{"schemaVersion":1,"id":"en-mechanism-kidneys","type":"mechanism","pillar":"body","locale":"en","translationGroupId":"mechanism-kidneys","slugPath":"body/systems/the-kidneys","title":"The kidneys and water balance","shortTitle":"The kidneys","summary":"They are described as waste filters. It is the other way round: they filter almost all the blood and then reclaim almost all of it, and the body's water balance is decided in what they choose to keep.","level":"intermediate","estimatedMinutes":8,"synonyms":["kidneys","water balance","filtration","urine","sodium","blood volume"],"blocks":[{"id":"b1","type":"heading","level":2,"text":"A regulator, not a bin","anchor":"a-regulator-not-a-bin"},{"id":"b2","type":"paragraph","content":[{"kind":"text","value":"The common image is of a filter holding back waste and letting the rest through. The real functioning is almost the reverse, and that reversal is what makes the organ interesting."}]},{"id":"b3","type":"paragraph","content":[{"kind":"text","value":"The kidney filters "},{"kind":"strong","value":"massively"},{"kind":"text","value":": a considerable share of the blood passing through it is poured into a system of tubules, along with the water, salts, glucose and amino acids it contains. Then it reclaims almost all of it. What was not reclaimed leaves in the urine."}]},{"id":"b4","type":"definition","term":"Filtration then reabsorption","definition":"The kidney first separates a large quantity of fluid from the blood, without sorting, then selectively reabsorbs what should be kept. The regulation happens in that second step, not the first.","alsoKnownAs":["the kidney's principle"]},{"id":"b5","type":"paragraph","content":[{"kind":"text","value":"The arrangement looks wasteful. It has a decisive advantage: the body needs no mechanism able to recognise every unwanted substance. It only has to recognise what it wants to "},{"kind":"strong","value":"keep"},{"kind":"text","value":" — and everything else leaves by default."}]},{"id":"b6","type":"heading","level":2,"text":"What the hydration page assumed","anchor":"what-the-hydration-page-assumed"},{"id":"b7","type":"paragraph","content":[{"kind":"text","value":"The Recovery pillar describes a water loss affecting performance, and the "},{"kind":"link","value":"skin","href":"/en/body/systems/the-skin","external":false},{"kind":"text","value":" page describes the sweating that causes it. Neither said how the body responds to that loss."}]},{"id":"b8","type":"paragraph","content":[{"kind":"text","value":"It is here. When water runs short, the kidney reabsorbs more of it: urine volume falls and its concentration rises. When water is plentiful, it lets more of it go."}]},{"id":"b9","type":"comparison","title":"Two situations, two responses","columns":["Situation","What the kidney does","Visible result"],"rows":[{"label":"Water plentiful","cells":["Reabsorbs less","Abundant, pale urine"]},{"label":"Water short","cells":["Reabsorbs more","Reduced, concentrated urine"]}]},{"id":"b10","type":"callout","tone":"caution","title":"What colour does not tell you","content":[[{"kind":"text","value":"That relationship is real, and it is often turned into a test. It is not one. Urine colour is influenced by diet, by some medicines and by vitamin supplements, independently of hydration status."}],[{"kind":"text","value":"Body Lab therefore proposes no indicator to follow, no threshold and no amount to drink: that would be personalised advice built on a sign too many factors disturb."}]]},{"id":"b11","type":"heading","level":2,"text":"Blood volume, not only water","anchor":"blood-volume-not-only-water"},{"id":"b12","type":"paragraph","content":[{"kind":"text","value":"The kidney does not only set water. It also sets "},{"kind":"strong","value":"sodium"},{"kind":"text","value":", and the two are linked: water follows salt. By keeping or discarding sodium, the kidney therefore acts on the volume of circulating fluid."}]},{"id":"b13","type":"paragraph","content":[{"kind":"text","value":"This is the junction with the "},{"kind":"link","value":"blood and circulation","href":"/en/body/systems/blood-and-circulation","external":false},{"kind":"text","value":" page. A reduced plasma volume forces the heart to beat faster to maintain the same output; the kidney is the organ working to prevent that reduction."}]},{"id":"b14","type":"evidence","level":"established","statement":"A substantial water loss during prolonged effort in the heat affects several water compartments of the body, including the water contained in muscle.","detail":"The study follows subjects put through prolonged effort in a hot environment, with measurement of muscle water and glycogen. It covers a limited sample and precise experimental conditions; it establishes a link between compartments, not an instruction about intake.","referenceIds":["fernandez2015"]},{"id":"b15","type":"heading","level":2,"text":"What changes during effort","anchor":"what-changes-during-effort"},{"id":"b16","type":"paragraph","content":[{"kind":"text","value":"The circulation page describes blood flow being redistributed towards the working muscles, at the expense of territories that can wait. The kidney is one of those territories."}]},{"id":"b17","type":"paragraph","content":[{"kind":"text","value":"Its urine production therefore falls during a sustained effort, and that fall adds to the one caused by water loss. Both mechanisms point the same way: conserve."}]},{"id":"b18","type":"heading","level":2,"text":"The other roles","anchor":"the-other-roles"},{"id":"b19","type":"paragraph","content":[{"kind":"text","value":"Two less well-known functions deserve a mention, because they link this page to others."}]},{"id":"b20","type":"paragraph","content":[{"kind":"text","value":"The kidney takes part in maintaining the acid-base balance of the blood, by discarding or keeping certain ions. And it produces a signal that stimulates the making of red cells — the ones whose role as oxygen carriers the "},{"kind":"link","value":"blood","href":"/en/body/systems/blood-and-circulation","external":false},{"kind":"text","value":" page describes."}]},{"id":"b21","type":"heading","level":2,"text":"Why no scene accompanies this page","anchor":"why-no-scene-accompanies-this-page"},{"id":"b22","type":"callout","tone":"limit","title":"A limit of rights, not of data","content":[[{"kind":"text","value":"As for the "},{"kind":"link","value":"digestive system","href":"/en/body/systems/the-digestive-system","external":false},{"kind":"text","value":", the reason is not an absence of data. The urinary system exists in the anatomical atlas Body Lab derives its scenes from, but it belongs to the visceral system, published under a non-commercial licence incompatible with the project's use."}],[{"kind":"text","value":"The absence is therefore declared as a limit of rights, not as a gap in the sources."}]]},{"id":"b23","type":"heading","level":2,"text":"What this page does not do","anchor":"what-this-page-does-not-do"},{"id":"b24","type":"callout","tone":"caution","title":"No tests, no quantities","content":[[{"kind":"text","value":"This page describes a general functioning. It addresses no kidney disease, allows no test result to be interpreted and gives no amount of drink or salt."}],[{"kind":"text","value":"A drop in urine output, cloudy, foamy or bloody urine, swelling of the legs or face, or severe low back pain belongs to a health professional."}]]},{"id":"b25","type":"heading","level":2,"text":"Sources","anchor":"sources"},{"id":"b26","type":"sourceList","title":"Main sources","referenceIds":["hall2020","standring2020","fernandez2015"]},{"id":"b27","type":"shapierAction","actionId":"hydration-performance","label":"Hydration and performance","description":"Adjusting water intake around exercise happens in Shapier; Body Lab only explains how the kidney decides what the body keeps.","webTarget":"https://shapier.app/en/conseils/hydratation-performance","appTarget":"shapier://conseils/hydratation-performance"},{"id":"b28","type":"quiz","title":"Check my understanding","questions":[{"id":"q1","prompt":"The kidney is often described as a filter that holds back waste. What does this page describe?","choices":[{"id":"q1c1","label":"It filters massively without sorting, then selectively reabsorbs what should be kept","correct":true,"explanation":"This is the page's central reversal: regulation happens in the second step, reabsorption, not in filtration."},{"id":"q1c2","label":"It sorts the blood to hold back only waste and let the rest through","correct":false,"explanation":"That is the common image the page corrects: the kidney first pours out a large quantity of fluid, then reclaims almost all of it, and what is not reclaimed leaves in the urine."},{"id":"q1c3","label":"It recognises every unwanted substance to eliminate them one by one","correct":false,"explanation":"The page says the opposite: the body does not need to recognise the unwanted, only what it wants to keep, and everything else leaves by default."}]},{"id":"q2","prompt":"Why can urine colour not serve as a hydration test?","choices":[{"id":"q2c1","label":"Because it is also influenced by diet, by some medicines and by vitamin supplements","correct":true,"explanation":"The page accepts that the link between available water and concentrated urine is real, but too many factors disturb this sign to make it an indicator."},{"id":"q2c2","label":"Because urine volume does not vary with hydration","correct":false,"explanation":"The page states the opposite: when water runs short, the kidney reabsorbs more, urine volume falls and its concentration rises."},{"id":"q2c3","label":"Because colour depends only on the amount of sodium discarded","correct":false,"explanation":"The sodium-water link concerns blood volume; the page attributes colour variations to diet, medicines and supplements, independently of hydration status."}]},{"id":"q3","prompt":"During a sustained effort, why does urine production fall?","choices":[{"id":"q3c1","label":"Because blood flow is redistributed towards the working muscles, at the expense of territories like the kidney","correct":true,"explanation":"That is the mechanism described by the circulation page, and it adds to the fall caused by water loss: both point towards conserving."},{"id":"q3c2","label":"Only because sweating makes the body lose water","correct":false,"explanation":"Water loss contributes, but the page adds a second mechanism, the redistribution of blood towards the muscles; reducing the answer to sweat misses half of it."},{"id":"q3c3","label":"Because the kidney stops filtering blood during effort","correct":false,"explanation":"The page describes no stop in filtration; it only says urine production falls during a sustained effort."}]}]},{"id":"b29","type":"relatedContent","title":"Read next","targetIds":["en-mechanism-hydration","en-mechanism-blood-circulation","en-mechanism-lymphatic-system"]}],"relations":[{"type":"prerequisite","targetId":"en-mechanism-blood-circulation"},{"type":"explains","targetId":"en-mechanism-hydration"},{"type":"explains","targetId":"en-mechanism-skin"},{"type":"next-step","targetId":"en-mechanism-lymphatic-system"}],"authors":["equipe-editoriale-shapier"],"reviewers":["Thanh Chau"],"references":[{"id":"hall2020","authors":"Hall JE, Hall ME","year":2020,"title":"Guyton and Hall Textbook of Medical Physiology, 14th edition","source":"Elsevier","kind":"book","url":"https://shop.elsevier.com/books/guyton-and-hall-textbook-of-medical-physiology/hall/978-0-323-59712-8"},{"id":"standring2020","authors":"Standring S","year":2020,"title":"Gray's Anatomy: The Anatomical Basis of Clinical Practice, 42nd edition","source":"Elsevier","kind":"book","url":"https://shop.elsevier.com/books/grays-anatomy/standring/978-0-7020-7705-0"},{"id":"fernandez2015","authors":"Fernández-Elías VE, Ortega JF, Nelson RK, Mora-Rodriguez R","year":2015,"title":"Relationship between muscle water and glycogen recovery after prolonged exercise in the heat in humans","source":"European Journal of Applied Physiology","kind":"journal-article","doi":"10.1007/s00421-015-3175-z"}],"review":{"publishedAt":"2026-08-02","reviewedAt":"2026-08-02","reviewDueAt":"2027-08-02","evidenceLevel":"established"},"limitations":["No scene accompanies this page: the urinary system belongs to the visceral system of the atlas used, published under a licence the project cannot use.","This page addresses no kidney disease and allows no test result to be interpreted.","Urine colour is influenced by diet, by some medicines and by supplements: it is not a measurement, and this page does not make it an indicator to follow.","A drop in urine output, cloudy or bloody urine, swelling or low back pain belongs to a health professional."],"seo":{"title":"The kidneys — a regulator, not a waste filter","description":"Massive filtration then near-total reclaiming: how the kidney sets the volume and composition of blood, and what the hydration page assumed.","canonicalPath":"/en/body/systems/the-kidneys","image":"/og/en/mechanism-kidneys.png"},"app":{"offlineEligible":true},"version":1}