blood leaving your gut goes to the liver first, before it is allowed anywhere else. There the liver checks it: it stores excess glucose and releases it later so your blood sugar stays steady, stores some vitamins and iron, breaks down substances the body does not want, and turns the waste from used proteins into urea — which it hands straight back to the blood for the kidneys to remove. A hand-over of its own.

The pancreas does two unrelated jobs. As a digestive gland it sends a powerful juice into the small intestine. As an endocrine gland it releases insulin and glucagon into the blood to control blood sugar. One organ, two systems.

Air moves for exactly one reason: a pressure difference. Air always flows from higher pressure to lower pressure, and the only way to get it into your chest is to make the pressure in there lower than the pressure outside.

Breathing in. The diaphragm contracts and flattens downwards. The muscles between the ribs contract and swing the ribcage up and out. The chest cavity gets bigger; the same amount of air now has more room; the pressure inside drops slightly below the air pressure outside — and the atmosphere pushes about 500 mL of air in through your nose.

Breathing out, at rest. Nothing contracts at all. The diaphragm relaxes back into its dome, the stretched ribcage and lungs spring back, the chest shrinks, the pressure rises above outside, and air leaves. Breathing out at rest is free.

Breathing out hard — blowing out a candle, shouting, coughing — does use muscle: the abdominal muscles pull the ribs down and shove the diaphragm up. Put a hand on your stomach and cough, and you will feel exactly which muscles just did that.

Blood does not go round your body in one big loop. It goes round two loops joined at the heart, and it passes through the heart twice on every complete journey.

The short loop (pulmonary). Right atrium → right ventricle → lungs → back to the left atrium. Distance: a few centimetres. Purpose: pick up oxygen, drop off carbon dioxide.

The long loop (systemic). Left atrium → left ventricle → the whole body → back to the right atrium. Distance: up to a metre and a half each way, against gravity on the return. Purpose: deliver everything, everywhere.

This explains the heart's lopsided shape. Both ventricles push out exactly the same volume with each beat — they must, or blood would pile up on one side within minutes. But the left one has to push it far further, so it must push much harder, so its muscular wall is roughly three times thicker. The heart is not symmetrical because its two jobs are not equal.

Spin a tube of blood in a fast centrifuge and it separates into two layers: a straw-coloured liquid on top, about 55%, and a dark red packed layer beneath, about 45%.

The straw-coloured liquid is plasma, and it is mostly water. Nearly everything that travels dissolved travels here: glucose from your breakfast, amino acids, salts, hormones, urea on its way to the kidneys, antibodies, and most of your carbon dioxide.

The dark layer is the cells, and there are three kinds.

Red cells carry oxygen, and they are strange little things: shaped like a squashed disc, flexible enough to fold through a capillary, and — in humans — they throw away their nucleus as they mature, so they can pack in more haemoglobin, the iron-containing red pigment that grabs oxygen where there is plenty and lets go where there is little. Having no nucleus, a red cell cannot repair itself; it wears out in about 120 days, and your bone marrow makes about 2.4 million replacements every second.

White cells are the immune system's soldiers: far fewer (about one for every 714 red cells), larger, and able to crawl out through capillary walls to reach trouble.

Platelets are cell fragments that start the clotting process at a cut, which is a hand-over of its own — a plug of platelets, then a mesh of fibres, then a scab.

A neuron is a nerve cell built for one purpose: sending a signal a long way, fast. It has a cell body, short branches that collect incoming signals, and one very long fibre that carries the signal onward. Some neurons run all the way from the base of your spine to your toe — a single cell nearly a metre long.

The signal is an electrical pulse travelling along that fibre. Its speed depends on two things: how thick the fibre is and whether it is wrapped in a fatty insulating sheath. Thick and wrapped: up to about 120 m/s, which is 432 km/h. Thin and bare: around 1 m/s, walking pace.

That difference is why pain arrives twice. Stub your toe and you feel a sharp stab almost at once — fast fibres — and then, a second or so later, a spreading dull ache that came along the slow ones.

Where one neuron meets the next there is a gap. The electrical signal cannot jump it. Instead the first neuron releases a chemical that drifts across the tiny space and triggers the next neuron. Every junction is therefore a hand-over — electrical, then chemical, then electrical again — and the gap is one-way, which is what stops signals running backwards.

A muscle pulls on a bone through a tendon, and the bone swings about a joint. That is a lever, and it obeys arithmetic you can do.

The biceps attaches to the forearm only about 4 cm from the elbow joint, while something held in your hand is about 32 cm from it. The muscle therefore works at a huge disadvantage: to hold a 5 kg weight in your hand, the biceps must pull with a force of roughly 5 × (32 ÷ 4) = 40.0 kg-worth, eight times the load.

Why build such a bad machine? Because you get something valuable in exchange: speed and range. Shorten the biceps by 1 cm and your hand moves 8 cm. A muscle can only shorten a little, and rather slowly — but the lever turns a small slow pull into a large fast movement. That trade is why you can bowl a cricket ball at 120 km/h using muscles that cannot contract anywhere near that fast.

Your body chose speed over strength, nearly everywhere. It is a design decision, and you can see it in the arithmetic.

The nervous system is a phone call: instant, aimed at one place, over in a moment. The endocrine system is a letter posted to every address in the country: slow to arrive, read by everyone, and its effects can last for hours, months or years.

Endocrine glands release hormones straight into the blood, which carries them everywhere. Only cells with the matching receiver respond — so a message that goes to the whole body still arrives at just the right places.

A few worth knowing. Insulin (from the pancreas) tells cells to take glucose out of the blood after a meal; glucagon (also pancreas) tells the liver to release stored glucose when blood sugar falls. Together they hold your blood sugar in a narrow band all day without you noticing. Adrenaline (from glands on top of the kidneys) prepares you for sudden effort: heart faster, breathing faster, blood sent to muscles — the jolt you feel when something startles you. Thyroid hormone sets the overall speed at which your body burns fuel.

Notice that the nervous and endocrine systems often do the same job twice, on two different timescales. Nerves speed your heart in a fraction of a second; adrenaline keeps it fast for minutes afterwards. That is why your heart is still thumping a full minute after the fright is over.

The immune system is your defence, and it works in layers.

The outer layers stop most trouble without any fuss: unbroken skin, the sticky mucus and tiny sweeping hairs in your nose and airways, tears, and the acid in your stomach. Most germs never get past these.

If something does get in, white blood cells arrive. Some engulf and digest invaders. Others make antibodies, proteins shaped to lock onto one particular germ and mark it for destruction.

And here is the part worth knowing: the immune system remembers. After an infection, some cells stay behind holding the recipe for that antibody. Meet the same germ years later and the response is so fast you may never notice you were infected at all.

A vaccine uses that memory deliberately. It shows the immune system a harmless version or a fragment of a germ, so the memory is built before the real thing ever arrives. No illness needed. The childhood vaccines you have had — and the ones given across India in enormous campaigns, which is how smallpox was ended worldwide and polio ended in India — are all that same idea.

Fever fits here too, and it is not a malfunction: see the next layer for why a raised temperature is a deliberate, controlled move.

You have been hot, cold, hungry, thirsty and out of breath today. Through all of it, the conditions inside you barely moved. Your core temperature stayed within about half a degree of 37.0 °C. Your blood sugar stayed in a narrow band. The amount of water in your blood hardly changed.

Holding the inside steady while the outside swings about is called homeostasis, and almost every system you have met is part of it.

The method is always the same, and it is beautifully simple. A sensor notices a change. A control centre compares it with the value it wants. An effector does something that pushes back the other way. Because the response always opposes the change, the value is pulled back towards where it started — this is called negative feedback, and it is the same idea as a fan regulator, or as steering a bicycle by constantly making small corrections you never think about.

Too hot? Sweat, and send blood to the skin. Too cold? Shiver, and keep blood in the core. Blood sugar high? Insulin. Blood sugar low? Glucagon. Short of water? Concentrate the urine, and make you thirsty. Every one of those is a push in the opposite direction.

One more thing your body does entirely without you: sleep.

Sleep is not the body switching off. Several systems get busier. The brain sorts through the day and moves what matters into longer-term memory, which is why a night's sleep genuinely improves what you learnt the day before. Growth hormone is released mostly during deep sleep. The immune system does much of its work then. Muscle repair happens then. Your temperature drops slightly, your heart slows, and your breathing becomes very regular.

Children of 9 to 13 need roughly 9 to 12 hours a night — more than adults, because there is more growing and more learning to consolidate. Over a lifetime, sleeping 9 hours a night comes to about 30 years of an 80-year life spent asleep. That is a colossal investment, and evolution does not make colossal investments in nothing.

The practical part is undramatic: a reasonably regular bedtime, a dark quiet room, and less bright screen light in the hour before bed, because bright light in the evening tells the body's internal clock that it is still daytime.