[{"data":1,"prerenderedAt":1258},["ShallowReactive",2],{"layer:body-systems:understand":3},{"layer":4,"contentHash":1237,"dependencyHashes":1238,"approval":1252,"releaseId":1257},{"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":1232,"reviewStatus":1233,"authoring":1234},1,"body-systems","en","understand","How the systems work, and how they hand over","One design used six times: thin wall, huge surface, steep difference","Go inside each system: enzymes and the chemical works, the pressure trick that moves air, two circuits through a four-chambered heart, filter-and-reclaim kidneys, the reflex arc and the nerve-to-muscle gap — then follow a breath all the way to a working cell.",[13,14,15,16,17],"State the three design rules every hand-over point in the body obeys, and find them at six different places.","Explain mechanical and chemical digestion, and why chewing changes how fast enzymes can work.","Explain how a pressure difference moves air, and why exhaling at rest needs no muscle.","Trace both circuits of the heart and say why the left ventricle wall is thicker.","Describe negative feedback and use it to explain sweating, shivering, thirst and blood-sugar control.",45,{"title":20,"rows":21},"Lesson plate",[22,25,28,31,34,37],{"label":23,"value":24},"Depth","Understand",{"label":26,"value":27},"Reading time","≈ 45 minutes",{"label":29,"value":30},"Prior knowledge","Discover: the seven systems",{"label":32,"value":33},"Chapters","10",{"label":35,"value":36},"Labs","6: three system-flows, sort, match, breath animation",{"label":38,"value":39},"Big idea","Thin wall, huge surface, steep difference",[41,45,51,54,75,81,121,126,131,134,138,171,174,219,238,254,259,262,281,286,304,309,314,317,346,350,397,421,443,448,451,492,496,509,514,517,551,568,609,675,680,683,688,694,699,702,734,737,741,746,808,813,846,851,854,882,886,907,960,963,1040,1199,1203,1220],{"id":42,"type":43,"markdown":44},"intro-understand","prose","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?**\n\nThe 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.\n\nAlong 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.",{"id":46,"type":47,"title":48,"eyebrow":49,"navLabel":50},"ch1","chapter","One design, used six times","Chapter 01","1 The hand-over rule",{"id":52,"type":43,"markdown":53},"three-rules","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.\n\nDiffusion 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.\n\nSo 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.",{"id":55,"type":56,"tone":57,"items":58},"spec-three-rules","spec","blue",[59,63,67,71],{"label":60,"big":61,"value":62},"Rule 1: thin","1 cell","The barrier is a single cell thick wherever it can be. Alveolus wall, capillary wall, villus wall.",{"label":64,"big":65,"value":66},"Rule 2: big","fold it","Surface area is multiplied by folding or branching. 300 million alveoli; millions of villi; 100,000 km of capillary.",{"label":68,"big":69,"value":70},"Rule 3: steep","keep a gap","Keep one side crowded and the other empty, so material keeps moving. Blood flow does this by constantly carrying arrivals away.",{"label":72,"big":73,"value":74},"The distance problem","5 L\u002Fmin","Diffusion cannot cross a body, so blood does the travelling: one full lap in about a minute at rest.",{"id":76,"type":77,"variant":78,"title":79,"markdown":80},"def-diffusion","callout","definition","Diffusion, and why blood flow matters so much","**Diffusion** is the spreading of a substance from where it is crowded to where it is scarce, by nothing more than random movement.\n\nHere is the part that is easy to miss. Diffusion stops the moment the two sides are equal. If blood simply sat still in the capillaries around an alveolus, it would fill with oxygen and then stop taking any.\n\n**Blood flow is what keeps the difference alive.** Loaded blood is swept away and replaced by hungry blood, so the alveolus is always facing an empty side and oxygen keeps crossing. The pump does not push the oxygen across. It keeps the *conditions* right for the oxygen to cross itself.",{"id":82,"type":83,"caption":84,"columns":85,"rows":90},"table-six-handovers","table","The same three rules, found at all six hand-overs.",[86,87,88,89],"Hand-over","Thin barrier","Big surface","What keeps the difference steep",[91,96,101,106,111,116],[92,93,94,95],"Alveolus ↔ blood","Alveolus + capillary wall, 2 cells total","≈ 70 m² from 300 million sacs","Breathing refreshes the air; blood flow removes the oxygen",[97,98,99,100],"Villus ↔ blood","Villus wall, 1 cell","≈ 30 m² from folds, villi and microvilli","Blood carries nutrients off to the liver at once",[102,103,104,105],"Capillary ↔ cell","Capillary wall, 1 cell","Every cell is within a fraction of a millimetre of one","The cell keeps consuming oxygen and making CO₂",[107,108,109,110],"Blood ↔ kidney filter","Filter membrane, very thin","2,000,000 nephrons, two kidneys","High blood pressure pushes fluid through, then it is reclaimed further along",[112,113,114,115],"Nerve → muscle","A gap a fraction of a micrometre wide","Thousands of junctions per muscle","The signal chemical is destroyed immediately, so the next signal is clean",[117,118,119,120],"Muscle → bone","Tendon (this one carries force, not substance)","Broad attachment spreads the load","Not diffusion at all — the one hand-over that works by pulling",{"id":122,"type":77,"variant":123,"title":124,"markdown":125},"nuance-not-all-diffusion","nuance","Not everything drifts — some things are carried","Diffusion explains the gases beautifully, and much else besides. But the body does not rely on it alone.\n\nGlucose does not simply drift out of the gut: special carrier proteins in the villus wall grab it and haul it across, spending energy to do so. That is why you can absorb sugar from a meal even when your blood already has plenty — something pure diffusion could never manage. The kidney's reclaiming step works the same way, actively pulling back sugar and salt against the flow.\n\nSo the honest summary is: **gases diffuse; many useful molecules are actively carried**. Both still need the same thin wall and huge surface. The rules survive; only the mechanism changes.",{"id":127,"type":47,"title":128,"eyebrow":129,"navLabel":130},"ch2","Digestion: machinery, then chemistry","Chapter 02","2 Digestion",{"id":132,"type":43,"markdown":133},"two-kinds-digestion","Digestion happens in two ways at once, and separating them makes the whole system make sense.\n\n**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.\n\n**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**.\n\nWhy 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.",{"id":135,"type":77,"variant":78,"title":136,"markdown":137},"def-enzyme","Enzyme","An **enzyme** is a protein that makes a particular chemical change happen far faster than it otherwise would, and is not used up in the process — so one enzyme molecule can do the job over and over.\n\nEnzymes are fussy. Each one fits one kind of molecule, the way one key fits one lock: amylase cuts starch and nothing else; protein-cutting enzymes ignore starch completely. They also have conditions they like. The enzyme in your stomach works best in strong acid; the ones in your small intestine would be destroyed by it, which is exactly why the pancreas sends in an alkaline juice to cancel the acid first.",{"id":139,"type":83,"caption":140,"columns":141,"rows":146},"table-enzymes","Where the chemistry happens, and who does it.",[142,143,144,145],"Where","Juice","What it does","Food acted on",[147,152,157,162,166],[148,149,150,151],"Mouth","Saliva, from three pairs of glands","Wets the food; **amylase** starts cutting starch into sugars","Starch",[153,154,155,156],"Stomach","Gastric juice: hydrochloric acid + pepsin","Acid unfolds proteins and kills most germs; **pepsin** cuts proteins into shorter chains","Protein",[158,159,160,161],"Small intestine","Bile, from the liver, stored in the gall bladder","No enzyme at all — it breaks fat into tiny droplets so enzymes can reach them","Fat (mechanically)",[158,163,164,165],"Pancreatic juice, from the pancreas","Cancels the acid, then finishes starch, protein **and** fat with three sets of enzymes","All three",[167,168,169,170],"Small intestine wall","Enzymes in the villus lining itself","The final snips, right at the doorstep, just before absorption","Sugars, short peptides",{"id":172,"type":43,"markdown":173},"liver-pancreas","Two organs sit beside the gut rather than in it, and send in chemicals through tubes. They are the chemical works.\n\nThe **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.\n\nBut 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.\n\nThe **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.",{"id":175,"type":176,"component":177,"componentVersion":5,"config":178,"objective":217,"textAlternative":218},"lab-flow-excretory","interactive","system-flow",{"system":179,"steps":180,"quiz":215,"handoverWith":216},"excretory",[181,186,191,196,200,205,210],{"id":182,"label":183,"text":184,"organ":185},"artery","Blood arrives","About 1.2 litres of blood a minute reaches the kidneys — roughly a fifth of everything the heart pumps.","renal artery",{"id":187,"label":188,"text":189,"organ":190},"filter","Filtering","Blood pressure pushes water, salt, sugar and urea out through 2,000,000 microscopic filters. Big things — blood cells and proteins — are too large to pass.","nephron",{"id":192,"label":193,"text":194,"organ":195},"reclaim","Reclaiming","Along a long looping tube, 99% of the water, all the sugar and most of the salt are actively pulled back into the blood.","tubule",{"id":197,"label":198,"text":199,"organ":195},"tune","Fine tuning","How much water comes back is adjusted by a hormone. Short of water, you reclaim more and pass less; full of water, the opposite.",{"id":201,"label":202,"text":203,"organ":204},"clean","Clean blood leaves","The hand-over point. Blood goes back into circulation lighter by its urea and with its water and salt set correctly.","renal vein",{"id":206,"label":207,"text":208,"organ":209},"urine","Urine collects","What was not reclaimed — about 1.5 litres a day — runs down the ureters to the bladder.","ureter",{"id":211,"label":212,"text":213,"organ":214},"store","Stored and released","The bladder stretches to hold it, so that excretion can wait for a convenient moment rather than happening continuously.","bladder",true,"circulatory","Follow blood through a kidney and watch it being filtered, reclaimed and handed back clean.","Seven steps through a kidney, with the **circulatory system** drawn beside it so you can see blood arriving dirty and leaving clean.\n\n**Arrives:** about 1.2 L of blood a minute, roughly 24% of everything the heart pumps, for organs that together weigh under half a kilogram.\n\n**Filtered:** pressure pushes fluid through 2,000,000 tiny filters. Water, salt, sugar and urea go through; blood cells and proteins are too big and stay behind. About 180 L a day crosses.\n\n**Reclaimed:** 178.5 L — 99.2% — is actively pulled back, including every gram of sugar.\n\n**Tuned:** a hormone sets how much water returns, which is why urine is dark when you are short of water and pale when you are not.\n\n**Handed back:** the hand-over is blood leaving lighter by its urea, with its water and salt at the right levels. Notice that the kidney is not only a bin. It is the body's **water and salt manager**, and the urine is simply what is left over once it has finished managing.\n\nThe quiz asks what would end up in the urine if the filter holes were slightly too big (protein and blood cells — which is exactly what a doctor tests urine for).",{"id":220,"type":221,"title":222,"problem":223,"steps":224,"help":233},"we-chewing-surface","worked_example","Why chewing changes how much you get out of a meal","Imagine a cube of paneer 2 cm on each side. Enzymes can only work on its **outside**. Now cut it into small cubes 0.5 cm on each side. How much more surface have you made — and how much more paneer?",[225,226,227,228,229,230,231,232],"A cube 2 cm on a side has 6 faces, each 2 × 2 = 4 cm². Surface = 6 × 4 = **24 cm²**.","Its volume is 2 × 2 × 2 = **8 cm³**.","Cutting it into 0.5 cm cubes: along each edge you get 2 ÷ 0.5 = 4 pieces, so 4 × 4 × 4 = **64 small cubes**.","Each small cube has surface 6 × (0.5 × 0.5) = 6 × 0.25 = **1.5 cm²**.","Total new surface = 64 × 1.5 = **96 cm²**, four times the original 24 cm².","Total volume = 64 × (0.5 × 0.5 × 0.5) = 64 × 0.125 = **8 cm³** — exactly the same as before.","**Four times the surface, not one gram more food.** Every enzyme in the gut now has four times as much to work on at once.","That is the whole argument for chewing. It is also, at a far more extreme scale, the argument for villi and for alveoli: fold a surface and you multiply what can cross it without making the organ any bigger.",{"simplerExplanation":234,"hints":235},"Cutting something up never changes how much there is, but it always makes more outside. Chemistry only happens on the outside.",[236,237],"Surface of a cube = 6 × (side × side).","Count how many small cubes fit along one edge, then cube that number.",{"id":239,"type":240,"itemId":241,"prompt":242,"check":243,"hints":248,"feedback":251},"p-surface-cubes","practice","body-systems.understand-surface","A 3 cm cube of food is cut into 1 cm cubes. How many small cubes are there?",{"kind":244,"answer":245,"tolerance":246,"unit":247},"number",27,0,"cubes",[249,250],"Along one edge, 3 ÷ 1 = 3 pieces.","It is 3 × 3 × 3.",{"correct":252,"incorrect":253},"Right: 3 × 3 × 3 = **27 cubes**. Their total surface is 27 × 6 = 162 cm², against the original 6 × 9 = 54 cm² — three times as much, for the same amount of food.","Work along one edge first: 3 cm ÷ 1 cm = 3 pieces. Then cube it: 3 × 3 × 3 = 27.",{"id":255,"type":47,"title":256,"eyebrow":257,"navLabel":258},"ch3","Breathing: how a muscle moves air","Chapter 03","3 Breathing",{"id":260,"type":43,"markdown":261},"breathing-mechanics","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.\n\n**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.\n\n**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.\n\n**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.",{"id":263,"type":264,"items":265},"formulas-breathing","formulas",[266,269,272,275,278],{"expression":267,"caption":268},"bigger chest → lower pressure","Same air in more space means lower pressure, so outside air pushes in.",{"expression":270,"caption":271},"15 × 60 × 24 = 21,600","Breaths in one day at fifteen a minute.",{"expression":273,"caption":274},"21,600 × 0.5 L = 10,800 L","Air moved through the chest in one day.",{"expression":276,"caption":277},"21% − 16% = 5%","Oxygen actually kept from each breath: 5% of 500 mL = 25 mL.",{"expression":279,"caption":280},"4% ÷ 0.04% = 100×","Exhaled air carries about a hundred times as much carbon dioxide as fresh air.",{"id":282,"type":77,"variant":283,"title":284,"markdown":285},"aha-exhaled-air","aha","Your breath out is still mostly good air","Air you breathe out contains about **16% oxygen** — only 5 percentage points less than the 21% you breathed in. You keep less than a quarter of the oxygen that comes in.\n\nThat sounds wasteful until you notice what it makes possible: **rescue breathing works**. If someone has stopped breathing, air from a rescuer's lungs still carries plenty of oxygen to keep that person's blood loaded. An inefficient-looking lung turns out to be a lifesaving spare.\n\nThe carbon dioxide tells the opposite story. Fresh air is 0.04% carbon dioxide; your breath out is about 4% — **100 times more**. Your body is far more decisive about getting rid of waste gas than about extracting oxygen.",{"id":287,"type":288,"prompt":289,"options":290,"explanation":303},"predict-alveoli-count","prediction","A pair of adult lungs holds about 300 million alveoli, giving roughly **70 m²** of surface. Suppose instead you had two plain smooth bags of exactly the same volume — no alveoli, just two balloons. Their inside surface would be about **0.1 m²**. What would happen?",[291,294,297,300],{"id":292,"label":293},"a","Nothing much; the same amount of air is there",{"id":295,"label":296},"b","You would absorb roughly 700 times less oxygen per breath and could not survive",{"id":298,"label":299},"c","You would breathe faster but manage",{"id":301,"label":302},"d","Oxygen would cross faster, because there is less wall in the way","**About 700 times less, and you could not survive.**\n\nHow much crosses a barrier depends on how much barrier there is. 70 m² ÷ 0.1 m² = **700**, so smooth bags would deliver roughly 700 times less oxygen per breath. No amount of breathing faster closes a gap like that.\n\nThis is the same arithmetic as the paneer cube, taken to an extreme. The volume of your lungs is nothing special — about 6 litres, the size of a small bucket. What is special is that the inside has been folded into 300 million bubbles, so that a bucket contains the area of a small classroom floor.",{"id":305,"type":77,"variant":306,"title":307,"markdown":308},"misconception-tennis-court","misconception","\"Your lungs have the surface area of a tennis court\"","You will read this in a great many books. Do the arithmetic and it does not hold up.\n\nA doubles tennis court measures 23.77 m × 10.97 m = **261 m²**. Careful measurements of adult lungs give a gas-exchange surface of about **70 m²** (the measured range is 50 to 75). That is about **27%** of a tennis court — roughly a quarter.\n\nBetter comparisons, worked out the same way: a **badminton** court is 81.7 m², so your lungs are about 86% of one. Or picture a classroom floor 7 m × 10 m — that is 70 m², almost exactly right.\n\nThe same thing has happened to the small intestine, which is often given as \"200 to 300 m², a tennis court\". Modern measurement puts it at about **30 m²** — the older figure is around 7 times too big.\n\nNone of this makes lungs less astonishing. A classroom floor folded inside your chest is remarkable enough without exaggeration — and checking a famous number for yourself is exactly what science is.",{"id":310,"type":47,"title":311,"eyebrow":312,"navLabel":313},"ch4","The heart: two pumps, four rooms, one beat","Chapter 04","4 The heart",{"id":315,"type":43,"markdown":316},"two-circuits","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.\n\n**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.\n\n**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.\n\nThis 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.",{"id":318,"type":319,"title":320,"items":321},"steps-one-beat","steps","One heartbeat, in order",[322,326,330,334,338,342],{"title":323,"tag":324,"text":325},"Filling","both sides at once","Blood returning from the body fills the right atrium; blood returning from the lungs fills the left atrium. The heart is relaxed.",{"title":327,"tag":328,"text":329},"Atria squeeze","a small push","Both atria contract gently, topping up the ventricles below them through open valves.",{"title":331,"tag":332,"text":333},"Valves shut","\"lub\"","The valves between atria and ventricles snap shut so blood cannot go backwards. This is the first heart sound.",{"title":335,"tag":336,"text":337},"Ventricles squeeze","the real push","Both ventricles contract hard and together. The right sends blood to the lungs; the left sends it to the whole body.",{"title":339,"tag":340,"text":341},"Valves shut again","\"dub\"","The valves at the exits snap shut so blood cannot fall back in. This is the second heart sound.",{"title":343,"tag":344,"text":345},"Rest","≈ 0.4 s","The heart relaxes and refills. At 72 beats a minute, the heart muscle is actually resting for roughly half of your life.",{"id":347,"type":77,"variant":283,"title":348,"markdown":349},"aha-heart-rests","Your heart rests more than you think","One beat at 72 beats a minute takes 60 ÷ 72 = about **0.83 seconds**. Of that, the ventricles are squeezing for roughly 0.3 s and relaxed for roughly 0.5 s.\n\nSo the hardest-working muscle in your body spends **more than half of every cycle resting**. That is exactly how it manages 103,680 beats a day for 3 billion beats in a lifetime without ever getting a day off.\n\nThere is a second reason it never tires. Heart muscle is supplied by its own dedicated arteries, the coronary arteries, which branch off the aorta within a centimetre of leaving the heart. The heart pays itself first.",{"id":351,"type":176,"component":177,"componentVersion":5,"config":352,"objective":395,"textAlternative":396},"lab-flow-circulatory",{"system":216,"steps":353,"quiz":215,"handoverWith":394},[354,359,364,369,374,379,384,389],{"id":355,"label":356,"text":357,"organ":358},"rightatrium","Right atrium","Dark, oxygen-poor blood arrives from the whole body through two big veins and collects here.","right atrium",{"id":360,"label":361,"text":362,"organ":363},"rightventricle","Right ventricle","Squeezes the blood a short distance to the lungs. Its wall is thin, because the journey is short.","right ventricle",{"id":365,"label":366,"text":367,"organ":368},"lungs","Lungs: the hand-over","At 300 million alveoli, carbon dioxide leaves the blood and oxygen joins it, clipping onto haemoglobin in the red cells.","alveoli",{"id":370,"label":371,"text":372,"organ":373},"leftatrium","Left atrium","Bright, oxygen-rich blood returns from the lungs and collects here.","left atrium",{"id":375,"label":376,"text":377,"organ":378},"leftventricle","Left ventricle","The strongest chamber, with a wall about three times thicker. It pushes blood to every part of the body.","left ventricle",{"id":380,"label":381,"text":382,"organ":383},"arteries","Arteries","Thick springy tubes carry the surge away, stretching with each beat and recoiling between beats. That recoil is your pulse.","aorta",{"id":385,"label":386,"text":387,"organ":388},"capillaries","Capillaries: the second hand-over","Walls one cell thick. Oxygen and food cross out to the cells; carbon dioxide and waste cross in. The alveolus in reverse.","capillary",{"id":390,"label":391,"text":392,"organ":393},"veins","Veins","Wide, low-pressure tubes with one-way valves. Walking squeezes them and the valves turn the squeeze into upward flow.","vena cava","respiratory","Follow one red cell through both loops of the circulation and find the two places where things cross.","Eight stations following one red blood cell all the way round, with the **respiratory system** drawn alongside.\n\nStart at the **right atrium** with dark blood back from the body. Right ventricle → **lungs** → left atrium → left ventricle → arteries → **capillaries** → veins → back to the start.\n\nTwo stations are marked as hand-overs, and they are the same event in opposite directions:\n\n- At the **alveoli**, oxygen joins the blood and carbon dioxide leaves it.\n- At a **capillary** next to a working cell, oxygen leaves the blood and carbon dioxide joins it.\n\nWatch the wall thickness as you go. The right ventricle's wall is thin; the left ventricle's is about three times thicker, because it pushes the same volume of blood a far greater distance. Watch the pressure readout too: high and surging in the arteries, low and steady in the capillaries (deliberately — crossing takes time), and almost nothing in the veins, which is why the valves and your calf muscles are needed.\n\nThe quiz asks you to name the two hand-over stations and explain why one red cell passes through the heart twice on every lap.",{"id":398,"type":56,"tone":399,"items":400},"spec-bp","copper",[401,405,409,413,417],{"label":402,"big":403,"value":404},"Blood pressure, typical adult","≈ 120\u002F80","Two numbers: the peak as the left ventricle squeezes, and the resting level between beats.",{"label":406,"big":407,"value":408},"What is being measured","mm Hg","The height in millimetres of a column of mercury the pressure could hold up — a very old unit that stuck.",{"label":410,"big":411,"value":412},"Why two numbers","push and rest","Arteries never fall to zero between beats, because their elastic walls keep squeezing. That is what the lower number shows.",{"label":414,"big":415,"value":416},"Where a pulse is easy","wrist, neck","Anywhere an artery runs close to the surface over a bone. Use fingertips, never the thumb.",{"label":418,"big":419,"value":420},"Pressure in the capillaries","very low","Deliberately: high pressure would damage a wall one cell thick, and slow flow gives substances time to cross.",{"id":422,"type":240,"itemId":423,"prompt":424,"check":425,"hints":437,"feedback":440},"p-thicker-wall","body-systems.understand-thick-wall","Why is the wall of the left ventricle about three times thicker than the wall of the right ventricle?",{"kind":426,"options":427,"correct":436},"choice",[428,430,432,434],{"id":292,"label":429},"It holds more blood",{"id":295,"label":431},"It must push blood much further, so it must push much harder",{"id":298,"label":433},"It beats more often",{"id":301,"label":435},"It carries oxygen-rich blood, which is heavier",[295],[438,439],"Where does each side send its blood?","Both push the same volume each beat.",{"correct":441,"incorrect":442},"Correct. Both ventricles push the **same volume**, but the right one sends it a few centimetres to the lungs while the left sends it to your toes and back. Further means harder means thicker muscle.","It is not about how much blood — both sides pump the same volume, or blood would pile up. It is about **distance**: the left ventricle supplies the whole body.",{"id":444,"type":47,"title":445,"eyebrow":446,"navLabel":447},"ch5","Blood: what it carries and who does what","Chapter 05","5 Blood",{"id":449,"type":43,"markdown":450},"blood-parts","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%**.\n\nThe 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.\n\nThe dark layer is the cells, and there are three kinds.\n\n**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**.\n\n**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.\n\n**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.",{"id":452,"type":83,"caption":453,"columns":454,"rows":459},"table-blood-cargo","Everything the blood is carrying past you right now.",[455,456,457,458],"Cargo","Picked up at","Delivered to","Carried by",[460,465,470,474,478,482,487],[461,462,463,464],"Oxygen","The alveoli","Every cell","Haemoglobin in red cells",[466,467,468,469],"Glucose, amino acids, vitamins","The villi of the small intestine","The liver first, then every cell","Dissolved in plasma",[471,463,472,473],"Carbon dioxide","The alveoli, to breathe out","Mostly dissolved in plasma",[475,476,477,469],"Urea (waste from used protein)","The liver","The kidneys",[479,480,481,469],"Hormones","Endocrine glands","Wherever the right receiver is",[483,484,485,486],"Heat","Busy muscles and the liver","The skin, to be lost","The water in the plasma",[488,489,490,491],"White cells and antibodies","Bone marrow and lymph tissue","Anywhere there is an infection","Travelling under their own power",{"id":493,"type":77,"variant":283,"title":494,"markdown":495},"aha-blood-carries-heat","Blood is also a heating system","It is easy to remember blood as a food-and-oxygen delivery van and forget that it carries **heat**.\n\nYour muscles and liver are furnaces. Blood flowing through them warms up, carries that heat away, and distributes it — which is how your fingers stay usable on a cold morning even though nothing in a finger produces much heat.\n\nIt is also how you cool down. Widen the vessels near the skin and hot blood is brought to the surface, where the heat escapes: that is why you go red when you run and why your ears glow after sport. Narrow them and blood is kept in the core, which is why your hands go pale and cold first when you are chilled. The body will sacrifice comfortable fingers to keep the brain and heart at 37 °C.\n\nSo the circulatory system is not one system serving the others. It is the road, the delivery van, the rubbish truck **and** the central heating.",{"id":497,"type":288,"prompt":498,"options":499,"explanation":508},"predict-carbon-monoxide","Haemoglobin picks up oxygen and lets go again easily. Carbon monoxide — a gas made when fuel burns without enough air, as in a closed room with a coal sigri — sticks to haemoglobin **hundreds of times more tightly**, and does not readily let go. What happens to someone breathing it?",[500,502,504,506],{"id":292,"label":501},"Nothing; there is still plenty of oxygen in the room",{"id":295,"label":503},"Their red cells fill up with carbon monoxide and can no longer carry oxygen, even though they look fine",{"id":298,"label":505},"Their lungs stop working",{"id":301,"label":507},"Their heart stops immediately","**Their red cells fill up and can no longer carry oxygen.** The lungs work perfectly. The heart works perfectly. The air may even be full of oxygen. But the delivery vehicles are all occupied, and every cell in the body starves.\n\nThis is a hand-over failure — and it shows how a body can fail at one tiny step while everything else is in perfect order.\n\nIt is also why carbon monoxide is so dangerous in practice: it has no smell, no colour, no taste, and the early signs are a headache and sleepiness, which is exactly when a person is least likely to act. The practical rule, worth knowing for life: **never burn coal, charcoal, wood or gas in a closed room for warmth, and never sleep near a burning sigri or angithi.** Keep a window open; it costs nothing.",{"id":510,"type":47,"title":511,"eyebrow":512,"navLabel":513},"ch6","Nerves: the fast messaging service","Chapter 06","6 Nerves",{"id":515,"type":43,"markdown":516},"neuron-and-synapse","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.\n\nThe 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\u002Fs**, which is 432 km\u002Fh. Thin and bare: around **1 m\u002Fs**, walking pace.\n\nThat 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.\n\nWhere 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.",{"id":518,"type":519,"title":520,"note":521,"scale":522,"rungs":523},"ladder-speeds","ladder","How fast is a nerve, really?","Metres per second, on a log scale. Nerves are fast for biology and slow for electronics.","log",[524,527,531,535,539,543,547],{"label":525,"value":5,"display":526},"Slow pain fibres (dull ache)","≈ 1 m\u002Fs",{"label":528,"value":529,"display":530},"A person walking briskly",1.5,"1.5 m\u002Fs",{"label":532,"value":533,"display":534},"Peristalsis moving food",0.02,"≈ 0.02 m\u002Fs",{"label":536,"value":537,"display":538},"Fast touch and motor nerves",120,"up to 120 m\u002Fs",{"label":540,"value":541,"display":542},"Sound in air",343,"343 m\u002Fs",{"label":544,"value":545,"display":546},"A signal in a copper wire",200000000,"≈ 2 × 10⁸ m\u002Fs",{"label":548,"value":549,"display":550},"Light in a vacuum",299792458,"≈ 3 × 10⁸ m\u002Fs",{"id":552,"type":221,"title":553,"problem":554,"steps":555,"help":563},"we-reflex-timing","Why the spinal cord decides, and not the brain","You touch something hot with your fingertip. Compare two possible routes: the **reflex** route (fingertip → spinal cord → back to the arm muscle) and the **conscious** route (fingertip → spinal cord → brain → decision → back down → arm muscle). Both use nerves at about 120 m\u002Fs.",[556,557,558,559,560,561,562],"**Reflex route.** Fingertip to spinal cord is roughly 0.8 m, and spinal cord back to the biceps is roughly 0.6 m: about 1.4 m in total.","Time in the nerves: 1.4 ÷ 120 = about 0.012 s, or 12 ms. Add the delay at the junctions and the measured knee-jerk-style reflex comes out around **20 ms**.","**Conscious route.** Add about 0.6 m up the cord to the brain and 0.6 m back down: roughly 1.2 m of extra travel, which is only another 10 ms.","But the travelling was never the slow part. Recognising a signal, deciding and issuing an order takes the brain roughly 200 ms — about **10 times** the whole reflex.","**The saving is about 180 ms**, nearly a fifth of a second, and it is bought entirely by *not thinking*.","A fifth of a second in contact with a hot vessel is the difference between a fright and a blister. So evolution wired the decision into the cord and let the brain find out afterwards.","For scale: in the 20 ms a reflex takes, light travels about **6,000 km** — from Delhi to well past Europe. Nerves are fast for biology, not fast for physics.",{"simplerExplanation":564,"hints":565},"Nerve travel is quick either way. Thinking is what takes time — about 200 ms of it. A reflex skips the thinking and saves nearly a fifth of a second.",[566,567],"Time = distance ÷ speed.","Compare the thinking time with the travelling time; one is far bigger.",{"id":569,"type":176,"component":177,"componentVersion":5,"config":570,"objective":607,"textAlternative":608},"lab-flow-nervous",{"system":571,"steps":572,"quiz":215,"handoverWith":606},"nervous",[573,578,583,588,592,597,602],{"id":574,"label":575,"text":576,"organ":577},"receptor","Receptor fires","Heat and pain sensors in the fingertip detect the hot vessel and start an electrical signal.","skin",{"id":579,"label":580,"text":581,"organ":582},"sensory","Sensory neuron","The signal travels up the arm at up to 120 m\u002Fs — about 432 km\u002Fh — towards the spinal cord.","arm nerve",{"id":584,"label":585,"text":586,"organ":587},"cord","Spinal cord decides","A connecting neuron passes the signal straight across to a motor neuron. The brain is not consulted. This is the whole decision.","spinal cord",{"id":589,"label":590,"text":591,"organ":582},"motor","Motor neuron","The order travels back down the arm. Total round trip so far: roughly 20 milliseconds.",{"id":593,"label":594,"text":595,"organ":596},"junction","Nerve meets muscle","The hand-over point. The nerve ending releases a chemical across a tiny gap; the muscle fibre receives it and contracts.","motor end plate",{"id":598,"label":599,"text":600,"organ":601},"muscle","Biceps contracts","The muscle shortens and pulls on the forearm bones. Your hand is already moving away.","biceps",{"id":603,"label":604,"text":605,"organ":603},"brain","The brain is informed","A copy of the signal continues up the cord. You feel the pain now — after you have moved. That gap is why you often notice a burn a moment late.","muscular","Follow a reflex from fingertip to muscle, and see the exact point where the nervous system hands over to the muscular system.","Seven steps through a withdrawal reflex, with the **muscular system** drawn alongside.\n\nA hot vessel touches the fingertip. Receptors fire. The signal runs up the arm nerve at up to 120 m\u002Fs and reaches the **spinal cord**, where a single connecting neuron shunts it straight across to a motor neuron. That shunt is the entire decision, and it is why the lab shows the brain greyed out at this step.\n\nThe **hand-over** is the last link: where the motor neuron ends on the muscle fibre. The electrical signal cannot cross the gap, so the nerve releases a chemical which drifts across and makes the fibre contract. It is one-way — muscles never send orders back to nerves — and the chemical is destroyed straight afterwards so the next signal arrives on a clean slate.\n\nTotal time from touch to movement: about **20 ms**. The final step shows the copy of the signal arriving at the **brain**, which is when you actually feel it — about 180 ms after your hand began to move.\n\nSwitch the quiz on and it asks you to say which step would fail if the gap at the muscle junction were blocked (the muscle would simply never receive the order, though the nerve is perfectly healthy).",{"id":610,"type":176,"component":611,"componentVersion":5,"config":612,"objective":673,"textAlternative":674},"lab-sort-control","sort-game",{"prompt":613,"bins":614,"items":624,"seconds":246},"Do you control this, or does your body handle it without asking? Some belong in both bins.",[615,618,621],{"id":616,"label":617},"voluntary","You decide",{"id":619,"label":620},"involuntary","Body decides",{"id":622,"label":623},"both","Either way",[625,629,633,637,641,645,649,653,657,661,665,669],{"id":626,"label":627,"bin":616,"why":628},"kick","Kicking a football","A chain of decisions from brain to leg muscles, and you can stop halfway.",{"id":630,"label":631,"bin":619,"why":632},"heartbeat","Your heart beating","The heart has its own pacemaker; nerves and hormones only speed it up or slow it down.",{"id":634,"label":635,"bin":619,"why":636},"digest","Pushing food along the gut","Peristalsis runs day and night with no instruction from you.",{"id":638,"label":639,"bin":616,"why":640},"writing","Writing your name","Learned and deliberate.",{"id":642,"label":643,"bin":622,"why":644},"breathing","Breathing","Automatic, but you can take over and hold, deepen or speed it — up to a point.",{"id":646,"label":647,"bin":622,"why":648},"blinking","Blinking","Protective and automatic, but you can also blink on purpose.",{"id":650,"label":651,"bin":619,"why":652},"sweating","Sweating","Temperature control decides. Willing yourself to stop does not work.",{"id":654,"label":655,"bin":619,"why":656},"pupil","Pupils narrowing in bright light","A reflex protecting the retina, with no conscious route at all.",{"id":658,"label":659,"bin":622,"why":660},"swallow","Swallowing","You start it; once food passes the throat the rest is automatic.",{"id":662,"label":663,"bin":619,"why":664},"shiver","Shivering","Fast involuntary twitching, switched on to make heat.",{"id":666,"label":667,"bin":619,"why":668},"kneejerk","The knee jerk","A spinal reflex the brain never gets to vote on.",{"id":670,"label":671,"bin":619,"why":672},"holdbreath","Holding your breath until you faint","You cannot. The automatic system takes the controls back first — which is a safety feature.","Sort twelve actions into voluntary, involuntary or both, and find where the boundary of your control really lies.","Twelve action cards, three bins: **You decide**, **Body decides**, **Either way**.\n\nKicking a ball and writing your name are voluntary. Heartbeat, peristalsis, sweating, shivering, pupil narrowing and the knee jerk are involuntary — and trying to will any of them to stop is a good demonstration in itself.\n\nThe interesting bin is **Either way**: breathing, blinking and swallowing. Each runs automatically but accepts a conscious override, and each takes the override back when it matters. Swallowing is the clearest: you start it deliberately, and the moment food passes your throat the rest happens without you, which is why you cannot change your mind halfway.\n\nThe last card is the sharpest. **Holding your breath until you faint** sounds voluntary and is not: the automatic system always wins before real harm is done.\n\nThe rule behind the whole split is **time**. Anything that must be decided in milliseconds, or must never be forgotten, is wired below consciousness. Anything needing judgement goes up to the brain.",{"id":676,"type":47,"title":677,"eyebrow":678,"navLabel":679},"ch7","Muscles, bones and the levers between them","Chapter 07","7 Levers",{"id":681,"type":43,"markdown":682},"levers","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.\n\nThe 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.\n\nWhy 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\u002Fh using muscles that cannot contract anywhere near that fast.\n\nYour body chose **speed over strength**, nearly everywhere. It is a design decision, and you can see it in the arithmetic.",{"id":684,"type":77,"variant":685,"title":686,"markdown":687},"example-jaw-exception","example","The jaw goes the other way","Not every lever in the body trades strength for speed. The muscle that closes your jaw attaches quite far from the hinge, relative to where your back teeth sit — the opposite arrangement to the biceps.\n\nThe result is the opposite trade: your jaw closes slowly and over a small range, but with enormous force. That is why your back teeth can crack a walnut or grind hard chana, and why the same bite at the front, further from the hinge, is much weaker. Try it with something firm: the difference is obvious immediately.\n\nSo the body uses both settings, and picks whichever the job needs. Arms and legs: speed. Jaws: force.",{"id":689,"type":690,"conceptId":691,"relation":692,"explanation":693},"conn-anatomy-levers","connection","human-body-anatomy","related_to","Which muscles attach where, and how each kind of joint moves, is set out properly in the anatomy topic.",{"id":695,"type":47,"title":696,"eyebrow":697,"navLabel":698},"ch8","The slow service and the defence","Chapter 08","8 Hormones, defence",{"id":700,"type":43,"markdown":701},"endocrine","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.\n\nEndocrine 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.\n\nA 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.\n\nNotice 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.",{"id":703,"type":83,"caption":704,"columns":705,"rows":709},"table-two-services","Two messaging services, compared.",[706,707,708],"Feature","Nervous system","Endocrine system",[710,714,718,722,726,730],[711,712,713],"Message is","An electrical pulse, then a chemical across a gap","A chemical dissolved in the blood",[715,716,717],"Speed","Up to 120 m\u002Fs — milliseconds","Seconds to minutes, carried by the blood",[719,720,721],"Aimed at","One precise muscle or gland","Everywhere; only cells with the receiver respond",[723,724,725],"Lasts","As long as the signal, and no longer","Minutes, hours, or years",[727,728,729],"Good for","Reflexes, movement, sensing","Growth, blood sugar, water balance, daily rhythms",[731,732,733],"Example","Pulling your hand off a hot vessel","Insulin lowering blood sugar after a meal",{"id":735,"type":43,"markdown":736},"immune","The **immune system** is your defence, and it works in layers.\n\nThe **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.\n\nIf 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.\n\nAnd 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.\n\nA **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.\n\nFever fits here too, and it is not a malfunction: see the next layer for why a raised temperature is a deliberate, controlled move.",{"id":738,"type":77,"variant":123,"title":739,"markdown":740},"nuance-immune-calm","A calm word about germs","Reading about defence can make the world sound hostile. It is not.\n\nThe overwhelming majority of the bacteria you meet are harmless, and trillions of them live inside you doing useful work. Your immune system handles small challenges constantly without you ever knowing — you have almost certainly fought off something this week without noticing.\n\nTwo ordinary habits do most of the practical work: **washing hands with soap** before eating and after the toilet, and **taking the vaccines** that are offered. Neither requires worrying about germs. That is rather the point of them.",{"id":742,"type":47,"title":743,"eyebrow":744,"navLabel":745},"ch9","Two complete journeys","Chapter 09","9 Two journeys",{"id":747,"type":748,"component":749,"componentVersion":5,"config":750,"textAlternative":807},"anim-breath-to-cell","animation","process-steps",{"title":751,"diagram":752,"steps":753},"A breath, from the air in the room to a working cell","none",[754,759,763,768,773,778,783,787,792,797,802],{"id":755,"label":756,"description":757,"highlight":758},"draw","The diaphragm contracts","The dome of muscle flattens and the ribs swing up and out. The chest cavity grows, the pressure inside drops below the pressure outside, and about 500 mL of air is pushed in through the nose by the atmosphere.",[],{"id":201,"label":760,"description":761,"highlight":762},"Warmed, moistened, filtered","In the nose the air is brought to body temperature, given moisture, and cleaned: hairs catch large particles and sticky mucus traps the rest, which tiny sweeping hairs move steadily back up the throat.",[],{"id":764,"label":765,"description":766,"highlight":767},"down","Down the branching tree","Through the trachea, held open by cartilage rings, then into two bronchi and about twenty further branchings into bronchioles thinner than a hair. The air slows down at every split.",[],{"id":769,"label":770,"description":771,"highlight":772},"sac","Into an alveolus","The air arrives in one of about 300 million microscopic sacs. The sac wall is one cell thick, and a capillary hugging it is also one cell thick. Two cell layers separate this air from your blood.",[],{"id":774,"label":775,"description":776,"highlight":777},"cross","HAND-OVER 1: oxygen crosses into the blood","There is more oxygen in the sac than in the arriving blood, so oxygen drifts across and clips onto haemoglobin inside the red cells, which turn from dark red to bright scarlet. Carbon dioxide crosses the other way at the same instant.",[],{"id":779,"label":780,"description":781,"highlight":782},"heart","Back to the heart, then out","Loaded blood returns to the left atrium, drops into the left ventricle and is pushed out into the aorta with the next beat. At rest, a full lap of the body takes about a minute.",[],{"id":388,"label":784,"description":785,"highlight":786},"Into a capillary in a calf muscle","The artery divides and divides until the vessel is so narrow that red cells pass almost single file. Blood slows right down here, which is exactly what is needed, because crossing takes time.",[],{"id":788,"label":789,"description":790,"highlight":791},"deliver","HAND-OVER 2: oxygen crosses into the muscle cell","The working muscle has been consuming oxygen, so there is less of it inside the cell than in the blood. Oxygen leaves the haemoglobin and drifts across the capillary wall into the cell.",[],{"id":793,"label":794,"description":795,"highlight":796},"burn","The cell uses it","Inside the cell, oxygen and glucose release energy, which the muscle fibre uses to pull. Carbon dioxide, water and heat are left over.",[],{"id":798,"label":799,"description":800,"highlight":801},"return","HAND-OVER 3: the waste gas crosses back","Carbon dioxide is now crowded inside the cell and scarce in the blood, so it drifts into the capillary and dissolves in the plasma. The blood darkens and carries it away.",[],{"id":803,"label":804,"description":805,"highlight":806},"out","HAND-OVER 4: and out into the room","Back at an alveolus the carbon dioxide crosses into the air sac, the diaphragm relaxes, the chest springs back, and it leaves. About five seconds of your life, four hand-overs, three systems.",[],"A step-through animation of a single breath from the room to a muscle cell and back.\n\n**1. The diaphragm contracts** — it flattens, the ribs swing up and out, the chest grows and the pressure inside drops, so the atmosphere pushes about 500 mL of air in.\n\n**2. Cleaned in the nose** — warmed to body temperature, moistened, and filtered by hairs and sticky mucus.\n\n**3. Down the tree** — trachea, two bronchi, about twenty further branchings into bronchioles thinner than a hair.\n\n**4. Into an alveolus** — one of about 300 million sacs. Its wall is one cell thick, and the capillary wrapped around it is one cell thick. Two cell layers, and no more, separate this air from your blood.\n\n**5. HAND-OVER 1** — oxygen drifts into the blood and clips onto haemoglobin, turning it bright scarlet; carbon dioxide drifts out at the same wall, at the same moment, in the opposite direction.\n\n**6–7. The journey** — back to the left side of the heart, out through the aorta, and down into a capillary in a calf muscle so narrow that red cells pass almost single file. Blood slows here deliberately: crossing takes time.\n\n**8. HAND-OVER 2** — the working muscle has used its oxygen up, so oxygen leaves the blood and enters the cell.\n\n**9. Used** — oxygen and glucose together release the energy the fibre uses to pull. Carbon dioxide, water and heat are left over.\n\n**10–11. HAND-OVERS 3 and 4** — the carbon dioxide crosses back into the blood, rides to the lungs, crosses into an alveolus, and leaves as you breathe out.\n\nFour hand-overs. Three systems. About five seconds. And you did not decide a single step of it.",{"id":809,"type":77,"variant":810,"title":811,"markdown":812},"observation-two-journeys-meet","observation","The two journeys meet at the same doorstep","Follow the roti from the Discover layer and follow the breath from the animation above, and notice where they end up: **the same capillary, beside the same muscle cell**.\n\nGlucose arrived from the gut, travelling in the plasma. Oxygen arrived from the lungs, travelling on the red cells. They come from opposite ends of the body by completely separate routes, and they meet at a doorstep a hundredth of a millimetre wide, at the right moment, in the right amounts, in every one of the roughly 30 trillion cells you have.\n\nNobody arranges the meeting. It happens because each substance keeps drifting away from crowding, and because the blood never stops moving. That is the whole trick.",{"id":814,"type":176,"component":815,"componentVersion":5,"config":816,"objective":844,"textAlternative":845},"lab-match-enzymes","match-pairs",{"prompt":817,"mode":818,"pairs":819},"Match each organ or juice to the job it does in digestion.","connect",[820,823,826,829,832,835,838,841],{"a":821,"b":822},"Saliva","Wets food and starts cutting starch",{"a":824,"b":825},"Stomach acid","Unfolds proteins and kills most germs",{"a":827,"b":828},"Bile from the liver","Breaks fat into droplets — no enzyme involved",{"a":830,"b":831},"Pancreatic juice","Cancels the acid, then finishes all three food types",{"a":833,"b":834},"Villi","The doorstep where nutrients cross into the blood",{"a":836,"b":837},"Large intestine","Takes back the water and feeds the gut bacteria",{"a":839,"b":840},"Liver, after absorption","Checks and stores everything before it goes on",{"a":842,"b":843},"Fibre","Gives the gut muscles something to push against","Connect eight parts of the digestive system with what each one actually does.","Eight cards on each side; draw the connections.\n\n- **Saliva** ↔ wets food and starts cutting starch (amylase, the first enzyme).\n- **Stomach acid** ↔ unfolds proteins and kills most germs. Note what it does *not* do: absorb.\n- **Bile from the liver** ↔ breaks fat into droplets. Bile contains no enzyme at all; it is a detergent, doing mechanical work by chemical means.\n- **Pancreatic juice** ↔ cancels the stomach acid first (the small intestine's enzymes would be destroyed by it), then finishes starch, protein and fat.\n- **Villi** ↔ the doorstep where nutrients cross into the blood. The only place food truly enters you.\n- **Large intestine** ↔ takes back the water and feeds the bacteria that live there.\n- **Liver, after absorption** ↔ checks and stores everything before it goes anywhere else. All blood from the gut goes here first.\n- **Fibre** ↔ gives the gut muscles something to push against. It is never digested, and that is exactly its job.\n\nThe pairs that people get wrong are bile (which everyone assumes is an enzyme) and the stomach (which everyone assumes absorbs food).",{"id":847,"type":47,"title":848,"eyebrow":849,"navLabel":850},"ch10","Keeping everything steady","Chapter 10","10 Staying steady",{"id":852,"type":43,"markdown":853},"homeostasis-intro","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.\n\nHolding the inside steady while the outside swings about is called **homeostasis**, and almost every system you have met is part of it.\n\nThe 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.\n\nToo 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.",{"id":855,"type":319,"title":856,"items":857},"steps-feedback","Getting too hot: the loop in five steps",[858,862,866,870,874,878],{"title":859,"tag":860,"text":861},"1. Change","the disturbance","You run for a bus in May. Your muscles make heat far faster than usual and your core temperature starts to rise.",{"title":863,"tag":864,"text":865},"2. Sensor","in the brain and skin","Temperature sensors in the brain and in the skin detect the rise within seconds.",{"title":867,"tag":868,"text":869},"3. Control centre","the hypothalamus","A small region at the base of the brain compares the reading with the value it holds, about 37 °C, and finds it too high.",{"title":871,"tag":872,"text":873},"4. Effectors","skin and sweat glands","Vessels near the skin widen, bringing hot blood to the surface (you go red). Sweat glands switch on.",{"title":875,"tag":876,"text":877},"5. Correction","heat leaves","Evaporating sweat carries away roughly 2,430 kJ for every litre. Temperature falls back, the sensors notice, and the response is turned down again.",{"title":879,"tag":880,"text":881},"The same loop, reversed","too cold","Vessels narrow, so your hands go pale and cold to protect the core; shivering switches on and can raise heat production about fivefold; goosebumps are a leftover from ancestors with more hair.",{"id":883,"type":77,"variant":306,"title":884,"markdown":885},"misconception-sweat-cools","\"Sweating cools you down\"","Sweat sitting on your skin cools you by almost nothing. What cools you is sweat **evaporating** — turning from liquid into vapour — because that change takes a large amount of heat from your skin to happen. About 2,430 kJ per litre, which is why half a litre of evaporated sweat removes roughly 1,215 kJ.\n\nThis explains something every Indian summer teaches. On a dry day in Jaipur at 40 °C you sweat and feel better, because the sweat evaporates fast. On a humid day in Kolkata at 34 °C you feel far worse, even though it is six degrees cooler — the air already holds so much water vapour that your sweat cannot evaporate. It runs off, taking almost no heat with it, and you are hot, wet and uncomfortable.\n\nIt is also why a fan helps even when it is only blowing warm air: it sweeps the humid layer off your skin and lets the next sweat evaporate. The fan is not cooling the air. It is helping you evaporate.",{"id":887,"type":240,"itemId":888,"prompt":889,"check":890,"hints":901,"feedback":904},"p-feedback","body-systems.understand-feedback","Blood sugar rises sharply after a large meal. Which response is an example of **negative feedback**?",{"kind":426,"options":891,"correct":900},[892,894,896,898],{"id":292,"label":893},"The pancreas releases insulin, so cells take sugar out of the blood",{"id":295,"label":895},"The liver releases more sugar into the blood",{"id":298,"label":897},"You feel hungrier, so you eat more",{"id":301,"label":899},"The kidneys stop filtering",[292],[902,903],"Negative feedback always pushes the *opposite* way to the change.","Which answer lowers blood sugar after it has risen?",{"correct":905,"incorrect":906},"Correct. Blood sugar goes up, so the response brings it **down** — insulin tells cells to take glucose in. The opposite response, glucagon, is used when blood sugar falls.","Negative feedback opposes the change. Sugar has risen, so the correct response must **lower** it. Options that raise it further, or make you eat more, would push in the same direction.",{"id":908,"type":909,"title":910,"prompt":911,"options":912},"explorer-homeostasis","explorer","Four things your body holds steady","Pick one and follow the loop.",[913,926,938,950],{"id":914,"label":915,"chain":916,"badge":922,"note":925},"temp","Temperature",[917,918,919,920,921],"Sensors in brain and skin","Hypothalamus compares","Sweat or shiver","Heat lost or made","Back to 37 °C",{"text":923,"tone":924},"Held near 37 °C","yes","Held within about half a degree of 37.0 °C, day and night, in Ladakh and in Chennai. Too hot: skin vessels widen, you flush, sweat glands open, and evaporation removes the heat. Too cold: skin vessels narrow so your hands go pale first (the body sacrifices fingers to protect the core), and shivering — fast involuntary muscle twitching — can raise heat production about 5-fold. The control centre is the **hypothalamus**, a region at the base of the brain about the size of an almond.",{"id":927,"label":928,"chain":929,"badge":935,"note":937},"water","Water",[930,931,932,933,934],"Blood gets concentrated","Brain detects it","Hormone to kidneys","More water reclaimed","Thirst as backup",{"text":936,"tone":924},"Two responses at once","A 30 kg child holds about 18 litres of water, roughly 60% of body mass. When the blood becomes slightly too concentrated, the brain notices and does two things at once. First, it sends a hormone to the kidneys telling them to reclaim more water, so less is lost: urine goes darker and there is less of it. Second, it makes you **thirsty** — an involuntary system recruiting a voluntary one, because the kidneys can only slow the loss and cannot add anything. Only drinking can do that.",{"id":939,"label":940,"chain":941,"badge":947,"note":949},"sugar","Blood sugar",[942,943,944,945,946],"Sugar rises after a meal","Pancreas senses it","Insulin released","Cells take sugar in","Level falls back",{"text":948,"tone":924},"Two opposite hormones","All the blood in an adult holds only about **4.5 g** of glucose — roughly one teaspoon in five litres. A meal could easily swamp it, and cells would starve between meals, so two hormones from the pancreas work against each other. **Insulin**, when sugar is high, tells cells and the liver to take glucose out. **Glucagon**, when sugar is low, tells the liver to release its stores. The liver acts as the warehouse. The result is a level that barely moves, whether you ate an hour ago or six hours ago.",{"id":951,"label":471,"chain":952,"badge":957,"note":959},"co2",[953,954,955,956,946],"CO₂ rises in blood","Sensed in the brain","Breathe faster and deeper","More CO₂ leaves",{"text":958,"tone":924},"Fastest loop of the four","The loop that runs every few seconds of your life. Carbon dioxide dissolves in blood to make it slightly acidic, and that acidity is watched far more closely than oxygen — because it changes quickly and reliably, while oxygen falls too slowly to be a useful alarm. When it rises, you automatically breathe faster and deeper and blow the excess off. This is the loop that makes holding your breath uncomfortable, that speeds your breathing when you run, and that keeps working all night while you sleep.",{"id":961,"type":43,"markdown":962},"sleep","One more thing your body does entirely without you: **sleep**.\n\nSleep 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.\n\nChildren 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.\n\nThe 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.",{"id":964,"type":965,"title":966,"terms":967},"glossary-understand","glossary","Words for this layer",[968,972,976,980,984,988,992,996,1000,1004,1008,1012,1016,1020,1024,1028,1032,1036],{"term":969,"meaning":970,"example":971},"Diffusion","The spreading of a substance from where it is crowded to where it is scarce, by random movement alone. It costs no energy and is hopeless over distance.","Oxygen crossing an alveolus wall.",{"term":973,"meaning":974,"example":975},"Mechanical digestion","Making food pieces smaller without changing what they are.","Chewing; the stomach churning; bile breaking fat into droplets.",{"term":977,"meaning":978,"example":979},"Chemical digestion","Cutting long food molecules into short ones, so they can be absorbed.","Starch → glucose; protein → amino acids.",{"term":981,"meaning":982,"example":983},"Bile","A greenish fluid made by the liver and stored in the gall bladder. It contains no enzyme; it breaks fat into droplets.","Squirted into the small intestine when fat arrives.",{"term":985,"meaning":986,"example":987},"Tidal volume","The amount of air moved in one ordinary resting breath, about 500 mL.","About the volume of a small water bottle.",{"term":989,"meaning":990,"example":991},"Pulmonary circuit","The short loop: heart → lungs → heart.","Where blood picks up oxygen.",{"term":993,"meaning":994,"example":995},"Systemic circuit","The long loop: heart → the whole body → heart.","Where blood delivers everything.",{"term":997,"meaning":998,"example":999},"Atrium","One of the two upper collecting chambers of the heart. Plural: atria.","Blood from the body arrives in the right atrium.",{"term":1001,"meaning":1002,"example":1003},"Ventricle","One of the two lower pumping chambers. The left one has a far thicker wall.","The left ventricle supplies the whole body.",{"term":1005,"meaning":1006,"example":1007},"Plasma","The straw-coloured liquid part of blood, about 55% of it, mostly water.","Glucose, urea, hormones and most carbon dioxide travel dissolved here.",{"term":1009,"meaning":1010,"example":1011},"Haemoglobin","The iron-containing red pigment in red blood cells that grabs oxygen where there is plenty and releases it where there is little.","It is what makes blood red.",{"term":1013,"meaning":1014,"example":1015},"Urea","The waste made by the liver from used-up protein, handed to the blood for the kidneys to remove.","The main solid waste dissolved in urine.",{"term":1017,"meaning":1018,"example":1019},"Nephron","One of the roughly two million microscopic filter-and-reclaim units in your kidneys.","Filter almost everything, then take back what is worth keeping.",{"term":1021,"meaning":1022,"example":1023},"Neuron","A nerve cell, built to send an electrical signal a long way, fast.","Some run from the base of the spine to the toe.",{"term":1025,"meaning":1026,"example":1027},"Synapse","The tiny gap where one neuron passes its signal to the next, or to a muscle, using a chemical.","One-way, which is what stops signals running backwards.",{"term":1029,"meaning":1030,"example":1031},"Hormone","A chemical message released into the blood, reaching the whole body but acting only where there is a matching receiver.","Insulin, glucagon, adrenaline.",{"term":1033,"meaning":1034,"example":1035},"Homeostasis","Holding the conditions inside the body steady while the outside changes.","Core temperature near 37 °C whatever the weather.",{"term":1037,"meaning":1038,"example":1039},"Negative feedback","A control loop in which the response always pushes in the opposite direction to the change.","Too hot → sweat; blood sugar too high → insulin.",{"id":1041,"type":1042,"title":1043,"questions":1044},"quiz-understand","quiz","Check yourself: mechanisms and joins",[1045,1058,1071,1084,1097,1110,1121,1134,1147,1160,1173,1186],{"itemId":1046,"prompt":1047,"options":1048,"correct":295,"why":1057},"body-systems.understand-q-diffusion","Why is every hand-over barrier in the body only one or two cells thick?",[1049,1051,1053,1055],{"id":292,"label":1050},"To save material",{"id":295,"label":1052},"Because diffusion is fast over tiny distances and hopeless over large ones",{"id":298,"label":1054},"So it can stretch",{"id":301,"label":1056},"So blood can pass through it","Diffusion across a hair's width is almost instant; across a centimetre it would take hours. Thin walls are what make diffusion usable.",{"itemId":1059,"prompt":1060,"options":1061,"correct":298,"why":1070},"body-systems.understand-q-flow","Why does blood have to keep flowing past an alveolus?",[1062,1064,1066,1068],{"id":292,"label":1063},"To keep the lungs warm",{"id":295,"label":1065},"To push oxygen across the wall",{"id":298,"label":1067},"To carry loaded blood away, so the difference across the wall stays steep",{"id":301,"label":1069},"It does not; still blood would work as well","Diffusion stops once both sides are equal. Flow removes loaded blood and brings hungry blood, so oxygen keeps crossing. The pump does not push the gas — it maintains the conditions.",{"itemId":1072,"prompt":1073,"options":1074,"correct":295,"why":1083},"body-systems.understand-q-bile","What does bile do?",[1075,1077,1079,1081],{"id":292,"label":1076},"It is the enzyme that digests fat",{"id":295,"label":1078},"It breaks fat into tiny droplets so enzymes can reach more surface",{"id":298,"label":1080},"It neutralises stomach acid only",{"id":301,"label":1082},"It absorbs fat into the blood","Bile contains **no enzyme**. It is a detergent that creates surface — mechanical digestion achieved chemically. The fat-cutting enzymes come from the pancreas.",{"itemId":1085,"prompt":1086,"options":1087,"correct":295,"why":1096},"body-systems.understand-q-chewing","Chopping a cube of food into smaller cubes changes which of these?",[1088,1090,1092,1094],{"id":292,"label":1089},"The amount of food",{"id":295,"label":1091},"The surface available to enzymes",{"id":298,"label":1093},"Both",{"id":301,"label":1095},"Neither","Cutting never changes how much there is; it always makes more outside. Since chemistry happens at surfaces, more outside means faster digestion — the same idea as villi and alveoli.",{"itemId":1098,"prompt":1099,"options":1100,"correct":295,"why":1109},"body-systems.understand-q-breathe-in","What actually makes air enter your lungs?",[1101,1103,1105,1107],{"id":292,"label":1102},"The lungs contracting",{"id":295,"label":1104},"The diaphragm flattening, enlarging the chest and lowering the pressure inside",{"id":298,"label":1106},"The heart pushing air",{"id":301,"label":1108},"Warm air rising up the trachea","Lungs have no muscle. A muscle makes the space; the atmosphere pushes the air in. Breathing out at rest needs no muscle at all.",{"itemId":1111,"prompt":1112,"options":1113,"correct":292,"why":1120},"body-systems.understand-q-ventricles","Both ventricles push out the same volume of blood per beat. Why is the left wall thicker?",[1114,1116,1117,1118],{"id":292,"label":1115},"It has to push the blood much further",{"id":295,"label":429},{"id":298,"label":433},{"id":301,"label":1119},"It contains the pacemaker","Same volume, far greater distance — to your toes and back rather than a few centimetres to the lungs. Further means harder, and harder means thicker muscle.",{"itemId":1122,"prompt":1123,"options":1124,"correct":295,"why":1133},"body-systems.understand-q-plasma","Glucose from your breakfast travels in the blood mainly as what?",[1125,1127,1129,1131],{"id":292,"label":1126},"Attached to haemoglobin",{"id":295,"label":1128},"Dissolved in the plasma",{"id":298,"label":1130},"Inside white blood cells",{"id":301,"label":1132},"Inside platelets","Haemoglobin carries oxygen. Glucose, amino acids, salts, hormones, urea and most carbon dioxide travel **dissolved in the plasma**.",{"itemId":1135,"prompt":1136,"options":1137,"correct":298,"why":1146},"body-systems.understand-q-reflex-saving","What does a reflex actually save time on?",[1138,1140,1142,1144],{"id":292,"label":1139},"The nerve signal travelling",{"id":295,"label":1141},"The muscle contracting",{"id":298,"label":1143},"The brain recognising, deciding and ordering",{"id":301,"label":1145},"The receptor firing","Nerve travel is quick on either route. The slow part is thinking — about 200 ms of it. Skipping it takes the response from roughly 200 ms down to about 20 ms.",{"itemId":1148,"prompt":1149,"options":1150,"correct":298,"why":1159},"body-systems.understand-q-synapse","How does a signal get from a nerve ending to a muscle fibre?",[1151,1153,1155,1157],{"id":292,"label":1152},"The electricity jumps the gap",{"id":295,"label":1154},"The nerve touches the muscle directly",{"id":298,"label":1156},"A chemical is released and drifts across a tiny gap",{"id":301,"label":1158},"A hormone travels there in the blood","The electrical signal cannot cross. A chemical is released, drifts across, triggers the fibre, and is then destroyed so the next signal arrives cleanly. The junction is strictly one-way.",{"itemId":1161,"prompt":1162,"options":1163,"correct":295,"why":1172},"body-systems.understand-q-sweat","Why do you feel so much worse at 34 °C in humid Kolkata than at 40 °C in dry Jaipur?",[1164,1166,1168,1170],{"id":292,"label":1165},"You sweat less in humid air",{"id":295,"label":1167},"Sweat cannot evaporate in humid air, and only evaporation removes heat",{"id":298,"label":1169},"Humid air is heavier",{"id":301,"label":1171},"Your body makes more heat in humid air","Sweat sitting on the skin cools you by almost nothing. Evaporation is what removes the heat, and humid air is already too full of water vapour to accept much more.",{"itemId":1174,"prompt":1175,"options":1176,"correct":295,"why":1185},"body-systems.understand-q-negfeed","Which of these is negative feedback?",[1177,1179,1181,1183],{"id":292,"label":1178},"Blood sugar rises, so the liver releases more sugar",{"id":295,"label":1180},"Body temperature rises, so you sweat",{"id":298,"label":1182},"You get thirsty, so you feel more thirsty",{"id":301,"label":1184},"You run, so your muscles make more heat","Negative feedback pushes the **opposite** way to the change. Temperature up → cooling response. The others either push the same way or are the disturbance itself.",{"itemId":1187,"prompt":1188,"options":1189,"correct":298,"why":1198},"body-systems.understand-q-endocrine","What is the main advantage of a hormone message over a nerve message?",[1190,1192,1194,1196],{"id":292,"label":1191},"It is much faster",{"id":295,"label":1193},"It is more precisely aimed",{"id":298,"label":1195},"It reaches the whole body and can last for hours or years",{"id":301,"label":1197},"It uses no energy","Nerves win on speed and aim. Hormones win on **reach and duration** — one release can act everywhere at once and keep acting long after the nerve signal would have ended.",{"id":1200,"type":1201,"prompt":1202},"reflect-understand","reflection","The alveolus and the capillary use the same design — a wall one cell thick, an enormous folded surface, and a difference kept steep by flowing blood.\n\nNow design a hand-over for something the body does *not* actually do: suppose you had to absorb a mineral directly from the air through your skin. Sketch what your skin would need to look like, using the three rules. Then say why real skin is built the opposite way, and what it is protecting you from.",{"id":1204,"type":1205,"title":1206,"points":1207},"cheat-understand","summary","Cheat sheet",[1208,1209,1210,1211,1212,1213,1214,1215,1216,1217,1218,1219],"**One design, six times:** every hand-over has (1) a barrier one or two cells thick, (2) an enormous folded surface, (3) a steep difference kept alive by flowing blood.","**Diffusion** moves things from crowded to scarce, free of charge, but only over tiny distances. Blood covers the distances; diffusion does the crossing.","**Digestion is mechanical then chemical.** Grinding makes surface; enzymes work only at surfaces. Amylase (starch, mouth), pepsin + acid (protein, stomach), bile (fat droplets, no enzyme), pancreatic juice (all three).","**Absorption happens at the villi only.** Blood from the gut goes to the **liver** first, which stores, sorts and makes urea.","**Breathing is a pressure trick:** the diaphragm flattens, the chest grows, pressure drops, and the atmosphere pushes 500 mL in. Breathing out at rest costs nothing.","**Check famous numbers:** lung surface is about 70 m² — a classroom floor, about 27% of a tennis court, not a whole one.","**The heart is two pumps, two circuits, four chambers.** Same volume each side, very different distances, so the left wall is about three times thicker. \"Lub-dub\" is valves shutting.","**Blood:** 55% plasma (glucose, urea, hormones, CO₂, heat), 45% cells — red (haemoglobin, oxygen), white (defence), platelets (clotting).","**Kidneys filter 180 L and reclaim 99%.** Filtering everything and choosing what to keep handles wastes it has never met before.","**A reflex saves the thinking, not the travelling:** about 20 ms instead of 200 ms. Nerve to muscle is a one-way chemical hand-over across a gap.","**Nerves vs hormones:** a phone call versus a letter to everyone. Fast, aimed and brief; or slow, everywhere and lasting.","**Homeostasis is negative feedback:** sensor → control centre → effector → a push the opposite way. Temperature, water, blood sugar, carbon dioxide.",{"id":1221,"type":1222,"sourceIds":1223},"sources-understand","sources",[1224,1225,1226,1227,1228,1229,1230,1231],"body-systems-britannica-digestive","body-systems-britannica-respiratory","body-systems-britannica-cardiovascular","body-systems-britannica-renal","body-systems-britannica-nervous","body-systems-helander-surface","body-systems-wiki-alveolus","body-systems-wiki-homeostasis",[1224,1225,1226,1227,1228,1229,1230,1231],"needs_review",{"generatedBy":1235,"notes":1236},"claude-code","Draft generated locally; pending owner review.","97e8c9df126325346d2b544778de1a63e2da0768596557ea8be1f71b71d604ae",{"component:system-flow@1":1239,"logic:practice":1240,"component:sort-game@1":1241,"component:process-steps@1":1242,"component:match-pairs@1":1243,"source:body-systems-britannica-cardiovascular":1244,"source:body-systems-britannica-digestive":1245,"source:body-systems-britannica-nervous":1246,"source:body-systems-britannica-renal":1247,"source:body-systems-britannica-respiratory":1248,"source:body-systems-helander-surface":1249,"source:body-systems-wiki-alveolus":1250,"source:body-systems-wiki-homeostasis":1251},"40eda343e2312ed1d0eed875e27ef482c65d948452c1be27522b9a4268085e71","3d6b0fe1b15255975a32b0fcd94e8019bc959ad45cbf12e136e86149549c6878","b164f45a2c8ca08f26c450768ff0231e113e9fe45381eddb34dc6d0548596c38","c2f918c426383c50d52939054780add1488f3f282c9d1a965c3a38345ddcb265","2a8ee4ac87460b4e1175a4bb13c96b03d577db06dde95670eb7fcfe4ad787899","8b7278004bfe64d7ad3e369fbfa62cada6c2ca19bdda240520d8cb96569d40a7","8b28eef54dd7327ab08588678c32674f0a6615182ae48d49d272ad156d0ad122","07d4488afad056b326893d48a8ecd74894b1e87c5b23c8f0bbd1268b77fdf252","5aac5e4ba60a1b43234a3f40d1a2f4a1fae961d335628e57236d7e84bd5073f3","dd8e7c0348ffd73f06866939a2804a19b80938f30a21e7c2b3e4ac922cb96f65","315ae0ee433a382e063169419964f84379d552e5957655fd04ced507ff2a6940","43da85a71de8cdd85dad5ae87aa0c57897396ecc1b5efde1f672fe7289321ecf","0e7d85baebb026691fbe6a4ef3c8aeaf14bd5c792553e4d59439acce7e4f81b3",{"state":1253,"reviewer":1254,"selfReview":215,"reviewedAt":1255,"method":1256},"approved","The library owner","2026-09-20T10:18:37.581Z","owner_bulk","preview-7e1cbbcc4f",1789899598025]