{"schemaVersion":1,"id":"en-mechanism-blood-circulation","type":"mechanism","pillar":"body","locale":"en","translationGroupId":"mechanism-blood-circulation","slugPath":"body/systems/blood-and-circulation","title":"Blood and circulation","shortTitle":"Blood and circulation","summary":"Thirty-two pages of this corpus mention blood without any of them saying what it carries or how it travels. Everything that matters happens in the smallest vessels, and the return journey poses a problem the outward one does not.","level":"beginner","estimatedMinutes":9,"synonyms":["blood","circulation","haemoglobin","red blood cells","capillaries","venous return","plasma"],"blocks":[{"id":"b1","type":"heading","level":2,"text":"A word used everywhere, never defined","anchor":"a-word-used-everywhere-never-defined"},{"id":"b2","type":"paragraph","content":[{"kind":"text","value":"Thirty-two pages of Body Lab mention blood. "},{"kind":"link","value":"Glycogen","href":"/en/energy/muscle-glycogen-and-water","external":false},{"kind":"text","value":" arrives through it, "},{"kind":"link","value":"amino acids","href":"/en/adaptations/what-happens-to-dietary-protein","external":false},{"kind":"text","value":" cross it, the "},{"kind":"link","value":"aerobic pathway","href":"/en/energy/the-aerobic-system","external":false},{"kind":"text","value":" depends on it, "},{"kind":"link","value":"hydration","href":"/en/recovery/hydration-and-performance","external":false},{"kind":"text","value":" changes its volume. None said what it is."}]},{"id":"b3","type":"heading","level":2,"text":"Two components, two roles","anchor":"two-components-two-roles"},{"id":"b4","type":"paragraph","content":[{"kind":"text","value":"Blood is not a uniform liquid. It is a suspension: a great many cells, carried by a fluid."}]},{"id":"b5","type":"comparison","title":"What blood is made of","columns":["Component","Nature","Main role"],"rows":[{"label":"Plasma","cells":["Liquid, overwhelmingly water","Carries everything dissolved: sugars, amino acids, hormones, heat"]},{"label":"Red cells","cells":["Cells without a nucleus, filled with haemoglobin","Carry oxygen and part of the carbon dioxide"]},{"label":"White cells","cells":["Defence cells","Act against threats"]},{"label":"Platelets","cells":["Cell fragments","Take part in stopping bleeding"]}]},{"id":"b6","type":"definition","term":"Haemoglobin","definition":"A protein contained in red cells, able to bind oxygen where it is abundant and release it where it is lacking. Without it, plasma alone would carry only a negligible share of the oxygen needed.","alsoKnownAs":["oxygen carrier"]},{"id":"b7","type":"paragraph","content":[{"kind":"text","value":"This protein is what makes the whole Energy pillar possible. Plasma without haemoglobin would dissolve only a tiny fraction of the oxygen a working muscle requires."}]},{"id":"b8","type":"heading","level":2,"text":"Three kinds of vessel, one place of exchange","anchor":"three-kinds-of-vessel-one-place-of-exchange"},{"id":"b9","type":"paragraph","content":[{"kind":"text","value":"Arteries and veins are plumbing. Nothing enters and nothing leaves them: their walls are too thick."}]},{"id":"b10","type":"paragraph","content":[{"kind":"text","value":"Everything happens in the "},{"kind":"strong","value":"capillaries"},{"kind":"text","value":" — vessels so fine that red cells sometimes pass in single file, with a wall reduced to a single layer of cells. It is there, and only there, that oxygen leaves the blood, that carbon dioxide enters it, that nutrients cross into the tissues."}]},{"id":"b11","type":"definition","term":"Capillary","definition":"The finest of vessels, set between an artery and a vein. Its thin wall is the only place in the body where exchange between blood and tissue can happen.","alsoKnownAs":["exchange vessel"]},{"id":"b12","type":"paragraph","content":[{"kind":"text","value":"One effect of endurance training is to increase the number of capillaries around muscle fibres. This is not a matter of flow but of surface: more capillaries means more contact between blood and muscle, and a shorter journey for the oxygen."}]},{"id":"b13","type":"heading","level":2,"text":"Blood does not go everywhere at once","anchor":"blood-does-not-go-everywhere-at-once"},{"id":"b14","type":"paragraph","content":[{"kind":"text","value":"At rest, a large share of the flow supplies the internal organs. During effort, the distribution changes."}]},{"id":"b15","type":"evidence","level":"established","statement":"During effort, blood flow is redistributed towards the working muscles, at the expense of territories that can wait.","detail":"The mechanism is established: the vessels of the muscles being used dilate, those of other territories narrow, under the control of the autonomic nervous system and of local signals produced by the muscle itself. The exact proportions depend on intensity, duration and temperature, and no single value holds for every situation.","referenceIds":["hall2020","bassett2000"]},{"id":"b16","type":"paragraph","content":[{"kind":"text","value":"This is the same autonomic command the "},{"kind":"link","value":"nervous system","href":"/en/body/systems/the-nervous-system","external":false},{"kind":"text","value":" page describes: nobody decides this redistribution, and it begins before the effort is genuinely under way."}]},{"id":"b17","type":"heading","level":2,"text":"The return, a problem the outward journey does not have","anchor":"the-return-a-problem-the-outward-journey-does-not-have"},{"id":"b18","type":"paragraph","content":[{"kind":"text","value":"The heart pushes blood down to the lower body without difficulty: pressure and gravity work the same way. The return has to climb, with a pressure that has almost entirely dropped away in the capillaries."}]},{"id":"b19","type":"paragraph","content":[{"kind":"text","value":"Two devices provide for it. The veins of the limbs contain "},{"kind":"strong","value":"valves"},{"kind":"text","value":" that stop blood from falling back. And the muscles surrounding them, as they contract, squeeze them and push their contents upwards."}]},{"id":"b20","type":"callout","tone":"insight","title":"Why standing still is more tiring than walking","content":[[{"kind":"text","value":"Walking works the "},{"kind":"link","value":"calves","href":"/en/body/muscles/calves","external":false},{"kind":"text","value":", which squeeze the veins at every step and restart the return. Standing without moving, that mechanism stops: blood pools in the legs and the volume coming back to the heart falls."}],[{"kind":"text","value":"This is what explains heavy legs after standing for a long time, and the vivid but inexact habit of calling the calf a \"second heart\". It pumps nothing itself: it squeezes a tube fitted with valves."}]]},{"id":"b21","type":"heading","level":2,"text":"Volume, and what makes it vary","anchor":"volume-and-what-makes-it-vary"},{"id":"b22","type":"paragraph","content":[{"kind":"text","value":"Plasma is overwhelmingly water, and its volume is not fixed. The "},{"kind":"link","value":"hydration","href":"/en/recovery/hydration-and-performance","external":false},{"kind":"text","value":" page describes a water loss that affects performance; here is where it acts."}]},{"id":"b23","type":"paragraph","content":[{"kind":"text","value":"A reduced plasma volume means a reduced blood volume, hence less blood to eject with each beat. To maintain the same output, the heart has to beat faster. That is what is observed: at constant effort, heart rate rises as dehydration sets in."}]},{"id":"b24","type":"heading","level":2,"text":"Why no scene accompanies this page","anchor":"why-no-scene-accompanies-this-page"},{"id":"b25","type":"callout","tone":"limit","title":"A network absent from the models","content":[[{"kind":"text","value":"The anatomical sources Body Lab uses supply the skeleton, the muscles, the joints and the regions of the body. No vessel appears there — checking their inventory confirms it."}],[{"kind":"text","value":"Capillaries would in any case pose a second problem: their diameter sits far below what an anatomical mesh represents."}]]},{"id":"b26","type":"heading","level":2,"text":"What this page does not do","anchor":"what-this-page-does-not-do"},{"id":"b27","type":"callout","tone":"caution","title":"No test interpretation, no diagnosis","content":[[{"kind":"text","value":"This page describes a general physiology. It addresses no abnormality of the blood or the circulation and allows no test result to be interpreted."}],[{"kind":"text","value":"A leg that swells and becomes painful, sudden breathlessness, chest pain or unusual bleeding calls for immediate care."}]]},{"id":"b28","type":"heading","level":2,"text":"Sources","anchor":"sources"},{"id":"b29","type":"sourceList","title":"Main sources","referenceIds":["hall2020","standring2020","bassett2000"]},{"id":"b30","type":"shapierAction","actionId":"hydration-blood-volume","label":"Drinking to preserve blood volume","description":"Organising day-to-day hydration happens in Shapier; Body Lab only explains why plasma volume matters.","webTarget":"https://shapier.app/en/conseils/hydratation-performance","appTarget":"shapier://conseils/hydratation-performance"},{"id":"b31","type":"quiz","title":"Check my understanding","questions":[{"id":"q1","prompt":"Why is standing still more tiring than walking?","choices":[{"id":"q1c1","label":"Because the calves no longer squeeze the veins: blood pools in the legs and the volume returning to the heart falls.","correct":true,"explanation":"This is the mechanism the page describes: walking restarts venous return at every step, standing still stops it."},{"id":"q1c2","label":"Because the heart has to pump harder when you do not move.","correct":false,"explanation":"Tempting, since the fatigue feels like effort, but the page locates the problem in venous return, not in the work of the heart."},{"id":"q1c3","label":"Because the valves in the veins end up blocking.","correct":false,"explanation":"The valves stop blood from falling back; the page describes no blockage, only the muscle pump switching off."}]},{"id":"q2","prompt":"Where does oxygen actually leave the blood?","choices":[{"id":"q2c1","label":"In the capillaries, the only vessels whose wall is thin enough for exchange.","correct":true,"explanation":"The page is categorical: that is where, and only where, exchange between blood and tissue happens."},{"id":"q2c2","label":"In the arteries, since they carry the oxygenated blood.","correct":false,"explanation":"Arteries do carry oxygen, but their walls are too thick: nothing enters and nothing leaves them."},{"id":"q2c3","label":"In the veins, on the way back to the heart.","correct":false,"explanation":"The return journey only carries blood back; veins, like arteries, are plumbing with no exchange."}]},{"id":"q3","prompt":"Who decides to redirect blood towards the working muscles during effort?","choices":[{"id":"q3c1","label":"Nobody: the redistribution belongs to the autonomic nervous system and local signals, and it begins before the effort itself.","correct":true,"explanation":"Exactly what the page stresses: no decision, and a command that precedes the actual effort."},{"id":"q3c2","label":"The heart, by speeding up to push more blood to the muscles.","correct":false,"explanation":"Plausible, since the heart does speed up during effort, but the page describes vessels dilating and narrowing, not a decision by the heart."},{"id":"q3c3","label":"The muscle, by directly drawing what it needs.","correct":false,"explanation":"The muscle does produce local signals, but it draws nothing itself: it is the vessels that change calibre."}]}]},{"id":"b32","type":"relatedContent","title":"Read next","targetIds":["en-mechanism-lungs","en-mechanism-heart","en-mechanism-hydration"]}],"relations":[{"type":"prerequisite","targetId":"en-mechanism-heart"},{"type":"explains","targetId":"en-mechanism-hydration"},{"type":"explains","targetId":"en-mechanism-aerobic-system"},{"type":"next-step","targetId":"en-mechanism-lungs"}],"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":"bassett2000","authors":"Bassett DR, Howley ET","year":2000,"title":"Limiting factors for maximum oxygen uptake and determinants of endurance performance","source":"Medicine and Science in Sports and Exercise","kind":"review","doi":"10.1097/00005768-200001000-00012"}],"review":{"publishedAt":"2026-08-02","reviewedAt":"2026-08-02","reviewDueAt":"2027-08-02","evidenceLevel":"established"},"limitations":["No scene accompanies this page: the circulatory system appears in none of the anatomical sources Body Lab uses.","The proportions of blood flow redistribution during effort depend on intensity, duration, temperature and hydration status: no single value holds everywhere.","This page addresses no abnormality of the blood or the circulation, and allows no test result to be interpreted.","A leg that swells and becomes painful, sudden breathlessness or chest pain calls for immediate care."],"seo":{"title":"Blood and circulation — what it carries and where it goes","description":"Haemoglobin, capillaries, redistribution during effort and venous return: what blood actually does, and why standing still is harder than walking.","canonicalPath":"/en/body/systems/blood-and-circulation","image":"/og/en/mechanism-blood-circulation.png"},"app":{"offlineEligible":true},"version":1}