[{"data":1,"prerenderedAt":787},["ShallowReactive",2],{"layer:body-systems:deepen":3},{"layer":4,"contentHash":770,"dependencyHashes":771,"approval":781,"releaseId":786},{"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":765,"reviewStatus":766,"authoring":767},1,"body-systems","en","deepen","Where the tidy rule bends","The mathematics of a thin wall, bone's double life, the lymphatic system, and why some hand-overs must be prevented","Quantify why hand-over barriers must be thin, meet the lymphatic system that returns leaked fluid and carries digested fat, see bone as a blood factory and calcium bank, and look at clotting and the blood-brain barrier as hand-overs the body deliberately controls or resists.",[13,14,15,16,17],"Explain and use the rule that diffusion time scales with the square of distance.","Describe bone marrow and calcium storage as two jobs of the skeletal system beyond being a lever.","Explain what the lymphatic system does, and why digested fat is absorbed differently from sugar.","Explain why a healthy vessel lining resists clotting, and what the blood-brain barrier trades away for protection.","Balance a day's water intake and output across several systems at once.",45,{"title":20,"rows":21},"Lesson plate",[22,25,28,31,34,37],{"label":23,"value":24},"Depth","Deepen",{"label":26,"value":27},"Reading time","≈ 45 minutes",{"label":29,"value":30},"Prior knowledge","Understand: the three-rule hand-over pattern",{"label":32,"value":33},"Chapters","11",{"label":35,"value":36},"Labs","3: skeletal flow, exceptions match, sort",{"label":38,"value":39},"Big idea","A rule this useful is worth knowing exactly where it bends",[41,45,51,54,67,81,87,92,95,134,161,166,169,174,203,217,222,225,255,268,273,278,281,285,289,316,321,324,337,351,357,362,365,419,424,427,440,444,449,452,495,499,504,507,519,533,547,551,556,559,584,597,626,735,739,756],{"id":42,"type":43,"markdown":44},"intro-deepen","prose","Understand gave you the rule: thin wall, huge surface, steep difference. This layer does two things with that rule that Understand did not have room for.\n\nFirst, it puts a number on **why** thin matters so much — not just \"thin is fast\", but *how much* faster, and what happens when a barrier that should be thin gets thicker than it should be. Second, it goes looking for the **exceptions**: places where the rule bends, is deliberately broken, or is joined by a second, quieter system nobody mentioned yet. Real biology, like real mathematics, gets more interesting exactly where the tidy pattern stops applying cleanly.",{"id":46,"type":47,"title":48,"eyebrow":49,"navLabel":50},"ch1","chapter","The mathematics of a thin wall","Chapter 01","1 Diffusion maths",{"id":52,"type":43,"markdown":53},"diffusion-scaling","Here is the fact Understand only stated in words: diffusion does not slow down in simple proportion to distance. It slows down with the **square** of the distance. Double the distance a substance must drift, and it takes **four** times as long, not two. Treble it, and it takes **nine** times as long.\n\nThis single relationship explains an enormous amount about the body's design. It is not merely nice that hand-over walls are one cell thick — it is close to essential, because doubling that thickness would not merely slow the crossing a little. It would slow it drastically, precisely because of this squared relationship.",{"id":55,"type":56,"items":57},"formulas-diffusion","formulas",[58,61,64],{"expression":59,"caption":60},"time ∝ distance²","Doubling the distance a substance diffuses multiplies the time needed by four, not two.",{"expression":62,"caption":63},"distance ×4 → time ×16","A barrier four times thicker than normal takes sixteen times longer to cross by diffusion alone.",{"expression":65,"caption":66},"10 μm (typical) vs 40 μm (swollen)","A capillary-to-cell gap widened by fluid build-up is a realistic example of exactly this effect.",{"id":68,"type":69,"title":70,"problem":71,"steps":72,"help":78},"we-swelling","worked_example","What swollen tissue does to a hand-over","A capillary normally sits about 10 micrometres from the cells it serves. Suppose an injury causes the surrounding tissue to swell with extra fluid, pushing that distance out to 40 micrometres. Using time ∝ distance², how much longer would oxygen take to diffuse across, all else being equal?",[73,74,75,76,77],"Find the ratio of the new distance to the old: 40 ÷ 10 = **4**.","Square it, because diffusion time scales with distance squared: 4² = **16**.","So the same oxygen molecule takes roughly sixteen times longer to make the same crossing.","This is a genuine piece of the reason swollen, bruised or waterlogged tissue heals slowly and can feel numb or sluggish: it is not just squashed, its hand-over distance has grown, and diffusion punishes distance savagely.","It is also why doctors and physiotherapists care about reducing swelling early (rest, raising the limb, gentle compression) — every millimetre of extra distance removed pays back disproportionately, because of the square.",{"simplerExplanation":79,"anotherExample":80},"If something has to travel twice as far, it does not take twice as long — it takes four times as long. Three times the distance is nine times the time.","The same idea explains why the alveolus wall being a single cell rather than two is worth far more than it sounds: doubling it would roughly quadruple the crossing time for every one of your 300 million alveoli, all day, every day.",{"id":82,"type":83,"variant":84,"title":85,"markdown":86},"nuance-not-purely-diffusion","callout","nuance","Two shortcuts the body takes around this problem","If diffusion punishes distance so harshly, how does the body ever move anything more than a hair's width? Two shortcuts.\n\nFirst, as Understand already showed, **the circulatory system does not rely on diffusion for the long haul at all**. Blood is physically carried in bulk by the heart's pumping, which does not slow down with distance the way diffusion does. Diffusion is only asked to do the last, tiny step — crossing the final wall — and blood flow handles every metre before that.\n\nSecond, some crossings are not diffusion at all but **active transport**: proteins in a cell membrane that grab a molecule and haul it across using energy, which can move things against a gradient and is not limited by the same distance-squared penalty over a single membrane. Understand mentioned this for glucose at the villi; it is the same idea.",{"id":88,"type":47,"title":89,"eyebrow":90,"navLabel":91},"ch2","Bone is not just scaffolding","Chapter 02","2 Skeletal system",{"id":93,"type":43,"markdown":94},"bone-more-than-frame","Every earlier layer treated bone as the thing muscles pull on — true, but a small fraction of what bone actually does. A living bone is a busy organ: hard on the outside for strength, and honeycombed and hollow on the inside, where two very different jobs happen that have nothing to do with movement.\n\n**Storage.** Bone is the body's calcium warehouse. About 99% of your body's calcium is locked up in your skeleton, and it is not inert there — it can be released into the blood when needed and restocked later, which is one more example of homeostasis at work, this time keeping blood calcium (needed for muscle contraction and blood clotting, among other things) steady.\n\n**Manufacture.** Inside the hollow shafts of the long bones — femur, humerus, pelvis, the vertebrae of the spine — is soft tissue called **bone marrow**, and marrow is where new blood cells are born: red cells, white cells and platelets all start here, at the staggering rate of millions of red cells alone every second, which you met back in Understand's blood chapter without being told where they actually come from.",{"id":96,"type":97,"component":98,"componentVersion":5,"config":99,"objective":132,"textAlternative":133},"lab-flow-skeletal","interactive","system-flow",{"system":100,"steps":101,"quiz":130,"handoverWith":131},"skeletal",[102,107,112,116,120,125],{"id":103,"label":104,"text":105,"organ":106},"frame","The frame","Bones give the body its shape, protect soft organs (skull over brain, ribcage over heart and lungs) and give muscles something to pull on.","skull",{"id":108,"label":109,"text":110,"organ":111},"marrow","Inside: the marrow","The hollow shaft of a long bone holds soft marrow tissue, a non-stop factory for blood cells.","femur",{"id":113,"label":114,"text":115,"organ":111},"make","New cells are made","Red cells, white cells and platelets are all produced here, at a rate of millions of red cells every second.",{"id":117,"label":118,"text":119,"organ":111},"release","HAND-OVER: cells enter the blood","Newly made blood cells cross from the marrow into the blood vessels running through it, joining circulation for the first time.",{"id":121,"label":122,"text":123,"organ":124},"store","Meanwhile: a calcium store","The hard mineral part of bone holds about 99% of the body's calcium, banked for whenever the blood needs to top up its own level.","pelvis",{"id":126,"label":127,"text":128,"organ":129},"lever","And still: the lever","The same bone that manufactures blood and stores calcium also swings as a lever every time a muscle pulls its tendon.","humerus",true,"circulatory","Follow what happens inside a long bone besides being a lever: making blood cells and banking calcium, both handed over to the blood.","Six steps through a long bone, with the **circulatory system** drawn alongside — because bone's two quieter jobs both end in a hand-over to blood.\n\nThe outer bone gives shape, protection and a surface for tendons. Step inside the hollow shaft and you find **marrow**, which manufactures red cells, white cells and platelets around the clock. The **hand-over** here is new blood cells leaving the marrow and entering the bloodstream for the very first time — the origin point of the \"2.4 million replacements a second\" fact from Understand.\n\nThe second thread is **calcium storage**: bone banks the vast majority of the body's calcium and releases it into blood on demand, the same negative-feedback logic met in the homeostasis chapter, applied to a mineral instead of heat or sugar.\n\nOnly the final step is the lever you already know. The quiz asks which of the three jobs — frame, factory, or bank — would fail first if a long bone were solid all the way through with no hollow marrow cavity (the factory: there would be nowhere for marrow to sit and make blood cells).",{"id":135,"type":136,"itemId":137,"prompt":138,"check":139,"hints":155,"feedback":158},"p-deepen-marrow","practice","body-systems.deepen-marrow","Where in the body are new red blood cells actually made?",{"kind":140,"options":141,"correct":154},"choice",[142,145,148,151],{"id":143,"label":144},"a","In the heart",{"id":146,"label":147},"b","In the liver, in an adult",{"id":149,"label":150},"c","In bone marrow, inside the hollow shafts of bones like the femur",{"id":152,"label":153},"d","In the blood itself",[149],[156,157],"Think about which organ system this chapter is about.","It is soft tissue inside a hard bone.",{"correct":159,"incorrect":160},"Correct. **Bone marrow**, inside long bones, makes red cells, white cells and platelets, and hands them straight to the blood passing through.","The heart pumps blood but does not make cells. In an adult, the main factory is **bone marrow**, inside bones such as the femur, pelvis and spine.",{"id":162,"type":47,"title":163,"eyebrow":164,"navLabel":165},"ch3","The quiet second network","Chapter 03","3 The lymphatic system",{"id":167,"type":43,"markdown":168},"lymph-intro","Every earlier layer said, more or less, \"everything crosses into the blood.\" That was a useful simplification and it is not quite the whole truth, and the exception is worth knowing because it is genuinely elegant.\n\nAs blood flows through capillaries under pressure, some plasma inevitably leaks out into the spaces between cells — roughly **3 litres a day**, a startlingly large number once you notice it is more than half of your entire blood volume. If that fluid simply stayed there, you would swell up like a water balloon within days. It does not, because a second network of vessels — the **lymphatic system** — collects it, cleans it by filtering it through small checkpoints called lymph nodes (where white blood cells patrol for germs), and quietly returns it to the blood near the base of the neck.\n\nThe lymphatic system has no pump of its own. It relies on exactly the same trick veins use: squeezing from nearby moving muscles and one-way valves, pushing the fluid gradually toward the point of return.",{"id":170,"type":83,"variant":171,"title":172,"markdown":173},"aha-fat-route","aha","Digested fat takes the scenic route","Understand told you that sugars and amino acids cross the villus wall straight into a capillary. Digested **fat** mostly does not. It is picked up instead by a tiny lymphatic vessel inside each villus, called a **lacteal**, and travels through the lymphatic system first, joining the bloodstream only later, near the neck, rather than heading straight to the liver like everything else absorbed at the villi.\n\nNobody designed this to be confusing — it works this way because fat droplets, even tiny ones, are a poor fit for a blood capillary but travel well in lymph. The result is that \"the villi hand everything to the blood\" from earlier layers was a simplification made for a reason: it is true for sugars, amino acids, vitamins and minerals, and only nearly true for fat.",{"id":175,"type":176,"caption":177,"columns":178,"rows":182},"table-two-networks","table","Blood and lymph, compared.",[179,180,181],"Feature","Blood (circulatory)","Lymph (lymphatic)",[183,187,191,195,199],[184,185,186],"Pump","The heart","None — muscle squeeze and one-way valves only",[188,189,190],"Direction","A full circuit, always returning to the heart","One way only: tissues → back to the blood near the neck",[192,193,194],"What it carries","Oxygen, food, hormones, waste, heat, cells","Leaked plasma, fats absorbed from the gut, white cells, germs and debris",[196,197,198],"Checkpoints","None along the way","Lymph nodes, where white cells inspect and filter the fluid",[200,201,202],"Volume moved per day","≈ 7,258 L, round and round","≈ 3 L, one-way, then rejoins blood",{"id":204,"type":205,"prompt":206,"options":207,"explanation":216},"predict-swollen-glands","prediction","A \"swollen gland\" in the neck or under the jaw during an infection is actually a swollen **lymph node**. Given what a lymph node does, why would it swell exactly when the body is fighting an infection?",[208,210,212,214],{"id":143,"label":209},"It fills up with germs that have nowhere else to go",{"id":146,"label":211},"White blood cells multiply rapidly inside it to fight the infection, and the extra cells take up space",{"id":149,"label":213},"It is a coincidence unrelated to the infection",{"id":152,"label":215},"It swells to block more lymph fluid from arriving","**White blood cells multiplying inside it.** A lymph node is a checkpoint where white cells sit and inspect the fluid draining from nearby tissue. When they detect an infection, they multiply rapidly right there, recruiting more defenders exactly where the germs are being filtered out — and that crowd of extra cells is what you can feel as a firm, tender swelling.\n\nFar from being a problem in itself, a swollen node is a visible sign the checkpoint system is doing precisely its job. It usually settles down within a couple of weeks as the infection is cleared, though a lump that is large, hard, painless or lasts a long time is a reasonable thing to have a doctor look at, exactly the \"ask an adult\" habit from Investigate.",{"id":218,"type":47,"title":219,"eyebrow":220,"navLabel":221},"ch4","The whole day's water, in and out","Chapter 04","4 Water balance",{"id":223,"type":43,"markdown":224},"water-balance-intro","Understand explained kidneys, sweat and breath separately. Put their numbers on the same page and something satisfying appears: over a full day, water leaving the body by every route balances water entering it, almost exactly.",{"id":226,"type":176,"caption":227,"columns":228,"rows":232},"table-water-balance","A typical day's water, in and out, in round litres.",[229,230,231,230],"Route out","Litres a day","Route in",[233,237,242,247,251],[234,235,236,235],"Urine (kidneys)","1.5","Drinking",[238,239,240,241],"Sweat (skin)","0.5","Food (fruit, vegetables, dal, milk all contain water)","0.7",[243,244,245,246],"Water vapour (breath)","0.4","Made inside cells as a by-product of using food for energy","0.3",[248,249,250,250],"Faeces (gut)","0.1","—",[252,253,254,253],"**Total out**","**2.5**","**Total in**",{"id":256,"type":69,"title":257,"problem":258,"steps":259,"help":265},"we-water-balance","Checking that the books balance","Add up the \"out\" column and the \"in\" column from the table separately, using the figures already established across this topic (urine, sweat, breath water from Understand, plus typical drinking, food and metabolic water). Do they match?",[260,261,262,263,264],"Out: 1.5 (urine) + 0.5 (sweat) + 0.4 (breath) + 0.1 (faeces) = **2.5 L**.","In: 1.5 (drink) + 0.7 (food) + 0.3 (made inside cells) = **2.5 L**.","Both totals come to **2.5 litres** — the body is, on an ordinary day, in balance.","Notice that three separate systems (excretory, respiratory, integumentary\u002Fskin) each quietly leak out water on the 'out' side, and three separate sources (a deliberate one — drinking — and two you rarely think about — food and your own metabolism) supply it on the 'in' side.","Change any one number — sweat a litre more on a hot day, say — and the body's thirst mechanism from Understand adjusts the 'in' side to compensate. The balance is not a coincidence; it is homeostasis, applied to water, tracked across every system in this topic at once.",{"simplerExplanation":266,"anotherExample":267},"Add up everything leaving on one side and everything arriving on the other. On an ordinary day they come out equal.","On a hot day sweat rises sharply, tipping the balance, which is exactly why thirst rises to bring drinking up to match.",{"id":269,"type":83,"variant":270,"title":271,"markdown":272},"model-limit-water","model_limit","Why these numbers are typical, not exact","Every figure in the table is a reasonable, typical value for an ordinary day at a moderate temperature and activity level — not a precise measurement of any one person. Body size, climate, diet and activity all shift the real numbers around, sometimes a great deal: someone doing hard physical work in Chennai in May will lose far more than half a litre to sweat.\n\nThe value of the table is not its exact digits. It is the **shape** of the balance: several small routes out, matched by several routes in, held level by feedback loops you have now met individually — thirst, the kidney's water-reclaiming, breathing rate. Deepening an idea often means checking that the pieces you learned separately actually add up together, and here they do.",{"id":274,"type":47,"title":275,"eyebrow":276,"navLabel":277},"ch5","When a hand-over must NOT happen","Chapter 05","5 Deliberate exceptions",{"id":279,"type":43,"markdown":280},"clotting-edge-case","Every hand-over so far in this topic is something the body wants to happen constantly, as fast as possible. Blood clotting is the deliberate opposite: a hand-over the body normally **prevents**, and only switches on at exactly the right place and moment.\n\nThe inner lining of a healthy blood vessel is smooth and actively discourages platelets (the cell fragments that start clotting) from sticking to it. Cut that lining, and the tissue just underneath — normally hidden — is exposed. Platelets recognise that hidden tissue immediately, stick to it, and trigger a cascade: more platelets pile on, a mesh of protein fibres forms around them, and a plug seals the leak, usually within minutes.\n\nThe whole system is a beautiful piece of engineering built around a single **model_limit**: the lining must be clot-resistant everywhere except exactly where it has been breached, updated within moments of the injury and switched off again once healing is under way.",{"id":282,"type":83,"variant":270,"title":283,"markdown":284},"model-limit-clotting","What goes wrong if the switch misfires","If the \"only clot where the lining is broken\" rule ever failed — clots forming inside intact, healthy vessels — the result would be a blockage exactly where blood is not supposed to stop. This is why the inner lining works so hard to actively repel platelets under normal conditions, not just fail to attract them.\n\nThis topic will not go into what causes such failures or into medical conditions — that is beyond its scope and beyond what a general body-systems lesson should try to explain. The lesson to take is a general one: a system built to let something cross easily (as every earlier hand-over in this topic was) sometimes needs an equally strong system built to **prevent** that same crossing everywhere else, and blood vessel walls carry both at once.",{"id":286,"type":83,"variant":84,"title":287,"markdown":288},"nuance-brain-barrier","Not every capillary is built the same","The three-rule pattern from Understand — thin wall, big surface, steep difference — describes a *typical* capillary. The capillaries serving the brain are a deliberate exception: their walls are joined far more tightly than elsewhere in the body, forming what is called the **blood-brain barrier**.\n\nThis barrier lets oxygen, glucose and a short list of essential substances through, and is deliberately far stricter about everything else, including many germs and chemicals that cross ordinary capillaries without trouble. The trade-off is protection for an organ that cannot afford visitors, at the cost of it being harder for useful medicines to reach the brain as well — engineers building brain treatments have to specifically solve this problem. It is a reminder that \"the rule\" in biology usually means \"the rule, unless there is a very good reason for an exception.\"",{"id":290,"type":97,"component":291,"componentVersion":5,"config":292,"objective":314,"textAlternative":315},"lab-match-exceptions","match-pairs",{"prompt":293,"mode":294,"pairs":295},"Match each exception to the ordinary rule it bends.","connect",[296,299,302,305,308,311],{"a":297,"b":298},"Digested fat","Travels by lymph first, not straight into a blood capillary",{"a":300,"b":301},"A healthy vessel lining","Actively repels platelets, so clotting normally does not happen",{"a":303,"b":304},"The blood-brain barrier","Far stricter capillary walls than the rest of the body",{"a":306,"b":307},"Bone","Also a blood-cell factory and calcium bank, not just a lever",{"a":309,"b":310},"A swollen lymph node","White cells multiplying at a checkpoint — the system working",{"a":312,"b":313},"Swollen tissue after injury","A wider gap that slows diffusion far more than it looks","Six exceptions to the tidy patterns from earlier layers, matched to what makes each one different.","Six cards, each an exception worth knowing precisely because it is an exception.\n\n- **Digested fat** ↔ takes the lymphatic route via lacteals, not the direct-to-blood route sugars and amino acids use.\n- **A healthy vessel lining** ↔ actively repels clotting, the opposite of every other hand-over's job of encouraging crossing.\n- **The blood-brain barrier** ↔ far stricter than an ordinary capillary, protecting an organ that cannot afford unwelcome visitors.\n- **Bone** ↔ a factory and a bank as well as a lever.\n- **A swollen lymph node** ↔ a sign the checkpoint system is working, not failing.\n- **Swollen tissue** ↔ where distance-squared diffusion maths turns a modest swelling into a much bigger slowdown.\n\nNone of these break the big idea from Understand. They sharpen it: the three-rule pattern is a *default*, and biology is full of good reasons to override a default on purpose.",{"id":317,"type":47,"title":318,"eyebrow":319,"navLabel":320},"ch6","A second lever, built the opposite way","Chapter 06","6 The calf lever",{"id":322,"type":43,"markdown":323},"achilles-lever","Understand's biceps example showed a lever that trades strength for speed: a small, fast muscle shortening produces a large, fast hand movement, at the cost of needing to pull much harder than the load itself weighs. The calf muscle, pulling on the Achilles tendon at the heel to lift you onto your toes, is built on the same idea but with different numbers, worth working through once more to see the pattern generalise.\n\nThe Achilles tendon attaches to the heel, about **4 cm** behind the ankle joint. The ball of the foot, which takes the body's weight when standing on tiptoe, is about **12 cm** in front of the joint on the other side.",{"id":325,"type":69,"title":326,"problem":327,"steps":328,"help":334},"we-achilles","How hard does your calf actually pull when you stand on tiptoe?","A person with a body weight of 45 kg rises onto their toes on one foot. The heel (where the calf pulls) is 4 cm from the ankle joint; the ball of the foot (which now carries the body weight) is 12 cm from the joint on the other side. Roughly how hard must the calf muscle pull?",[329,330,331,332,333],"Find the lever ratio: 12 ÷ 4 = **3**.","The muscle's force must be the load multiplied by this ratio: 45 × 3 = **135 kg-worth of force**.","So lifting your own 45 kg onto your toes asks your calf to pull with a force equivalent to about 135 kg — roughly three times your own body weight, from one calf, for a completely ordinary movement.","This is exactly why calf muscles are among the strongest, thickest muscles in the body relative to their job: standing on tiptoe, walking, running and jumping all load them this heavily, every single step.","The trade for that hard-working muscle is, once again, speed and range: a small shortening of the calf lifts the whole heel a good distance, fast enough to run and jump.",{"simplerExplanation":335,"anotherExample":336},"The ball of the foot is three times further from the ankle joint than the heel is. So the calf must pull about three times as hard as your body weight to balance it.","It is the same arithmetic as the biceps example: a longer \"load arm\" than \"muscle arm\" always means the muscle works harder than the load itself weighs, in exchange for speed and reach.",{"id":338,"type":136,"itemId":339,"prompt":340,"check":341,"hints":345,"feedback":348},"p-deepen-achilles","body-systems.deepen-achilles","Using the same 4 cm and 12 cm lever arms, roughly how many kilograms-worth of force must the calf produce to lift a 60 kg person onto their toes?",{"kind":342,"answer":343,"tolerance":5,"unit":344},"number",180,"kg-worth",[346,347],"Find the ratio 12 ÷ 4 first.","Multiply the body weight by that ratio.",{"correct":349,"incorrect":350},"Right: the ratio is 3, so 60 × 3 = **180 kg-worth**.","Ratio = 12 ÷ 4 = 3. Force = load × ratio = 60 × 3.",{"id":352,"type":353,"conceptId":354,"relation":355,"explanation":356},"conn-anatomy-lever2","connection","human-body-anatomy","related_to","The exact shape of the ankle joint and where the Achilles tendon attaches is anatomy; the arithmetic of the lever it makes is covered here.",{"id":358,"type":47,"title":359,"eyebrow":360,"navLabel":361},"ch7","Naming the mechanism, not just the crossing","Chapter 07","7 Sort the mechanism",{"id":363,"type":43,"markdown":364},"mechanisms-recap","This topic has now used four different mechanisms to move things around the body, and it is easy to blur them together because they all end in something crossing somewhere. Naming which one is at work in a given hand-over is a genuinely deeper level of understanding than just knowing that a crossing happens.",{"id":366,"type":97,"component":367,"componentVersion":5,"config":368,"objective":417,"textAlternative":418},"lab-sort-mechanism","sort-game",{"prompt":369,"bins":370,"items":383,"seconds":416},"Sort each crossing by the mechanism that actually moves it — not by which system it involves.",[371,374,377,380],{"id":372,"label":373},"diffusion","Diffusion",{"id":375,"label":376},"active","Active transport",{"id":378,"label":379},"bulk","Bulk flow (a pump or pressure)",{"id":381,"label":382},"lymph","Lymph drainage",[384,388,392,396,400,404,408,412],{"id":385,"label":386,"bin":372,"why":387},"o2alv","Oxygen crossing the alveolus wall","Simply drifts from the crowded air sac to the less-crowded blood. No energy spent, no pump.",{"id":389,"label":390,"bin":372,"why":391},"co2alv","Carbon dioxide crossing the alveolus wall","Drifts the opposite way, from crowded blood to less-crowded air, at the same wall, same moment.",{"id":393,"label":394,"bin":375,"why":395},"glucosevilli","Glucose crossing the villus wall","Carrier proteins haul it across, spending energy — which is why it works even against a gradient.",{"id":397,"label":398,"bin":378,"why":399},"bloodheart","Blood moving from the heart to the toes","Physically pushed in bulk by the heart's pumping — not drifting, and not a protein carrier.",{"id":401,"label":402,"bin":378,"why":403},"filtratekidney","Fluid pushed out of the blood in a kidney filter","Blood pressure physically forces fluid through the filter — a pressure-driven bulk process, like diffusion's big cousin.",{"id":405,"label":406,"bin":381,"why":407},"fatvilli","Digested fat leaving a villus","Picked up by a lacteal and carried away in lymph fluid, not diffusing or being pumped directly into a blood capillary.",{"id":409,"label":410,"bin":381,"why":411},"leakedplasma","Plasma that has leaked into tissue, returning to the blood","Collected by lymph vessels and carried, valve by valve, back to a large vein near the neck.",{"id":413,"label":414,"bin":375,"why":415},"saltkidney","Salt reclaimed from the filtered fluid back into the blood in a kidney tubule","Actively pulled back against the flow — the same style of carrier-protein work as glucose at the villi.",0,"Sort nine real crossings from this topic by which of four mechanisms actually moves them: diffusion, active transport, bulk flow or lymph drainage.","Nine crossings, four bins, and the point of the game is that **the mechanism, not the location, is what should decide the bin**.\n\n**Diffusion** (free, no energy, drifts from crowded to scarce): both gases at the alveolus.\n\n**Active transport** (energy spent, carrier proteins, can work against a gradient): glucose at the villi, and salt being reclaimed in the kidney tubule — two different organs, the same mechanism.\n\n**Bulk flow** (pressure or a pump physically moves a volume of fluid): blood pushed by the heart, and fluid pushed out of a kidney's filter by blood pressure — again, two different organs, the same mechanism.\n\n**Lymph drainage** (a separate, pump-free network): digested fat leaving a villus, and leaked plasma finding its way back to the blood.\n\nGrouping this way reveals that the body only has a handful of basic tricks for moving things, reused again and again across completely different organs — which is a very deep-sounding idea that this whole topic has actually been demonstrating from the very first lesson.",{"id":420,"type":47,"title":421,"eyebrow":422,"navLabel":423},"ch8","Turning one number into calories","Chapter 08","8 Energy accounting",{"id":425,"type":43,"markdown":426},"energy-accounting-intro","Every system in this topic has been building toward one final delivery: a cell using oxygen and glucose to release usable energy. That means the oxygen numbers from Understand's breathing chapter can be turned directly into an energy figure — kilocalories, the same unit printed on a food packet — with one more piece of information: each litre of oxygen consumed releases, very roughly, about **5 kilocalories**, whatever food it happens to be burning.\n\nThis is the calculation that connects the respiratory system's numbers to the digestive system's numbers, and it is worth doing once, carefully, because it shows the two chapters were always describing one and the same energy budget from two different ends.",{"id":428,"type":69,"title":429,"problem":430,"steps":431,"help":437},"we-energy-oxygen","From litres of oxygen to kilocalories burned","At rest, you use about 375 mL of oxygen a minute (from Understand's breathing chapter). During hard exercise that rises to about 3375 mL a minute. Using 5 kcal released per litre of oxygen, find the energy cost of each, per minute and per hour.",[432,433,434,435,436],"Convert millilitres to litres: 375 mL = 0.375 L at rest; 3375 mL = 3.375 L during hard exercise.","Multiply each by 5 kcal per litre: resting cost = 0.375 × 5 = **1.88 kcal a minute**.","Hard-exercise cost = 3.375 × 5 = **16.88 kcal a minute** — about **9 times** the resting rate, matching the oxygen-use multiple from Understand almost exactly, because the energy conversion factor is the same for both.","Scale the hard-exercise rate up to an hour: 16.88 × 60 = **1013 kcal an hour**.","Since one roti was set at 120 kcal back in Discover, that hour of hard exercise costs about 1013 ÷ 120 = **8.4 rotis' worth of energy** — a genuinely large appetite, entirely explained by multiplying together numbers this topic had already produced separately.",{"simplerExplanation":438,"anotherExample":439},"Oxygen used per minute, turned into litres, multiplied by about 5 kcal per litre, gives an energy cost per minute. Multiply by 60 for an hour.","This is exactly why a day of hard physical work or sport genuinely requires more food, not just more willpower — the energy really has been spent, litre of oxygen by litre of oxygen.",{"id":441,"type":83,"variant":84,"title":442,"markdown":443},"nuance-approx-energy","Why 5 kcal per litre is \"roughly\", not exactly","The 5 kcal-per-litre figure is a typical, widely used approximation, not a fixed constant. Its true value depends slightly on **what** is being burned: fat, carbohydrate (like the starch in a roti) and protein each release a slightly different amount of energy per litre of oxygen used, generally somewhere between about 4.7 and 5.0.\n\nFor working out the *shape* of the story — that hard exercise costs roughly nine times the resting energy rate, because it costs roughly nine times the oxygen — a single typical value is perfectly good enough. For a food label's exact kilocalorie count, a food scientist uses the more detailed, ingredient-specific figures. Knowing which precision a question actually needs is itself part of thinking clearly with numbers.",{"id":445,"type":47,"title":446,"eyebrow":447,"navLabel":448},"ch9","One feeling, several possible causes","Chapter 09","9 One symptom, many",{"id":450,"type":43,"markdown":451},"one-symptom-intro","Every chapter so far has gone from a system to its effects. Real bodies are often investigated the other way round: you notice one feeling, and have to work out which system, if any, is behind it. This is genuinely harder, because several completely different systems can produce the same feeling.\n\nTake **dizziness** — a light-headed, unsteady feeling. This topic alone gives you at least three unrelated ways to arrive at it, and telling them apart needs exactly the kind of systems thinking this whole topic has been building.",{"id":453,"type":454,"title":455,"prompt":456,"options":457},"explorer-dizzy","explorer","Three roads to the same feeling","Pick a cause and see how it leads to dizziness.",[458,471,483],{"id":459,"label":460,"chain":461,"badge":467,"note":470},"standup","Standing up too fast",[462,463,464,465,466],"Blood pools in the legs","Brain gets less blood, briefly","Brain briefly short of fuel","A few seconds of light-headedness","Circulation catches up",{"text":468,"tone":469},"Circulatory","yes","Gravity pulls blood toward the legs the instant you stand, and it takes the circulatory system a second or two to adjust vessel width and heart rate to compensate. The brief gap is felt as dizziness, and it passes as soon as blood flow catches up — usually within seconds, and it is why standing up slowly after lying down is a genuinely sensible habit, not an old wives' tale.",{"id":472,"label":473,"chain":474,"badge":480,"note":482},"lowsugar","Not eating for a long time",[475,476,477,478,479],"Blood glucose drifts low","Brain cells run low on their main fuel","Thinking and balance both suffer","Light-headedness and shakiness","Eating restores it",{"text":481,"tone":469},"Digestive \u002F endocrine","The brain depends almost entirely on a steady glucose supply and has very little fuel stored of its own. Go a long time without eating and blood glucose can drift toward the low end of its usual range, leaving the brain under-fuelled — felt as light-headedness, shakiness or trouble concentrating, and fixed quickly by eating.",{"id":484,"label":485,"chain":486,"badge":492,"note":494},"innerear","Spinning around and stopping",[487,488,489,490,491],"Fluid in the inner ear keeps moving","Sensors report motion that has stopped","The brain gets conflicting signals","A spinning, dizzy feeling","Settles once the fluid stills",{"text":493,"tone":469},"Nervous (balance)","Balance sensors in the inner ear detect movement using fluid that swirls when you spin. Stop suddenly and the fluid keeps moving for a moment, so the balance system keeps reporting motion your eyes and joints say has stopped — the mismatch between signals is exactly what a spinning, dizzy feeling is.",{"id":496,"type":83,"variant":84,"title":497,"markdown":498},"nuance-doctor-questions","Why a doctor asks so many questions","None of the three roads above look alike from the inside — a doctor working out which is which asks about timing (did it happen on standing up?), about food (when did you last eat?), and about spinning or ear symptoms — precisely because \"dizzy\" alone does not say which system is involved.\n\nThis is the practical, grown-up version of the systems thinking this whole topic has practised: naming a feeling is the beginning of an investigation, not the end of one, and matching a symptom to the right system is a skill, not a guess. It is also, once again, a case for the simple childhood rule: if something like this happens and does not pass quickly, or happens often, telling an adult is the right next step — not working it out alone.",{"id":500,"type":47,"title":501,"eyebrow":502,"navLabel":503},"ch10","Why blood must slow down as vessels branch","Chapter 10","10 Flow conservation",{"id":505,"type":43,"markdown":506},"flow-conservation-intro","Understand stated, almost in passing, that blood slows down in the capillaries \"which sounds like a fault but is the design\". Here is the physics that makes that slowdown unavoidable, not just convenient.\n\nWhatever volume of blood leaves the heart each second must arrive somewhere each second — nothing piles up or vanishes along the way. That single fact, called conservation of flow, links a vessel's cross-sectional area and the speed of blood inside it: **area × speed stays the same** as one wide vessel divides into many narrower ones, so if the *combined* area of all the branches is bigger than the original vessel, the speed in each branch must be correspondingly smaller.",{"id":508,"type":56,"items":509},"formulas-flow",[510,513,516],{"expression":511,"caption":512},"flow rate = area × speed","The same total flow rate must pass every stage of a branching network, from a wide vessel to its narrow branches.",{"expression":514,"caption":515},"3 cm² × 30 cm\u002Fs","Illustrative wide vessel: flow rate = 90 cm³\u002Fs.",{"expression":517,"caption":518},"10 cm² (combined) × 9 cm\u002Fs","Illustrative narrow branches: same flow rate, 90 cm³\u002Fs, but a much bigger combined area, so a much lower speed.",{"id":520,"type":69,"title":521,"problem":522,"steps":523,"help":530},"we-flow-conservation","An illustrative branching network","These numbers are a made-up but structurally realistic illustration, not a measurement of a real vessel. One vessel of cross-sectional area 3 cm², with blood moving at 30 cm\u002Fs, splits into 1,000 tiny vessels, each with a cross-sectional area of only 0.01 cm². Find the combined area of the branches, and work out how fast blood must move through each one.",[524,525,526,527,528,529],"Combined area of the branches: 0.01 × 1,000 = **10 cm²** — more than three times the original 3 cm², even though each individual branch is far narrower.","The flow rate leaving the wide vessel is area × speed = 3 × 30 = **90 cm³ a second**.","That same flow rate must be shared across the branches, so their speed must be 90 ÷ 10 = **9.0 cm\u002Fs** — a little under a third of the original speed.","Check it: 10 × 9.0 ≈ 90 cm³\u002Fs, matching the original flow rate. ✓","This is the real reason blood arrives at a capillary bed moving slowly: not because something is pushing less hard, but because the total 'width' available for the blood to spread across has grown, even though any one capillary is far narrower than the vessel it branched from.","It is exactly the kind of trade Understand's three-rule pattern needs: slow flow gives diffusion the time it needs to work, and this arithmetic shows the slowdown was never optional — it follows automatically from the branching itself.",{"simplerExplanation":531,"anotherExample":532},"The same amount of blood has to get through every stage. If the combined width of the branches is bigger than the original pipe, the blood must move slower through them to still get through the same amount each second.","A wide river slows to a crawl the moment it spreads out into a broad delta with many channels, for exactly the same reason — the same volume of water, sharing a much bigger total cross-section.",{"id":534,"type":136,"itemId":535,"prompt":536,"check":537,"hints":541,"feedback":544},"p-deepen-flow","body-systems.deepen-flow","A vessel of area 2 cm² carrying blood at 40 cm\u002Fs branches into vessels with a combined area of 8 cm². What speed (in cm\u002Fs) must blood have in the branches, to keep the flow rate the same?",{"kind":342,"answer":538,"tolerance":539,"unit":540},10,0.5,"cm\u002Fs",[542,543],"Flow rate = area × speed. Find the flow rate first using the original vessel.","Divide that flow rate by the new combined area.",{"correct":545,"incorrect":546},"Right: flow rate = 2 × 40 = 80 cm³\u002Fs. New speed = 80 ÷ 8 = **10 cm\u002Fs**.","Compute the flow rate from the first vessel (area × speed), then divide it by the new combined area to find the new speed.",{"id":548,"type":83,"variant":84,"title":549,"markdown":550},"nuance-illustrative-numbers","Why these numbers are illustrative, not measured","The 3 cm², 1000 vessels and 0.01 cm² figures above were chosen to make the arithmetic clean and the pattern obvious, not measured from a real body — a fair comparison in style, if not in scale, to the aorta genuinely being far narrower than the combined width of all its downstream capillaries.\n\nThe lesson to take is the **relationship**, not the digits: total cross-sectional area grows enormously as vessels branch, and the physics of conserved flow rate then forces the speed to fall by the same enormous factor. That relationship is real, well established, and exactly what makes a capillary a slow, patient place for a hand-over to happen, however you choose your illustrative numbers.",{"id":552,"type":47,"title":553,"eyebrow":554,"navLabel":555},"ch11","Not every rise is the same size","Chapter 11","11 Comparing multiples",{"id":557,"type":43,"markdown":558},"comparing-multiples-intro","This topic has computed a \"how many times more\" figure for exercise again and again: pulse, breathing, cardiac output, blood flow to muscle, oxygen use, energy burned. Laid out side by side, an interesting pattern appears — they do **not** all rise by the same factor, and the differences are themselves informative.",{"id":560,"type":176,"caption":561,"columns":562,"rows":566},"table-exercise-multiples","Every exercise multiple in this topic, side by side.",[563,564,565],"Measurement","Rest → hard exercise","Multiple",[567,570,573,576,579,582],[568,250,569],"Pulse (bpm)","×1.7",[571,250,572],"Breathing rate","×1.9",[574,250,575],"Cardiac output (L\u002Fmin)","×4.0",[577,250,578],"Blood flow to muscle","×16",[580,250,581],"Oxygen used per minute","×9.0",[583,250,581],"Energy burned (kcal\u002Fmin)",{"id":585,"type":69,"title":586,"problem":587,"steps":588,"help":594},"we-why-different-multiples","Why does blood flow to muscle rise so much more than pulse does?","Pulse rises by only about ×1.7 during hard exercise, yet blood flow specifically to the muscles rises by about ×16. Both numbers describe blood being delivered faster — so why is one so much bigger than the other?",[589,590,591,592,593],"Pulse rising ×1.7 and cardiac output rising ×4 together explain only part of the story: **more blood overall** is being pumped, but that alone would only multiply muscle flow by about the same ×4.","The rest of the gap comes from **redistribution**, from Discover's exercise chapter: the *share* of that blood sent to muscle rises from about 20% to 80%, a further ×4 on top.","Multiply the two effects together: ×4 (more total blood) × ×4 (more of it sent to muscle) = **×16**, matching the number from Discover exactly.","So a modest rise in pulse is compounded by a much bigger rise in cardiac output, which is compounded again by the vessels themselves widening near muscle and narrowing near the gut — three separate systems' worth of adjustment stacked on top of each other for one combined effect.","This is the real reason different measurements in this topic rise by such different amounts during exercise: each one sits at a different point in a chain of compounding effects, not because some systems are simply trying harder than others.",{"simplerExplanation":595,"anotherExample":596},"A bigger heart output multiplied by a bigger share going to muscle gives a much bigger increase than either change on its own.","It is the same style of reasoning as compound interest: two moderate multipliers, applied one after another, combine into one much larger multiplier.",{"id":598,"type":599,"title":600,"terms":601},"glossary-deepen","glossary","Words for this layer",[602,606,610,614,618,622],{"term":603,"meaning":604,"example":605},"Bone marrow","Soft tissue inside the hollow shaft of a long bone, where red cells, white cells and platelets are made.","Found in the femur, pelvis, spine and other long bones.",{"term":607,"meaning":608,"example":609},"Lymphatic system","A one-way network of vessels that collects leaked plasma from body tissues, filters it through lymph nodes, and returns it to the blood.","No pump of its own — muscle squeeze and one-way valves move it, like veins.",{"term":611,"meaning":612,"example":613},"Lymph node","A checkpoint along the lymphatic system where white blood cells inspect fluid for germs.","Swells when it is busy fighting an infection nearby.",{"term":615,"meaning":616,"example":617},"Lacteal","A tiny lymphatic vessel inside a villus that absorbs digested fat, instead of the fat going straight into a blood capillary.","The one thing at the villi that does not head straight to the liver.",{"term":619,"meaning":620,"example":621},"Blood-brain barrier","An unusually tight capillary wall around the brain, far stricter than an ordinary capillary about what it lets through.","Protects the brain, but also makes it harder for medicines to reach it.",{"term":623,"meaning":624,"example":625},"Diffusion time ∝ distance²","A rule of physics: diffusion time rises with the square of the distance, not in simple proportion to it.","Doubling a gap does not double the crossing time — it quadruples it.",{"id":627,"type":628,"title":629,"questions":630},"quiz-deepen","quiz","Check yourself: mathematics, exceptions and the lymph network",[631,644,657,670,683,696,709,722],{"itemId":632,"prompt":633,"options":634,"correct":149,"why":643},"body-systems.deepen-q-square","If a hand-over barrier becomes three times thicker than normal, roughly how much longer does diffusion take across it?",[635,637,639,641],{"id":143,"label":636},"Three times as long",{"id":146,"label":638},"Six times as long",{"id":149,"label":640},"Nine times as long",{"id":152,"label":642},"No longer — thickness does not matter","Diffusion time scales with distance squared, so three times the distance means 3² = 9 times the time.",{"itemId":645,"prompt":646,"options":647,"correct":146,"why":656},"body-systems.deepen-q-marrow2","Besides being a lever, what does bone do that this layer added to the picture?",[648,650,652,654],{"id":143,"label":649},"It produces hormones that control mood",{"id":146,"label":651},"It manufactures blood cells and stores most of the body's calcium",{"id":149,"label":653},"It filters the blood like a kidney",{"id":152,"label":655},"It stores extra oxygen","Bone marrow makes red cells, white cells and platelets, and the hard mineral part of bone banks roughly 99% of the body's calcium.",{"itemId":658,"prompt":659,"options":660,"correct":146,"why":669},"body-systems.deepen-q-lymph","What does the lymphatic system mainly do?",[661,663,665,667],{"id":143,"label":662},"Pumps blood around the body",{"id":146,"label":664},"Collects fluid that leaks from capillaries and returns it to the blood, filtering it on the way",{"id":149,"label":666},"Carries oxygen to the lungs",{"id":152,"label":668},"Digests fat directly","It has no pump of its own, but it collects leaked plasma, filters it through lymph nodes, and returns it to the bloodstream near the neck.",{"itemId":671,"prompt":672,"options":673,"correct":146,"why":682},"body-systems.deepen-q-fat","Digested fat is absorbed differently from sugars and amino acids at the villi. How?",[674,676,678,680],{"id":143,"label":675},"It is not absorbed at all",{"id":146,"label":677},"It enters a lacteal (lymph vessel) rather than a blood capillary directly",{"id":149,"label":679},"It is absorbed in the stomach instead",{"id":152,"label":681},"It is absorbed twice as fast as sugar","Fat droplets are picked up by a lacteal inside the villus and travel through the lymphatic system before joining the blood, unlike sugars and amino acids.",{"itemId":684,"prompt":685,"options":686,"correct":146,"why":695},"body-systems.deepen-q-clot","Why does a healthy blood vessel lining normally resist clotting?",[687,689,691,693],{"id":143,"label":688},"Because platelets are too small to stick to anything",{"id":146,"label":690},"So that clots only form exactly where the lining has actually been broken",{"id":149,"label":692},"Because the blood is moving too fast for clots to form",{"id":152,"label":694},"It does not resist clotting; clots form randomly","An intact lining actively discourages platelets from sticking, so that clotting — a hand-over the body normally prevents — switches on only at a genuine injury.",{"itemId":697,"prompt":698,"options":699,"correct":146,"why":708},"body-systems.deepen-q-brain-barrier","How does a brain capillary differ from a typical capillary described in Understand?",[700,702,704,706],{"id":143,"label":701},"It has no wall at all",{"id":146,"label":703},"Its wall is joined far more tightly, letting far less through",{"id":149,"label":705},"It only carries carbon dioxide",{"id":152,"label":707},"It is much wider than usual","The blood-brain barrier is stricter than an ordinary capillary, protecting the brain at the cost of making it harder for many substances, including some medicines, to get through.",{"itemId":710,"prompt":711,"options":712,"correct":146,"why":721},"body-systems.deepen-q-flow-conserve","Why must blood slow down as a wide vessel branches into many narrow capillaries?",[713,715,717,719],{"id":143,"label":714},"The heart pumps less hard further from itself",{"id":146,"label":716},"The combined cross-sectional area of the branches is bigger than the original vessel, so the same flow rate needs a lower speed",{"id":149,"label":718},"Capillaries actively brake the blood",{"id":152,"label":720},"Blood gets heavier as it travels","Flow rate (area × speed) must stay the same through a branching network. A bigger combined area forces a lower speed to carry the same volume each second.",{"itemId":723,"prompt":724,"options":725,"correct":146,"why":734},"body-systems.deepen-q-multiples","Why does blood flow to muscle rise by a much bigger factor during exercise than pulse alone does?",[726,728,730,732],{"id":143,"label":727},"Pulse does not actually matter",{"id":146,"label":729},"Several effects — more cardiac output and more of it redirected to muscle — compound together",{"id":149,"label":731},"Muscles pump their own blood",{"id":152,"label":733},"It is a measurement error","A rise in cardiac output and a rise in the share of blood sent to muscle multiply together, producing a much bigger combined effect than either change alone.",{"id":736,"type":737,"prompt":738},"reflect-deepen","reflection","This layer found four places where a tidy rule from Understand had at least one exception: fat's lymphatic route, the vessel lining that resists clotting, the extra-strict blood-brain barrier, and bone's double life as a blood factory and calcium bank.\n\nPick one of the four and explain, in your own words, why the exception exists — what problem would the *ordinary* rule have caused if it had been followed without exception in that particular place?",{"id":740,"type":741,"title":742,"points":743},"cheat-deepen","summary","Cheat sheet",[744,745,746,747,748,749,750,751,752,753,754,755],"**Diffusion time scales with distance squared.** Double the gap, quadruple the time — this is most of the reason hand-over walls must be as thin as possible.","**Bone is a factory and a bank as well as a lever.** Marrow makes blood cells; the mineral part stores about 99% of body calcium.","**The lymphatic system is a one-way, pump-free network** that returns about 3 L a day of leaked plasma to the blood, filtering it through lymph nodes on the way.","**Digested fat travels by lymph (lacteals), not straight into a blood capillary** like sugars and amino acids do.","**A healthy vessel lining actively resists clotting**, so the hand-over of platelets sticking only happens exactly where the lining is broken.","**The blood-brain barrier is a stricter-than-usual capillary**, trading easy access for protection of an organ that cannot afford visitors.","**Water balances across systems**: about 2.5 L out (urine, sweat, breath, faeces) matches about 2.5 L in (drink, food, metabolism) on an ordinary day.","**A second lever, the calf and Achilles tendon**, needs about 3 times body weight in force to rise onto the toes — the same speed-for-strength trade as the biceps, with different numbers.","**Oxygen use converts directly to energy**, roughly 5 kcal per litre — turning hard exercise's ninefold rise in oxygen use into a ninefold rise in kilocalories burned.","The same feeling (like dizziness) can come from several unrelated systems — naming it is the start of an investigation, not the end of one.","**Flow rate (area × speed) is conserved** through a branching network, which is the real, physical reason blood must slow down as vessels branch into capillaries.","**Exercise multiples compound**: a ×4 rise in cardiac output and a redirected blood share together produce the much bigger ×16 rise in blood flow to muscle.",{"id":757,"type":758,"sourceIds":759},"sources-deepen","sources",[760,761,762,763,764],"body-systems-britannica-cardiovascular","body-systems-britannica-respiratory","body-systems-wiki-homeostasis","body-systems-britannica-digestive","body-systems-britannica-nervous",[760,761,762,763,764],"needs_review",{"generatedBy":768,"notes":769},"claude-code","Draft generated locally; pending owner review.","246b14ab3c2f6bca9eb6485523b0573b81ffbc1b64030ae0ad1bb53070c459a5",{"component:system-flow@1":772,"logic:practice":773,"component:match-pairs@1":774,"component:sort-game@1":775,"source:body-systems-britannica-cardiovascular":776,"source:body-systems-britannica-digestive":777,"source:body-systems-britannica-nervous":778,"source:body-systems-britannica-respiratory":779,"source:body-systems-wiki-homeostasis":780},"40eda343e2312ed1d0eed875e27ef482c65d948452c1be27522b9a4268085e71","3d6b0fe1b15255975a32b0fcd94e8019bc959ad45cbf12e136e86149549c6878","2a8ee4ac87460b4e1175a4bb13c96b03d577db06dde95670eb7fcfe4ad787899","b164f45a2c8ca08f26c450768ff0231e113e9fe45381eddb34dc6d0548596c38","8b7278004bfe64d7ad3e369fbfa62cada6c2ca19bdda240520d8cb96569d40a7","8b28eef54dd7327ab08588678c32674f0a6615182ae48d49d272ad156d0ad122","07d4488afad056b326893d48a8ecd74894b1e87c5b23c8f0bbd1268b77fdf252","dd8e7c0348ffd73f06866939a2804a19b80938f30a21e7c2b3e4ac922cb96f65","0e7d85baebb026691fbe6a4ef3c8aeaf14bd5c792553e4d59439acce7e4f81b3",{"state":782,"reviewer":783,"selfReview":130,"reviewedAt":784,"method":785},"approved","The library owner","2026-09-20T10:18:37.581Z","owner_bulk","preview-7e1cbbcc4f",1789899598330]