[{"data":1,"prerenderedAt":1146},["ShallowReactive",2],{"layer:body-systems:discover":3},{"layer":4,"contentHash":1125,"dependencyHashes":1126,"approval":1140,"releaseId":1145},{"schemaVersion":5,"conceptId":6,"locale":7,"depth":8,"revision":5,"title":9,"subtitle":10,"summary":11,"objectives":12,"estimatedMinutes":18,"plate":19,"blocks":40,"sourceIds":1120,"reviewStatus":1121,"authoring":1122},1,"body-systems","en","discover","Seven teams, one body","What each system does, and where it hands the work to the next one","Meet the organ systems one at a time — digestive, respiratory, circulatory, excretory, nervous, muscular and skeletal — then follow a roti and a breath across the hand-over points where each system passes its work to the next.",[13,14,15,16,17],"Explain what an organ system is, and place cell, tissue, organ and system in order.","Describe the main job of each of the seven systems and name its headline organs.","Point to the exact place where each system hands over to the next, and say what crosses there.","Trace one mouthful of roti from plate to a working muscle, naming the systems in order.","Measure your own pulse and explain why every system changes together when you run.",35,{"title":20,"rows":21},"Lesson plate",[22,25,28,31,34,37],{"label":23,"value":24},"Depth","Discover",{"label":26,"value":27},"Reading time","≈ 35 minutes",{"label":29,"value":30},"Prior knowledge","None — but the anatomy topic helps",{"label":32,"value":33},"Chapters","9",{"label":35,"value":36},"Labs","6: two system-flows, sort, explorer, match, data",{"label":38,"value":39},"Big idea","The hand-overs matter more than the teams",[41,45,51,57,63,66,71,97,142,145,227,232,259,264,267,294,299,337,342,363,368,371,399,404,434,448,453,458,461,489,531,535,552,566,571,574,578,592,621,625,630,633,660,665,678,682,687,690,694,737,742,745,784,788,806,839,844,847,851,902,907,911,968,1090,1094,1109],{"id":42,"type":43,"markdown":44},"intro-roti","prose","Tear a piece of **roti**, chew it, swallow. Ten seconds later it has gone somewhere you cannot see, and you stop thinking about it.\n\nBut 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.\n\nThis 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.**",{"id":46,"type":47,"variant":48,"title":49,"markdown":50},"how-to-read","callout","observation","How to use this lesson","Read the chapters in order the first time; each one adds a team to the relay. When you meet a **prediction**, answer before you read on — being wrong for ten seconds is how an idea sticks.\n\nA companion topic, [Anatomy of the human body](\u002Ftopics\u002Fhuman-body-anatomy), shows you **what each organ is and where it sits**. This one is about **what each one does and who it passes the work to**. If you have not met the organs yet, that is a good place to start.",{"id":52,"type":53,"conceptId":54,"relation":55,"explanation":56},"conn-anatomy-intro","connection","human-body-anatomy","helps_understand","Anatomy names the parts and shows where they sit; this topic follows the work as it passes between them.",{"id":58,"type":59,"title":60,"eyebrow":61,"navLabel":62},"ch1","chapter","What a system actually is","Chapter 01","1 What is a system",{"id":64,"type":43,"markdown":65},"what-is-system","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.\n\nYour 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**.\n\nThat 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.",{"id":67,"type":47,"variant":68,"title":69,"markdown":70},"def-system","definition","Organ system","An **organ system** is a group of organs that work together on one job — digesting food, moving air, carrying blood, sending signals.\n\nThe ladder goes: **cell → tissue → organ → organ system → organism (you)**. Each step up can do something the step below cannot.",{"id":72,"type":73,"title":74,"note":75,"scale":76,"rungs":77},"ladder-levels","ladder","From one cell to one you","Rough numbers for an adult. Each rung is built out of the rung below it.","log",[78,81,85,89,93],{"label":79,"value":5,"display":80},"One cell (a red blood cell)","1 cell",{"label":82,"value":83,"display":84},"Cells in a sip of blood (1 mL)",5000000,"≈ 5 million cells",{"label":86,"value":87,"display":88},"Cells in one heart",2000000000,"≈ 2 billion cells",{"label":90,"value":91,"display":92},"Cells in the whole body",30000000000000,"≈ 30 trillion cells",{"label":94,"value":95,"display":96},"Bacteria living in and on you",38000000000000,"≈ 38 trillion cells",{"id":98,"type":99,"caption":100,"columns":101,"rows":106},"table-seven","table","The main organ systems, their one big job, and their headline organs.",[102,103,104,105],"System","Its one job","Main organs","Hands over to",[107,112,117,122,127,132,137],[108,109,110,111],"**Digestive**","Break food into pieces small enough to absorb","Mouth, oesophagus, stomach, small and large intestine, liver, pancreas","Blood (nutrients)",[113,114,115,116],"**Respiratory**","Bring oxygen in and push carbon dioxide out","Nose, trachea, bronchi, lungs, diaphragm","Blood (gases)",[118,119,120,121],"**Circulatory**","Carry everything to everywhere","Heart, arteries, veins, capillaries, blood","Every cell in you",[123,124,125,126],"**Excretory**","Clean the blood and control water","Kidneys, ureters, bladder; also skin and lungs","Outside the body",[128,129,130,131],"**Nervous**","Sense, decide and command — fast","Brain, spinal cord, nerves","Muscles (orders)",[133,134,135,136],"**Muscular**","Pull, to make movement and force","Skeletal muscles, heart muscle, gut muscle","Bones (movement)",[138,139,140,141],"**Skeletal**","Support, protect, and give muscles something to pull on","Bones, joints, cartilage","The whole body (shape)",{"id":143,"type":43,"markdown":144},"two-more-systems","Two more teams matter to this story, and you should meet them now even though they are quieter.\n\nThe **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.\n\nThe **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.",{"id":146,"type":147,"component":148,"componentVersion":5,"config":149,"objective":225,"textAlternative":226},"lab-sort-organ-system","interactive","sort-game",{"prompt":150,"bins":151,"items":167,"seconds":224},"Put each organ into the system it belongs to. Some organs belong to a system you might not expect.",[152,155,158,161,164],{"id":153,"label":154},"digestive","Digestive",{"id":156,"label":157},"respiratory","Respiratory",{"id":159,"label":160},"circulatory","Circulatory",{"id":162,"label":163},"excretory","Excretory",{"id":165,"label":166},"nervous","Nervous",[168,172,176,180,184,188,192,196,200,204,208,212,216,220],{"id":169,"label":170,"bin":153,"why":171},"stomach","Stomach","It churns food and adds acid, but absorbs almost nothing.",{"id":173,"label":174,"bin":153,"why":175},"pancreas","Pancreas","It sends a jet of digestive juice into the small intestine. It is also an endocrine gland.",{"id":177,"label":178,"bin":153,"why":179},"smallint","Small intestine","Where nearly all absorption happens, through millions of villi.",{"id":181,"label":182,"bin":153,"why":183},"liver","Liver","It makes bile for fats and processes everything absorbed from the gut.",{"id":185,"label":186,"bin":156,"why":187},"trachea","Trachea (windpipe)","A ringed tube that keeps the air path open all the way to the lungs.",{"id":189,"label":190,"bin":156,"why":191},"alveoli","Alveoli","The tiny air sacs where oxygen finally crosses into blood.",{"id":193,"label":194,"bin":156,"why":195},"diaphragm","Diaphragm","The muscle sheet under the lungs that does most of the work of breathing.",{"id":197,"label":198,"bin":159,"why":199},"heart","Heart","A double pump: right side to the lungs, left side to the rest of you.",{"id":201,"label":202,"bin":159,"why":203},"capillaries","Capillaries","Vessels thin enough for substances to cross in and out.",{"id":205,"label":206,"bin":162,"why":207},"kidneys","Kidneys","They filter the blood and decide what to keep.",{"id":209,"label":210,"bin":162,"why":211},"bladder","Bladder","A stretchy store for urine until it is convenient to empty it.",{"id":213,"label":214,"bin":162,"why":215},"skin","Skin (sweat)","Sweat removes water, salt and a little waste — and cools you as a bonus.",{"id":217,"label":218,"bin":165,"why":219},"brain","Brain","It decides, remembers and sends orders.",{"id":221,"label":222,"bin":165,"why":223},"spinalcord","Spinal cord","The main cable, and the place where reflexes are decided without the brain.",0,"Sort fourteen organs into five systems, and read why each one belongs where it does.","This game deals you fourteen organ cards and five bins: Digestive, Respiratory, Circulatory, Excretory and Nervous. Drop each card into a bin and it tells you why it belongs there.\n\nThe straightforward ones: stomach, small intestine and pancreas are **digestive**; trachea, alveoli and diaphragm are **respiratory**; heart and capillaries are **circulatory**; kidneys and bladder are **excretory**; brain and spinal cord are **nervous**.\n\nThree cards are worth arguing about. The **liver** is filed under digestive because it makes bile, but it also cleans the blood, stores sugar and handles almost everything absorbed from the gut. The **pancreas** makes digestive juice *and* the hormone insulin, so it belongs to two systems at once. **Skin** counts as excretory because sweat carries out water, salt and a little urea — it is also part of your temperature control and your first defence against germs.\n\nThe lesson of the game is that the bins are useful but not walls. Real organs often work for two teams.",{"id":228,"type":47,"variant":229,"title":230,"markdown":231},"aha-no-walls","aha","The bins are labels, not walls","You will see the seven systems drawn as seven neat charts, and that is a useful way to learn them. But nothing inside you is labelled. The pancreas does not know it is \"digestive\". The skin is excretory, protective and part of temperature control at the same time.\n\nTreat the systems as **seven good questions** to ask about a body — how does it get food in? how does it get air in? how does it move things around? — rather than seven separate machines.",{"id":233,"type":234,"itemId":235,"prompt":236,"check":237,"hints":253,"feedback":256},"p-levels","practice","body-systems.discover-levels","Put these in order, from smallest to largest: organ, cell, organ system, tissue.",{"kind":238,"options":239,"correct":252},"choice",[240,243,246,249],{"id":241,"label":242},"a","cell → tissue → organ → organ system",{"id":244,"label":245},"b","cell → organ → tissue → organ system",{"id":247,"label":248},"c","tissue → cell → organ → organ system",{"id":250,"label":251},"d","organ → organ system → tissue → cell",[241],[254,255],"Start with the smallest living unit.","Many of the same cells together make a tissue.",{"correct":257,"incorrect":258},"Correct. Cell → tissue → organ → organ system. Muscle cells make muscle tissue, muscle tissue helps make the heart, and the heart is part of the circulatory system.","Start from the smallest: a **cell**. Same-kind cells make a **tissue**; several tissues make an **organ**; organs sharing a job make an **organ system**.",{"id":260,"type":59,"title":261,"eyebrow":262,"navLabel":263},"ch2","The digestive system: a road, not a bag","Chapter 02","2 Digestion",{"id":265,"type":43,"markdown":266},"gut-is-a-tube","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.\n\nFood 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.\n\nSo 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.",{"id":268,"type":269,"title":270,"items":271},"steps-food-road","steps","The food road, station by station",[272,276,280,283,286,290],{"title":273,"tag":274,"text":275},"Mouth","≈ 30 seconds","Teeth cut and grind; the tongue mixes. Saliva wets the food and adds **amylase**, the first enzyme, which starts breaking starch into sugar. Chewing is not optional — it makes the surface the chemicals can work on.",{"title":277,"tag":278,"text":279},"Oesophagus","≈ 8 seconds","A muscular tube. Rings of muscle squeeze behind the food and relax in front of it, pushing it down. This wave is called **peristalsis** and it works even upside down.",{"title":170,"tag":281,"text":282},"2–4 hours","A stretchy bag that churns. It adds strong acid and an enzyme that attacks protein. The acid also kills most germs that arrived with the food.",{"title":178,"tag":284,"text":285},"3–5 hours","About 6 metres of narrow, folded tube. Bile from the liver and juice from the pancreas finish the job, and **absorption happens here** through millions of tiny fingers called villi.",{"title":287,"tag":288,"text":289},"Large intestine","12–48 hours","Wider and shorter. Most of the water is taken back, and the bacteria living here feed on what is left, making some vitamins in return.",{"title":291,"tag":292,"text":293},"Out","the end","What remains is fibre, dead cells and bacteria. Fibre was never digestible — its job was to give the muscles something to push against all the way along.",{"id":295,"type":47,"variant":296,"title":297,"markdown":298},"try-chew","try_it","Chew a piece of roti for a full minute","Take a small piece of plain roti or plain rice — nothing with salt, oil or sugar on it. Put it in your mouth and chew slowly, without swallowing, while you count to sixty.\n\nIt starts tasting **sweet**.\n\nNothing sweet was added. Roti is mostly **starch**, which is a long chain of sugar units joined together, and long chains have no taste. The amylase in your saliva has been snipping the chain into short pieces, and short pieces taste sweet. You have just watched the very first step of digestion happen on your own tongue.\n\n(Do it with a friend and time who notices the sweetness first. Everybody makes slightly different amounts of amylase.)",{"id":300,"type":147,"component":301,"componentVersion":5,"config":302,"objective":335,"textAlternative":336},"lab-flow-digestive","system-flow",{"system":153,"steps":303,"quiz":334,"handoverWith":159},[304,307,310,312,316,321,325,329],{"id":305,"label":273,"text":306,"organ":305},"mouth","Teeth grind the roti into a paste; saliva wets it and amylase starts cutting starch into sugars.",{"id":308,"label":277,"text":309,"organ":308},"oesophagus","Peristalsis: muscle rings squeeze behind the ball of food and push it down in about eight seconds.",{"id":169,"label":170,"text":311,"organ":169},"Churning plus acid and protein-cutting enzymes turn the food into a thick soup over two to four hours.",{"id":313,"label":314,"text":315,"organ":181},"juices","Liver and pancreas","Bile from the liver breaks fat into droplets; pancreatic juice adds enzymes for starch, protein and fat, and cancels the acid.",{"id":317,"label":318,"text":319,"organ":320},"villi","Small intestine: villi","The hand-over point. Sugars, amino acids, vitamins and minerals cross the one-cell-thick villus wall into the capillaries inside it.","small intestine",{"id":322,"label":323,"text":324,"organ":181},"blood","Into the blood","Nutrient-rich blood leaves the gut and goes first to the liver, which sorts, stores and cleans before sending it on.",{"id":326,"label":287,"text":327,"organ":328},"largeint","Water and salts are pulled back out of the leftovers; gut bacteria feed and make some vitamin K and B vitamins.","large intestine",{"id":330,"label":331,"text":332,"organ":333},"out","Leftovers out","Fibre, dead gut-lining cells and bacteria leave the body, typically one to three days after the meal.","rectum",true,"Follow one roti from mouth to blood and find the exact place where the digestive system hands over to the circulatory system.","This lab lays the digestive system out as eight stations you can step through, with the **circulatory system** drawn alongside so you can see where the two touch.\n\nStep through mouth, oesophagus, stomach, liver and pancreas, small intestine, large intestine, and out. At each station the lab shows what is added and what changes: saliva and amylase at the mouth, acid at the stomach, bile and pancreatic juice in the small intestine.\n\nOnly **one** station is marked as a hand-over: the **villi of the small intestine**. Here the drawing shows the two systems joined. The wall of a villus is a single cell thick, and inside each villus is a loop of capillary. Sugars, amino acids, vitamins and minerals cross that wall and are gone — into the blood, out of the gut, away to the liver.\n\nNotice what the digestive system does *not* do. It never delivers anything. It prepares a parcel and leaves it at the gate. The circulatory system does every kilometre of the delivery.\n\nTurn the quiz on and it asks you to point at the hand-over station, and to say what would go wrong if the villi were flat instead of folded.",{"id":338,"type":47,"variant":339,"title":340,"markdown":341},"nuance-gut-bacteria","nuance","You are carrying trillions of helpers","Your large intestine is home to an enormous population of bacteria. Careful counting in 2016 put it at roughly **38 trillion** bacterial cells against about **30 trillion** human cells — close to **1.3 bacteria for every one of your own cells**.\n\nYou may have read that the ratio is **10 to 1**. That was an old estimate, repeated for forty years because it sounded striking, until somebody measured it properly. The honest number is far less dramatic and far more interesting.\n\nThese bacteria are not passengers. They digest fibre your own enzymes cannot touch, make vitamin K and several B vitamins, and crowd out germs that would otherwise settle in.",{"id":343,"type":234,"itemId":344,"prompt":345,"check":346,"hints":357,"feedback":360},"p-absorb-where","body-systems.discover-absorb-where","At which station do the useful parts of your food actually enter your body?",{"kind":238,"options":347,"correct":356},[348,350,352,354],{"id":241,"label":349},"The mouth, as soon as chewing starts",{"id":244,"label":351},"The stomach, where the acid is",{"id":247,"label":353},"The small intestine, through the villi",{"id":250,"label":355},"The large intestine, with the water",[247],[358,359],"\"Entering your body\" means crossing into the blood.","Which organ is lined with millions of tiny fingers?",{"correct":361,"incorrect":362},"Yes. Almost all absorption happens through the **villi of the small intestine**. The stomach absorbs a little water and a few medicines, and the large intestine takes back water — but the main crossing is in the small intestine.","The stomach churns and part-digests; it barely absorbs. The crossing point is the **small intestine**, whose wall is folded into millions of villi to make the surface enormous.",{"id":364,"type":59,"title":365,"eyebrow":366,"navLabel":367},"ch3","The respiratory system: air in, air out","Chapter 03","3 Breathing",{"id":369,"type":43,"markdown":370},"breathing-path","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.\n\nAir 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.\n\nAt 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.",{"id":372,"type":373,"tone":374,"items":375},"spec-breathing","spec","blue",[376,380,384,388,392,395],{"label":377,"big":378,"value":379},"Breaths per minute, resting","20","Typical for a 9–13 year old. Adults are slower, around 12–18; a newborn is much faster.",{"label":381,"big":382,"value":383},"Breaths per day","21,600","At 15 a minute. You never decide to take a single one of them.",{"label":385,"big":386,"value":387},"Air per breath","≈ 500 mL","A \"tidal\" breath at rest — about the volume of a small water bottle. A deep breath moves far more.",{"label":389,"big":390,"value":391},"Air per day","10,800 L","21,600 breaths × 0.5 L. That is about eleven thousand litres through your chest, every day.",{"label":190,"big":393,"value":394},"≈ 300 million","Tiny sacs, each wrapped in capillaries. Together about 70 m² of surface for gases to cross.",{"label":396,"big":397,"value":398},"Oxygen in vs out","21% → 16%","You keep only about a quarter of the oxygen you breathe in — which is why rescue breathing works.",{"id":400,"type":47,"variant":401,"title":402,"markdown":403},"misconception-lungs-suck","misconception","\"Lungs pull air in by themselves\"","Lungs have **no muscle of their own**. They are soft, floppy and completely passive — they cannot pull, push or squeeze.\n\nThe work is done by the **diaphragm**, a dome-shaped sheet of muscle under your lungs, helped by the muscles between your ribs. When the diaphragm contracts it flattens downward and the ribs swing up and out. The chest gets bigger, the pressure inside drops, and air pushes *in from outside* to fill the space.\n\nBreathing out at rest needs no effort at all: the diaphragm simply relaxes back into its dome, the stretched lungs spring back, and air leaves.\n\nSo air is not sucked in by the lungs. It is **pushed in by the atmosphere**, into a space a muscle has just made.",{"id":405,"type":147,"component":301,"componentVersion":5,"config":406,"objective":432,"textAlternative":433},"lab-flow-respiratory",{"system":156,"steps":407,"quiz":334,"handoverWith":159},[408,412,415,419,421,425,429],{"id":409,"label":410,"text":411,"organ":409},"nose","Nose","Air is warmed, moistened and filtered. Hairs catch dust; sticky mucus traps the rest.",{"id":185,"label":413,"text":414,"organ":185},"Trachea","A tube held open by C-shaped cartilage rings, so it cannot collapse when you bend your neck.",{"id":416,"label":417,"text":418,"organ":416},"bronchi","Bronchi and bronchioles","The pipe splits in two, then keeps splitting — about twenty branchings down to tubes thinner than a hair.",{"id":189,"label":190,"text":420,"organ":189},"The hand-over point. 300 million air sacs, each wrapped in capillaries, with a wall only one cell thick.",{"id":422,"label":423,"text":424,"organ":189},"oxygenin","Oxygen crosses in","There is more oxygen in the air sac than in the arriving blood, so oxygen moves across and clips onto haemoglobin in the red cells.",{"id":426,"label":427,"text":428,"organ":189},"co2out","Carbon dioxide crosses out","There is more carbon dioxide in the arriving blood than in the air sac, so it moves the other way and is breathed out.",{"id":193,"label":430,"text":431,"organ":193},"The diaphragm does the work","Contract: the dome flattens, the chest grows, air pushes in. Relax: the chest shrinks, air leaves. No effort needed to breathe out at rest.","Trace a breath from nose to alveolus and watch both gases cross in opposite directions at the same wall.","This lab steps through the air path — nose, trachea, bronchi and bronchioles, alveoli — with the **circulatory system** shown beside it.\n\nThe hand-over station is the **alveolus**. The lab zooms in on one: a bubble about a fifth of a millimetre across, with a wall a single cell thick, hugged by a capillary whose wall is also a single cell thick. Two cell layers is the entire distance between the air you just breathed and your blood.\n\nTwo arrows cross that wall in **opposite directions at the same moment**. Oxygen goes from the air sac into the blood, because there is more of it in the sac. Carbon dioxide goes from the blood into the sac, because there is more of it in the blood. Nothing pumps them; each simply drifts from where it is crowded to where it is not.\n\nThe last step shows the diaphragm. Contract, and the chest cavity grows and air pushes in. Relax, and it leaves. Turn the quiz on and it asks which way each gas moves, and what the alveoli would have to look like for the exchange to fail.",{"id":435,"type":436,"prompt":437,"options":438,"explanation":447},"predict-hold-breath","prediction","Hold your breath for as long as you comfortably can (stop the moment it stops being comfortable). After about thirty seconds an urgent feeling builds up. **What is your body actually reacting to?**",[439,441,443,445],{"id":241,"label":440},"Running out of oxygen",{"id":244,"label":442},"Carbon dioxide building up in the blood",{"id":247,"label":444},"The lungs getting too empty",{"id":250,"label":446},"The heart slowing down","**Carbon dioxide building up.** This surprises almost everyone.\n\nYour brain has sensors that watch the amount of carbon dioxide in the blood far more closely than the amount of oxygen. Carbon dioxide dissolves to make the blood slightly acidic, and that change is fast, reliable and easy to detect. Oxygen levels fall much more slowly, and by the time they are low you may already be in trouble — a poor alarm to rely on.\n\nSo the urgent, uncomfortable feeling is not \"I need oxygen\". It is \"there is too much waste gas in here\". The alarm fires early and safely, with plenty of oxygen still left.\n\n**Never** try to beat this feeling by breathing hard and fast first, especially near water. Doing so blows off carbon dioxide and disables the alarm without adding any useful oxygen — people have drowned that way.",{"id":449,"type":47,"variant":450,"title":451,"markdown":452},"careful-breath-safety","careful","A rule for breath-holding games","Breath-holding for a few seconds on dry land, sitting down, is harmless and interesting. Two rules make it stay that way:\n\n1. **Stop when it becomes uncomfortable.** The discomfort is the safety alarm, not the enemy.\n2. **Never do it in or under water**, and never take fast deep breaths beforehand to \"prepare\". That combination is genuinely dangerous.\n\nIf you ever feel dizzy, just breathe normally and sit still until it passes.",{"id":454,"type":59,"title":455,"eyebrow":456,"navLabel":457},"ch4","The circulatory system: the delivery service","Chapter 04","4 Circulation",{"id":459,"type":43,"markdown":460},"heart-double-pump","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.\n\nThe **heart** is the pump, and it is really **two pumps stuck together**, side by side, beating at the same instant.\n\n- The **right** pump takes tired blood arriving from the body and sends it a short distance to the **lungs**.\n- The **left** pump takes freshly loaded blood back from the lungs and sends it to **everywhere else** — brain, fingers, kidneys, toes.\n\nEach 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.\n\nThe 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.",{"id":462,"type":373,"tone":463,"items":464},"spec-heart","copper",[465,469,473,477,481,485],{"label":466,"big":467,"value":468},"Resting pulse, age 9–13","80 bpm","Anywhere from about 70 to 100 is ordinary. Fitter people tend to be slower, because each beat does more.",{"label":470,"big":471,"value":472},"Beats per day","103,680","At 72 beats a minute — the origin of the famous \"about 100,000 beats a day\".",{"label":474,"big":475,"value":476},"Pumped per beat","≈ 70 mL","Roughly a third of a cup, from the left ventricle alone, every single beat.",{"label":478,"big":479,"value":480},"Pumped per day","≈ 7,300 L","103,680 × 70 mL = 7,257.6 L — over seven full 1,000-litre overhead tanks.",{"label":482,"big":483,"value":484},"Blood in an adult","≈ 5 L","About 70 mL for every kilogram of body mass, so a 30 kg child has around 2.1 L.",{"label":486,"big":487,"value":488},"All the vessels, end to end","≈ 100,000 km","About 2.5 times around the Earth. Almost all of it is capillary.",{"id":490,"type":491,"title":492,"prompt":493,"options":494},"explorer-vessels","explorer","Three kinds of pipe","Pick a vessel to see what it is built for.",[495,508,519],{"id":496,"label":497,"chain":498,"badge":504,"note":507},"artery","Arteries",[499,500,501,502,503],"Heart squeezes","Thick springy wall stretches","Wall recoils","Blood pushed onward","Pulse you can feel",{"text":505,"tone":506},"Away from the heart","yes","Arteries carry blood **away** from the heart, at high pressure and in surges. Their walls are thick, muscular and elastic: they balloon slightly with each beat and spring back between beats, which smooths the flow and keeps blood moving even while the heart is refilling. That stretch-and-recoil is exactly what you feel as a **pulse** at your wrist or neck. Because the pressure inside is high, arteries are buried deep where they are protected.",{"id":509,"label":202,"chain":510,"badge":516,"note":518},"capillary",[511,512,513,514,515],"Artery narrows","Wall becomes one cell thick","Substances cross both ways","Cells served","Vein collects",{"text":517,"tone":506},"Where the work happens","Capillaries are the point of the whole system. They are so narrow (about 8 micrometres, roughly a ninth the width of a hair) that red cells travel through them almost single file, and their wall is a **single cell thick**. That thinness is deliberate: oxygen and food slip out to the surrounding cells, and carbon dioxide and waste slip in. No cell in your body is more than a fraction of a millimetre from one. Blood also slows right down here — which sounds like a fault but is the design, because crossing takes time. Arteries and veins are just the roads; the capillary is the doorstep.",{"id":520,"label":521,"chain":522,"badge":528,"note":530},"vein","Veins",[523,524,525,526,527],"Capillaries join up","Wide, floppy tube","Muscles squeeze the vein","One-way valves open","Blood returns to heart",{"text":529,"tone":506},"Back to the heart","Veins carry blood **back** to the heart. By the time blood reaches them the push from the heartbeat has almost gone, so veins face a real problem: how do you get blood up from your feet with no pressure left? Two answers. First, veins have **one-way valves** along their length, so blood can move towards the heart but not back down. Second, every time you walk, the calf muscles squeeze the veins between them, and the valves turn that squeeze into upward movement. Your calf is sometimes called the second heart — which is why standing still makes your feet swell.",{"id":532,"type":47,"variant":401,"title":533,"markdown":534},"misconception-blue-blood","\"Blood is blue until it meets the air\"","Blood is **never** blue. Not in your veins, not anywhere, not for a moment.\n\nBlood loaded with oxygen is bright scarlet. Blood that has given most of its oxygen away is a deep dark red, the colour of a ripe jamun. Both are red.\n\nThe veins on the back of your hand look blue-green for a reason that has nothing to do with blood: it is about **skin and light**. Skin scatters and absorbs the different colours of light unequally. Red light penetrates deeply and is mostly swallowed by the dark blood; blue light bounces back from the upper layers of skin before it ever reaches the vein. So what returns to your eye from over a vein is relatively rich in blue — and your brain, comparing it with the pinker skin next to it, calls it blue.\n\nThe diagrams that draw arteries in red and veins in blue are using a **colour code**, exactly like a red hot tap and a blue cold one. Nobody thinks cold water is blue.",{"id":536,"type":537,"title":538,"problem":539,"steps":540,"help":547},"we-beats-day","worked_example","How many times will your heart beat today?","A resting pulse of **72 beats a minute** is a fair average for an adult. How many beats is that in a full day, and how much blood is that, if each beat pushes out about 70 mL?",[541,542,543,544,545,546],"Beats in one hour: 72 × 60 = **4,320**.","Beats in one day: 4,320 × 24 = **103,680** — which is where \"about 100,000 beats a day\" comes from.","Blood per day: 103,680 × 70 mL = 7,257,600 mL.","Convert to litres: 7,257,600 ÷ 1,000 = **7,257.6 L**.","Picture it: a large domestic overhead tank holds 1,000 L, so your heart moves about **7.3 tankfuls a day** — while you sit still.","And none of it is new blood. The same 5 litres goes round and round, about once a minute.",{"simplerExplanation":548,"hints":549},"Beats per minute × 60 = per hour. × 24 = per day. Multiply by 70 mL and divide by 1,000 to get litres.",[550,551],"Do the hour first, then the day.","1,000 mL = 1 L.",{"id":553,"type":234,"itemId":554,"prompt":555,"check":556,"hints":560,"feedback":563},"p-beats-hour","body-systems.discover-beats-hour","A classmate counts a resting pulse of **80 beats a minute**. How many times will her heart beat in one hour?",{"kind":557,"answer":558,"tolerance":224,"unit":559},"number",4800,"beats",[561,562],"There are 60 minutes in an hour.","80 × 60.",{"correct":564,"incorrect":565},"Right: 80 × 60 = **4,800 beats an hour**. In a day that would be 115,200.","Multiply beats per minute by the number of minutes in an hour: 80 × 60 = 4,800.",{"id":567,"type":59,"title":568,"eyebrow":569,"navLabel":570},"ch5","The excretory system: keeping the blood clean","Chapter 05","5 Cleaning up",{"id":572,"type":43,"markdown":573},"kidneys-intro","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.\n\nA 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.\n\n**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.\n\n**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.",{"id":575,"type":47,"variant":229,"title":576,"markdown":577},"aha-filter-then-reclaim","Why throw out what you mean to keep?","It sounds like madness: filter 180 litres, then take 99% of it straight back.\n\nBut think about how you would clean a bag of rice with a few small stones in it. Picking out each stone by hand is slow and you will miss some. Tipping everything into a sieve and then choosing what to keep is quick, and — this is the point — it deals with stones you have **never seen before**.\n\nYour kidneys work the same way. They do not need a special detector for every possible waste chemical, including ones from a medicine invented last year. They dump everything small, then actively reclaim the short list of things worth keeping. Anything not on the list leaves. It is a filter that does not need to know what it is filtering.",{"id":579,"type":537,"title":580,"problem":581,"steps":582,"help":589},"we-kidney-glasses","What if the kidneys did not take the water back?","Your kidneys filter about **180 litres** of fluid a day and give back all but **1.5 litres**. Suppose they filtered exactly the same amount but reclaimed nothing. How much would you have to drink each day just to stay level — and how many 250 mL glasses is that?",[583,584,585,586,587,588],"Losing 180 L a day, you would have to drink 180 L a day.","In millilitres: 180 × 1,000 = 180,000 mL.","Glasses: 180,000 ÷ 250 = **720 glasses a day**.","That is one glass every two minutes, awake and asleep, forever.","Now the real figures: you actually reclaim 178.5 L, which is 99.2% of everything filtered, and pass only 1.5 L.","The kidney's real skill is not throwing things away. It is knowing what to keep.",{"simplerExplanation":590,"anotherExample":591},"180 litres is 180,000 mL. Divide by 250 mL per glass: 720 glasses.","Your whole blood volume is about 5 L, so filtering 180 L a day means the kidneys process the equivalent of your entire blood supply 36 times over.",{"id":593,"type":99,"caption":594,"columns":595,"rows":600},"table-out-routes","Four ways things leave the body — only some of them count as excretion.",[596,597,598,599],"Route","What leaves","How much a day","Is it excretion?",[601,606,611,616],[602,603,604,605],"**Kidneys** → urine","Water, salts, urea (protein waste)","≈ 1.5 L","Yes — the main route, and the only adjustable one",[607,608,609,610],"**Lungs** → breath","Carbon dioxide and water vapour","≈ 0.4 L of water, plus all your CO₂","Yes — you excrete with every breath out",[612,613,614,615],"**Skin** → sweat","Water, salt, a little urea","≈ 0.5 L at rest; far more in the heat","Yes, though cooling is its main purpose",[617,618,619,620],"**Gut** → faeces","Fibre, dead cells, gut bacteria","Varies","Mostly **no** — this never entered the body, so it is elimination, not excretion. (Bile pigments are a genuine exception.)",{"id":622,"type":47,"variant":48,"title":623,"markdown":624},"observation-urine-colour","What the colour is telling you","Pale straw-yellow urine usually means you have had plenty of water and your kidneys are comfortably passing the extra. Dark yellow usually means the kidneys are conserving water, so the same waste is dissolved in less of it.\n\nIt is a rough signal, not a medical test — some foods (beetroot) and vitamin tablets change the colour completely, and so do some medicines. But as a habit it is a useful nudge: if it is dark and you feel thirsty, drink some water.\n\nThirst itself is the more reliable guide. Drinking when you are thirsty, and a little extra on a hot day or after sport, is the whole rule. There is no prize for drinking more than that.",{"id":626,"type":59,"title":627,"eyebrow":628,"navLabel":629},"ch6","The nervous system: sensing and commanding","Chapter 06","6 Nerves",{"id":631,"type":43,"markdown":632},"nerves-intro","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**.\n\nIt 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.\n\nSignals travel along the thickest, best-insulated nerve fibres at up to about **120 metres a second** — that is 432 km\u002Fh, 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.",{"id":634,"type":269,"title":635,"items":636},"steps-reflex-arc","The reflex arc: five steps, and the brain is not one of them",[637,641,645,649,652,656],{"title":638,"tag":639,"text":640},"1. Receptor","the skin","You touch a hot vessel. Heat and pain sensors in your fingertip fire immediately.",{"title":642,"tag":643,"text":644},"2. Sensory neuron","in ≈ 10 ms","The signal races up the arm nerve to the spinal cord — not to the brain.",{"title":646,"tag":647,"text":648},"3. Spinal cord","the decision","A connecting neuron in the cord passes the signal straight across to a motor neuron. This is the whole \"decision\", and it takes almost no time.",{"title":650,"tag":330,"text":651},"4. Motor neuron","The order travels back down the arm to the biceps.",{"title":653,"tag":654,"text":655},"5. Effector","the muscle","The biceps contracts and your hand is already moving away — before you have consciously felt anything.",{"title":657,"tag":658,"text":659},"And then","the brain","A copy of the signal continues up to the brain, which is how you become aware of the pain **after** you have moved. Your body saved your hand while you were still catching up.",{"id":661,"type":47,"variant":662,"title":663,"markdown":664},"example-two-reflexes","example","Two reflexes you can meet today","**The knee jerk.** Sit with one leg crossed loosely over the other so the lower leg hangs free. Have someone tap gently, once, just below the kneecap with the side of a hand. The leg kicks — and you could not stop it if you tried. The tap stretches a tendon, the muscle's stretch sensors fire, the signal reaches the spinal cord and comes straight back as \"contract\". Total time: about **20 milliseconds**, roughly **10 times faster** than the 200 ms it takes you to react to something you see.\n\nDoctors use this test because a normal jerk tells them the nerves, the cord and the muscle are all connected properly. It is not a party trick; it is a circuit test.\n\n**Pulling back from something hot.** The same shape of circuit, with a much more obvious purpose. A quarter of a second saved is the difference between a fright and a burn.",{"id":666,"type":436,"prompt":667,"options":668,"explanation":677},"predict-blink","Somebody claps their hands suddenly in front of your face. You blink, even though you knew they were going to do it and told yourself not to. **Why can you not stop it?**",[669,671,673,675],{"id":241,"label":670},"Your brain decides to blink too quickly to cancel",{"id":244,"label":672},"The order is generated below the level of conscious decision, and your decision arrives too late",{"id":247,"label":674},"Eyelids move on their own with no nerve involved",{"id":250,"label":676},"It is a habit you could unlearn with practice","**The order is generated below conscious control.** A protective reflex like blinking is wired to run *first* and inform you afterwards. The pathway that closes the eyelid is short and fast; the pathway that would let you veto it goes up to the brain, gets considered, and comes back — far too slowly.\n\nThis is not a flaw. Reflexes protect exactly the things that cannot afford a delay: eyes, skin, balance, airway. Evolution's rule seems to be that when damage is possible in a tenth of a second, you do not get a vote.\n\nYou *can* train some reflexes to weaken — a cricket wicketkeeper learns not to flinch — but that is the brain slowly turning the volume down on a circuit, not switching it off.",{"id":679,"type":47,"variant":339,"title":680,"markdown":681},"nuance-voluntary","Who is actually in charge?","Some of what your body does, you decide. Most of it, you do not.\n\nYou choose to kick a ball, write your name or jump. You do **not** choose to beat your heart, push food along your gut, sweat, shiver, or narrow your pupils in bright light — and willing any of them to stop simply does not work. Try it with sweating on a hot day and see.\n\nA few sit in between. **Breathing, blinking and swallowing** all run automatically but accept a conscious override. Breathing is the clearest case: it is the only life-support process you can take the controls of at all. But only up to a point — hold your breath long enough and the automatic system takes them straight back, which is why you cannot hold your breath until you faint by willpower alone.\n\nThe rule behind the split is **time**. Anything that must be decided in milliseconds, or must never be forgotten, is wired below consciousness. Anything that needs judgement is sent up to the brain.",{"id":683,"type":59,"title":684,"eyebrow":685,"navLabel":686},"ch7","Muscles and bones: the machine that moves","Chapter 07","7 Muscles and bones",{"id":688,"type":43,"markdown":689},"muscles-bones","Muscles can do exactly one thing: **pull**. A muscle cannot push. It shortens, and that is its entire vocabulary.\n\nSo 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.\n\nBones 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.\n\nThere 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.",{"id":691,"type":53,"conceptId":54,"relation":692,"explanation":693},"conn-anatomy-bones","related_to","Which bone is which, where the big muscles attach and how joints are shaped are covered properly in the anatomy topic.",{"id":695,"type":147,"component":696,"componentVersion":5,"config":697,"objective":735,"textAlternative":736},"lab-body-explorer","body-explorer",{"views":698,"parts":702,"modes":732},[699,700,701],"organs","skeleton","muscles",[703,705,708,710,712,715,717,719,721,724,727,730],{"id":197,"note":704},"A double pump the size of your fist. Right side to the lungs, left side to the rest of you.",{"id":706,"note":707},"lungs","Two soft sponges with no muscle of their own. The diaphragm below them does the work.",{"id":193,"note":709},"The dome of muscle that flattens to pull air in. Hiccups are this muscle twitching.",{"id":169,"note":711},"A stretchy churn with acid. It part-digests; it barely absorbs.",{"id":713,"note":714},"intestines","Six metres of small intestine where absorption happens, then the shorter, wider large intestine that recovers water.",{"id":181,"note":716},"The chemical works. It makes bile, stores sugar, and processes everything absorbed from the gut.",{"id":205,"note":718},"Two filters that process about 180 litres of fluid a day and hand back almost all of it.",{"id":217,"note":720},"The decider. About 2% of your mass and about 20% of your energy use.",{"id":722,"note":723},"ribcage","A cage that protects the heart and lungs and swings up and out every time you breathe in.",{"id":725,"note":726},"femur","The thigh bone: the longest, strongest lever in your body.",{"id":728,"note":729},"biceps","Bends the elbow by pulling. To straighten it, the triceps behind pulls the other way.",{"id":213,"note":731},"Barrier, temperature control and excretory route all at once — your largest organ.",[733,734],"explore","find-it","Switch between organs, skeleton and muscles, and see which system each part belongs to.","A model of the body with three views you can switch between — **organs**, **skeleton** and **muscles** — and twelve parts you can tap for a note.\n\nIn **explore** mode, tapping a part tells you what it does and which system it serves. Tapping the liver, for example, tells you it belongs to the digestive system but also cleans the blood and stores sugar. Tapping the skin tells you it is a barrier, a cooling system and an excretory route.\n\nIn **find-it** mode the lab names a part and you have to find it: \"find the muscle that flattens to pull air into your lungs\" (the diaphragm), \"find the organ that filters about 180 litres a day\" (the kidneys), \"find the longest bone\" (the femur).\n\nTwo things are worth noticing. First, how **packed** the body is — there is no spare space, and every organ presses against its neighbours. Second, how the protective bones line up with what needs protecting: skull over brain, ribcage over heart and lungs, spine around the spinal cord.",{"id":738,"type":59,"title":739,"eyebrow":740,"navLabel":741},"ch8","The hand-overs: where systems touch","Chapter 08","8 The hand-overs",{"id":743,"type":43,"markdown":744},"handovers-intro","Now for the heart of this whole topic.\n\nEvery 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.\n\nEvery 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:\n\n1. **A very thin barrier.** Usually one cell thick, sometimes two. Anything thicker is too slow.\n2. **A huge surface area**, achieved by folding, branching or ballooning. Millions of alveoli; millions of villi; kilometres of capillary.\n3. **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.\n\nLook 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.",{"id":746,"type":99,"caption":747,"columns":748,"rows":753},"table-handovers","The six hand-overs that keep you alive.",[749,750,751,752],"Hand-over","Where exactly","What crosses","Which way",[754,759,764,769,774,779],[755,756,757,758],"Lungs ↔ blood","Alveolus wall, one cell thick","Oxygen and carbon dioxide","O₂ in, CO₂ out — at the same wall, at the same time",[760,761,762,763],"Gut ↔ blood","Villus wall in the small intestine","Sugars, amino acids, fats, vitamins, minerals","Into the blood, then straight to the liver",[765,766,767,768],"Blood ↔ every cell","Capillary wall, everywhere in you","Oxygen and food out; carbon dioxide and waste in","Both ways at once — the reverse of the alveolus",[770,771,772,773],"Blood ↔ kidneys","A million filters per kidney","Water, salt, sugar, urea filtered out; nearly all taken back","Out, then mostly back in",[775,776,777,778],"Nerves ↔ muscles","The junction where a nerve ending meets a muscle fibre","A chemical signal telling the fibre to contract","One way only: nerve to muscle",[780,781,782,783],"Muscles ↔ bones","Tendons, at the joint","Force — a pull","One way: muscle pulls, bone swings",{"id":785,"type":47,"variant":229,"title":786,"markdown":787},"aha-same-trick","The alveolus and the capillary are the same idea, reversed","At the **alveolus**, oxygen leaves the air and enters the blood, and carbon dioxide leaves the blood and enters the air.\n\nAt a **capillary** beside a working muscle, oxygen leaves the blood and enters the cell, and carbon dioxide leaves the cell and enters the blood.\n\nSame thin wall, same two gases, same rule — *each gas drifts from where it is crowded to where it is not* — and the direction simply flips, because at one end the air is oxygen-rich and at the other end the muscle is oxygen-hungry.\n\nNothing steers the gases. There is no valve, no pump, no decision. The only reason oxygen ends up in your toe is that your toe keeps using it up, and so keeps the difference alive.",{"id":789,"type":537,"title":790,"problem":791,"steps":792,"help":803},"we-roti-journey","The journey of one roti, from plate to working muscle","Follow a single mouthful of roti all the way to the moment it powers a step. Roughly how long does each stage take, and which system is in charge at each point?",[793,794,795,796,797,798,799,800,801,802],"**Mouth, about 30 seconds.** Teeth grind; saliva wets it; amylase starts cutting starch into sugar. *Digestive system.*","**Oesophagus, about 8 seconds.** Peristalsis pushes the ball of food down. *Digestive.*","**Stomach, 2–4 hours.** Churned with acid into a thick soup. *Digestive.*","**Small intestine, 3–5 hours.** Bile and pancreatic juice finish the breakdown. Starch is now **glucose**. **HAND-OVER 1:** glucose crosses the villus wall into a capillary. *Digestive → circulatory.*","**Liver, minutes.** All blood from the gut goes here first. The liver keeps the blood's sugar level steady, storing the excess and releasing it later. *Circulatory, with the liver as manager.*","**Around the body, about 60 seconds per lap.** Glucose is carried in the plasma, oxygen on the red cells, going round and round until something needs them. *Circulatory.*","**A capillary in your calf muscle. HAND-OVER 2:** glucose and oxygen cross the capillary wall into a muscle cell. *Circulatory → muscular.*","**Inside the muscle cell.** Glucose plus oxygen release energy; the muscle fibre uses it to pull. Carbon dioxide and water are left over. **HAND-OVER 3:** the carbon dioxide crosses back into the blood. *Muscular → circulatory.*","**Back at the lungs. HAND-OVER 4:** carbon dioxide crosses into an alveolus and you breathe it out — that same roti, leaving as a gas. *Circulatory → respiratory.*","**And the rest.** Fibre never left the tube; it travels on to the large intestine and leaves 17 to 57 hours after the meal. Four hand-overs, five systems, one roti.",{"simplerExplanation":804,"anotherExample":805},"Digestive takes the roti apart → blood collects the pieces → blood delivers them to a muscle → the muscle burns them → blood carries the waste gas back → lungs breathe it out.","Follow a breath the other way: air → alveolus → blood → capillary → the same muscle cell, arriving to meet the glucose.",{"id":807,"type":147,"component":808,"componentVersion":5,"config":809,"objective":837,"textAlternative":838},"lab-match-handovers","match-pairs",{"prompt":810,"mode":811,"pairs":812},"Match each hand-over point to what actually crosses there.","connect",[813,816,819,822,825,828,831,834],{"a":814,"b":815},"Alveolus wall","Oxygen in, carbon dioxide out",{"a":817,"b":818},"Villus wall","Glucose and amino acids into the blood",{"a":820,"b":821},"Capillary beside a muscle","Food and oxygen out, waste in",{"a":823,"b":824},"Kidney filter","Water and waste out of the blood",{"a":826,"b":827},"Nerve-to-muscle junction","A chemical \"contract now\" signal",{"a":829,"b":830},"Tendon at a joint","A pull, from muscle to bone",{"a":832,"b":833},"Gut wall to liver","Nutrient-rich blood, for sorting and storing",{"a":835,"b":836},"Skin surface","Water and salt out, as sweat","Connect each place where two systems touch with the thing that crosses there.","Eight hand-over places on one side, eight things that cross on the other; draw the lines.\n\n- **Alveolus wall** ↔ oxygen in, carbon dioxide out (both at once, both by drifting).\n- **Villus wall** ↔ glucose and amino acids into the blood.\n- **Capillary beside a muscle** ↔ food and oxygen out, waste in — the alveolus in reverse.\n- **Kidney filter** ↔ water and waste out of the blood.\n- **Nerve-to-muscle junction** ↔ a chemical \"contract now\" signal, and it only goes one way.\n- **Tendon at a joint** ↔ a pull, from muscle to bone. This is the one hand-over that passes **force** rather than a substance.\n- **Gut wall to liver** ↔ nutrient-rich blood, sent for sorting and storing before it goes anywhere else.\n- **Skin surface** ↔ water and salt out, as sweat.\n\nWhen you have finished, look at the list again and notice how many of them are the circulatory system. Six of the eight involve blood. The circulatory system is not one team among seven — it is the road every other team's work travels on.",{"id":840,"type":59,"title":841,"eyebrow":842,"navLabel":843},"ch9","All at once: what happens when you run","Chapter 09","9 Running",{"id":845,"type":43,"markdown":846},"exercise-all-at-once","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.\n\n**Muscles** demand far more glucose and oxygen, and start producing far more carbon dioxide and heat.\n**Breathing** goes from about 20 breaths a minute to 45 or more, and each breath gets deeper — so far more air moves per minute.\n**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.\n**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.)\n**Skin** flushes and **sweat** starts, to dump the extra heat.\n\nWhat 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.",{"id":848,"type":47,"variant":296,"title":849,"markdown":850},"try-pulse","Find your own pulse, then change it","Place two fingertips — **not your thumb**, which has a pulse of its own — on the thumb side of the inside of your wrist, and press gently. Move them about until you feel a soft, regular tap. Your neck, just beside the windpipe, is often easier.\n\n1. **Count for 15 seconds** and multiply by 4. That is your resting pulse in beats per minute. For someone your age, anywhere from about 70 to 100 is ordinary.\n2. Now **do 30 star jumps**, sit straight back down and count again immediately.\n3. Keep counting for 15 seconds at a time, once a minute, until you are back to where you started. Write down how long that took.\n\nThat recovery time is genuinely interesting: on average, the fitter someone is, the faster their pulse comes back down. Compare with a friend — but compare *changes*, not raw numbers, because resting pulses differ between people for all sorts of reasons.\n\nTwo rules: stop if you feel dizzy or unwell, and do not do this instead of anything a doctor has told you.",{"id":852,"type":147,"component":853,"componentVersion":5,"config":854,"objective":900,"textAlternative":901},"lab-pulse-data","data-lab",{"datasets":855,"valueRange":885,"step":5,"challenges":888},[856,870],{"label":857,"values":858,"unit":869},"Resting pulse, one class of 12",[859,860,861,862,863,864,865,866,861,867,868,860],72,76,80,68,84,78,74,82,70,88,"bpm",{"label":871,"values":872,"unit":869},"Pulse straight after 1 minute of star jumps",[873,874,875,876,877,878,879,880,881,882,883,884],124,132,140,118,146,136,128,142,138,122,150,134,{"min":886,"max":887},50,170,[889,893,897],{"measure":890,"target":891,"prompt":892},"mean",77.3,"What is the mean resting pulse of the class? (Answer: 77.3 bpm.)",{"measure":894,"target":895,"prompt":896},"range",20,"How spread out are the resting pulses? Find the range. (Answer: 20 bpm.)",{"measure":890,"target":898,"prompt":899},134.2,"Now the mean after exercise. How much has it risen? (Answer: 134.2 bpm.)","Plot one class's pulse before and after exercise, and read off the mean, median, mode and range of each.","Two sets of real-shaped data from a class of twelve, plotted as dots you can drag, with the mean, median, mode and range updating live.\n\n**Resting pulses:** 72, 76, 80, 68, 84, 78, 74, 82, 80, 70, 88, 76 bpm. Mean **77.3**, median **77**, mode **76**, range **20**.\n\n**Straight after one minute of star jumps:** 124, 132, 140, 118, 146, 136, 128, 142, 138, 122, 150, 134 bpm. Mean **134.2**, range **32**.\n\nThe mean rose by **56.9 bpm** — about **1.74 times** the resting value. Notice the second thing too: the dots **spread out**, range 20 before and 32 after. At rest, twelve children are fairly similar. Push them, and their differences show.\n\nSwap in your own class's numbers and the statistics recalculate. The challenges ask you to find each measure and explain what a big range actually means about a group of people.",{"id":903,"type":53,"conceptId":904,"relation":905,"explanation":906},"conn-data-handling","data-handling","applied_in","Pulse and breathing rates before and after exercise are real measurements to average, compare and graph — mean, median, mode and range with a reason to care.",{"id":908,"type":47,"variant":339,"title":909,"markdown":910},"nuance-health-links","What all this suggests about looking after yourself","No lecture, no rules, no shame about anybody's body. Just four things that follow directly from how the systems work.\n\n- **Move.** The heart is a muscle, and muscles get stronger when they are used. Anything that gets you a bit out of breath counts: cycling, running about, climbing stairs, kho-kho.\n- **Eat a variety.** Different foods bring different building blocks, and no single food brings them all. Fibre matters for a reason you have just met — it gives the gut muscles something to push against and feeds the bacteria that help you.\n- **Drink when thirsty**, a little more when it is hot or you have been running. Thirst is a very good sensor.\n- **Wash your hands** before eating. Stomach acid kills many germs, but hands are how most of them arrive in the first place. Soap and twenty seconds does most of the work.\n\nBodies come in every shape and size, and always have. None of this is about how anyone looks.",{"id":912,"type":913,"title":914,"terms":915},"glossary-discover","glossary","Words for this topic",[916,920,924,928,931,935,939,943,946,949,953,957,961,964],{"term":917,"meaning":918,"example":919},"Cell","The smallest living unit of the body. You have roughly 30 trillion of them.","A red blood cell, a muscle cell, a nerve cell.",{"term":921,"meaning":922,"example":923},"Tissue","A group of similar cells doing one job together.","Muscle tissue, nerve tissue, bone tissue.",{"term":925,"meaning":926,"example":927},"Organ","A structure made of several tissues, with a job of its own.","The heart, the stomach, a kidney.",{"term":69,"meaning":929,"example":930},"A group of organs sharing one big job.","The digestive system: mouth, stomach, intestines, liver, pancreas.",{"term":932,"meaning":933,"example":934},"Enzyme","A chemical that speeds up the breaking-down (or building-up) of other chemicals without being used up itself.","Amylase in your saliva, which cuts starch into sugar.",{"term":936,"meaning":937,"example":938},"Peristalsis","The wave of muscle squeezing that pushes food along the gut. It works even if you are upside down.","Why an astronaut can swallow in orbit.",{"term":940,"meaning":941,"example":942},"Villi","Millions of tiny finger-shaped folds in the small intestine wall that make its surface enormous. Singular: villus.","Each one is about a millimetre tall and holds a loop of capillary.",{"term":190,"meaning":944,"example":945},"The microscopic air sacs at the end of the smallest air tubes, where gases cross into and out of the blood.","About 300 million in a pair of adult lungs.",{"term":194,"meaning":947,"example":948},"The sheet of muscle under the lungs that flattens to pull air in.","A hiccup is this muscle twitching suddenly.",{"term":950,"meaning":951,"example":952},"Capillary","The narrowest blood vessel, with a wall one cell thick — where everything actually crosses.","No cell of yours is far from one.",{"term":954,"meaning":955,"example":956},"Reflex","An automatic response decided in the spinal cord, before the brain is involved.","The knee jerk; pulling your hand off something hot.",{"term":958,"meaning":959,"example":960},"Excretion","Getting rid of waste that was made inside your body.","Urine, and the carbon dioxide in every breath out.",{"term":749,"meaning":962,"example":963},"A place where two systems touch closely enough for something to cross between them.","The alveolus wall, the villus wall, a capillary.",{"term":965,"meaning":966,"example":967},"Hormone","A chemical message carried slowly in the blood to the whole body, rather than fast along a nerve.","Insulin, which tells cells to take sugar out of the blood.",{"id":969,"type":970,"title":971,"questions":972},"quiz-discover","quiz","Check yourself: the seven teams and the joins between them",[973,986,999,1012,1025,1038,1051,1064,1077],{"itemId":974,"prompt":975,"options":976,"correct":244,"why":985},"body-systems.discover-q-absorb","Where does food actually cross into your blood?",[977,979,981,983],{"id":241,"label":978},"The stomach",{"id":244,"label":980},"The villi of the small intestine",{"id":247,"label":982},"The large intestine",{"id":250,"label":984},"The liver","The **villi** of the small intestine. The wall is one cell thick and each villus holds a capillary loop, so nutrients have almost no distance to travel.",{"itemId":987,"prompt":988,"options":989,"correct":247,"why":998},"body-systems.discover-q-diaphragm","What actually pulls air into your lungs?",[990,992,994,996],{"id":241,"label":991},"The lungs squeezing themselves",{"id":244,"label":993},"The heart pushing air along",{"id":247,"label":995},"The diaphragm flattening and making the chest bigger",{"id":250,"label":997},"The trachea sucking","Lungs have no muscle. The **diaphragm** flattens, the chest grows, pressure inside drops, and the atmosphere pushes air in.",{"itemId":1000,"prompt":1001,"options":1002,"correct":247,"why":1011},"body-systems.discover-q-alveoli","What crosses the alveolus wall, and in which direction?",[1003,1005,1007,1009],{"id":241,"label":1004},"Only oxygen, into the blood",{"id":244,"label":1006},"Only carbon dioxide, out of the blood",{"id":247,"label":1008},"Oxygen into the blood and carbon dioxide out of it, at the same time",{"id":250,"label":1010},"Blood, into the air sac","Both gases cross the same wall at the same moment, in opposite directions, each drifting from where it is crowded to where it is not.",{"itemId":1013,"prompt":1014,"options":1015,"correct":247,"why":1024},"body-systems.discover-q-double","Why is the heart called a double pump?",[1016,1018,1020,1022],{"id":241,"label":1017},"It beats twice for every pulse you feel",{"id":244,"label":1019},"It has two valves",{"id":247,"label":1021},"One side sends blood to the lungs and the other to the rest of the body",{"id":250,"label":1023},"There are two hearts, one behind the other","The right side pumps to the **lungs**; the left side pumps to **everywhere else**. Both squeeze at the same instant, but the left has much further to push, so its wall is thicker.",{"itemId":1026,"prompt":1027,"options":1028,"correct":244,"why":1037},"body-systems.discover-q-blue","Why do the veins on your hand look blue?",[1029,1031,1033,1035],{"id":241,"label":1030},"The blood in them is blue until it meets air",{"id":244,"label":1032},"Because of how skin scatters and absorbs light; the blood is dark red",{"id":247,"label":1034},"Veins have a blue lining",{"id":250,"label":1036},"They carry a different liquid, not blood","Blood is **never** blue. Skin lets red light through to be absorbed by the dark blood, while blue light bounces back from the upper skin layers — so what reaches your eye looks bluish.",{"itemId":1039,"prompt":1040,"options":1041,"correct":244,"why":1050},"body-systems.discover-q-kidney","Your kidneys filter about 180 litres a day but you pass only about 1.5 litres of urine. What happens to the rest?",[1042,1044,1046,1048],{"id":241,"label":1043},"It evaporates through the skin",{"id":244,"label":1045},"It is taken back into the blood along the kidney tubes",{"id":247,"label":1047},"It is stored in the bladder for later",{"id":250,"label":1049},"It is breathed out","About 178.5 litres — over 99% — is reabsorbed as the filtered fluid travels along the tubule. Filtering everything and then reclaiming what is worth keeping is simpler than picking out each waste chemical.",{"itemId":1052,"prompt":1053,"options":1054,"correct":247,"why":1063},"body-systems.discover-q-reflex","In a reflex like pulling your hand off something hot, what is the brain doing?",[1055,1057,1059,1061],{"id":241,"label":1056},"Deciding to move the hand",{"id":244,"label":1058},"Nothing at all, ever",{"id":247,"label":1060},"Being informed afterwards, while the spinal cord has already ordered the movement",{"id":250,"label":1062},"Sending the pain signal to the hand","The spinal cord completes the circuit and the hand is already moving. A copy of the signal reaches the brain a moment later, which is why you feel the pain *after* you have moved.",{"itemId":1065,"prompt":1066,"options":1067,"correct":247,"why":1076},"body-systems.discover-q-handover","Which feature do all the body's hand-over points share?",[1068,1070,1072,1074],{"id":241,"label":1069},"A thick, strong wall",{"id":244,"label":1071},"A pump to push things across",{"id":247,"label":1073},"A very thin wall and a very large folded surface",{"id":250,"label":1075},"They are all in the chest","Thin barrier, huge surface area, and a difference across it. Alveoli, villi and capillaries are the same design solved three times.",{"itemId":1078,"prompt":1079,"options":1080,"correct":244,"why":1089},"body-systems.discover-q-exercise","When you run hard, what mostly triggers the faster breathing?",[1081,1083,1085,1087],{"id":241,"label":1082},"Your brain deciding you need more air",{"id":244,"label":1084},"Rising carbon dioxide in your blood",{"id":247,"label":1086},"Your muscles pulling on the ribs",{"id":250,"label":1088},"Falling temperature","Sensors watch **carbon dioxide** far more closely than oxygen, because it changes quickly and reliably. The same signal is what makes holding your breath uncomfortable.",{"id":1091,"type":1092,"prompt":1093},"reflect-discover","reflection","Pick any one of the seven systems and imagine it simply stopped — no damage, no pain, it just paused.\n\nWrite down, in order, the other systems that would run into trouble and why, until you reach one that would fail completely. How many steps did it take? Try a second system and see whether it takes more or fewer.",{"id":1095,"type":1096,"title":1097,"points":1098},"cheat-discover","summary","Cheat sheet",[1099,1100,1101,1102,1103,1104,1105,1106,1107,1108],"**Ladder:** cell → tissue → organ → organ system → you. Each step can do something the one below cannot.","**Digestive:** mouth (chewing, amylase) → oesophagus (peristalsis) → stomach (acid, churning) → small intestine (**absorption at the villi**) → large intestine (water back) → out, one to three days later.","**Respiratory:** nose → trachea → bronchi → 300 million alveoli. The **diaphragm** does the work; lungs have no muscle of their own.","**Circulatory:** a double pump, four chambers, about 5 L of blood, 103,680 beats and 7,258 L a day, through about 100,000 km of vessels. Blood is never blue.","**Excretory:** kidneys filter 180 L a day and take 99% of it back, leaving 1.5 L of urine. Lungs and skin excrete too.","**Nervous:** brain decides, spinal cord relays — and in a **reflex** the cord decides alone, in about 20 ms, before the brain hears about it.","**Muscular and skeletal:** muscles only pull, so they work in opposing pairs; bones are the levers they pull on.","**Hand-overs are the point:** alveolus ↔ blood (gases), villus ↔ blood (food), capillary ↔ every cell (both), kidney ↔ blood (cleaning), nerve → muscle (orders), muscle → bone (pull).","**Every hand-over has the same three features:** a wall one cell thick, an enormous folded surface, and a difference across it so things move by themselves.","**Exercise moves all of them at once:** faster heart, deeper breathing, blood sent to muscles, sweat to lose the heat — triggered mainly by rising carbon dioxide.",{"id":1110,"type":1111,"sourceIds":1112},"sources-discover","sources",[1113,1114,1115,1116,1117,1118,1119],"body-systems-britannica-digestive","body-systems-britannica-respiratory","body-systems-britannica-cardiovascular","body-systems-britannica-renal","body-systems-britannica-nervous","body-systems-wiki-alveolus","body-systems-sender-cells",[1113,1114,1115,1116,1117,1118,1119],"needs_review",{"generatedBy":1123,"notes":1124},"claude-code","Draft generated locally; pending owner review.","ccde84075ae864fb5cd75c7ec8999b2f0bfddf5d182c4d037436cfae50a254b0",{"component:sort-game@1":1127,"logic:practice":1128,"component:system-flow@1":1129,"component:body-explorer@1":1130,"component:match-pairs@1":1131,"component:data-lab@1":1132,"source:body-systems-britannica-cardiovascular":1133,"source:body-systems-britannica-digestive":1134,"source:body-systems-britannica-nervous":1135,"source:body-systems-britannica-renal":1136,"source:body-systems-britannica-respiratory":1137,"source:body-systems-sender-cells":1138,"source:body-systems-wiki-alveolus":1139},"b164f45a2c8ca08f26c450768ff0231e113e9fe45381eddb34dc6d0548596c38","3d6b0fe1b15255975a32b0fcd94e8019bc959ad45cbf12e136e86149549c6878","40eda343e2312ed1d0eed875e27ef482c65d948452c1be27522b9a4268085e71","b44b6ef91c082fccbb2437932390f32c3cdec3d3b00c1261c62e6f7a421e644b","2a8ee4ac87460b4e1175a4bb13c96b03d577db06dde95670eb7fcfe4ad787899","466896cc37735f48db03875fe9c9ce42fc8bcb7e5f937c9779d70513703b91bd","8b7278004bfe64d7ad3e369fbfa62cada6c2ca19bdda240520d8cb96569d40a7","8b28eef54dd7327ab08588678c32674f0a6615182ae48d49d272ad156d0ad122","07d4488afad056b326893d48a8ecd74894b1e87c5b23c8f0bbd1268b77fdf252","5aac5e4ba60a1b43234a3f40d1a2f4a1fae961d335628e57236d7e84bd5073f3","dd8e7c0348ffd73f06866939a2804a19b80938f30a21e7c2b3e4ac922cb96f65","275141a01bad918372489d869091a22c69fcb823ac1a7a523ba14b5ab8494acc","43da85a71de8cdd85dad5ae87aa0c57897396ecc1b5efde1f672fe7289321ecf",{"state":1141,"reviewer":1142,"selfReview":334,"reviewedAt":1143,"method":1144},"approved","The library owner","2026-09-20T10:18:37.581Z","owner_bulk","preview-7e1cbbcc4f",1789899597374]