{"schemaVersion":1,"id":"en-mechanism-heart","type":"mechanism","pillar":"body","locale":"en","translationGroupId":"mechanism-heart","slugPath":"body/systems/the-heart","title":"The heart","shortTitle":"The heart","summary":"A muscle that obeys nobody and never stops. What training improves is not its top speed — that barely moves — but how much it shifts with every beat.","level":"beginner","estimatedMinutes":9,"synonyms":["heart","cardiac muscle","cardiac output","heart rate","stroke volume","maximum heart rate"],"blocks":[{"id":"b1","type":"heading","level":2,"text":"A muscle apart","anchor":"a-muscle-apart"},{"id":"b2","type":"paragraph","content":[{"kind":"text","value":"The heart is a muscle, but no other one resembles it. It is striated like the "},{"kind":"link","value":"skeletal muscles","href":"/en/body/muscle-fibres","external":false},{"kind":"text","value":", which sets it apart from the walls of the internal organs. And yet it obeys no voluntary command."}]},{"id":"b3","type":"paragraph","content":[{"kind":"text","value":"Its peculiarity is its autonomy. It has its own trigger: a group of cells that spontaneously produce a rhythm without any nerve needing to ask them to. A heart isolated from every nerve keeps beating."}]},{"id":"b4","type":"paragraph","content":[{"kind":"text","value":"The "},{"kind":"link","value":"autonomic nervous system","href":"/en/body/systems/the-nervous-system","external":false},{"kind":"text","value":" therefore does not create the rhythm: it "},{"kind":"strong","value":"modulates"},{"kind":"text","value":" it. It speeds things up at the start of an effort and slows them at rest, but the underlying beat does not come from it."}]},{"id":"b5","type":"heading","level":2,"text":"Two circuits, in series","anchor":"two-circuits-in-series"},{"id":"b6","type":"paragraph","content":[{"kind":"text","value":"The common image is a single loop. There are two, mounted one behind the other, and the heart is a double pump feeding both."}]},{"id":"b7","type":"comparison","title":"The two circuits","columns":["Circuit","Route","What it does"],"rows":[{"label":"Pulmonary","cells":["Right heart → lungs → left heart","Blood releases carbon dioxide and takes on oxygen"]},{"label":"Systemic","cells":["Left heart → the whole body → right heart","Blood delivers oxygen and collects carbon dioxide"]}]},{"id":"b8","type":"paragraph","content":[{"kind":"text","value":"The consequence is simple: "},{"kind":"strong","value":"all"},{"kind":"text","value":" the blood passes through the lungs on every circuit. There is no shortcut allowing part of the blood to go straight from body to body."}]},{"id":"b9","type":"heading","level":2,"text":"The equation the Energy pillar was missing","anchor":"the-equation-the-energy-pillar-was-missing"},{"id":"b10","type":"paragraph","content":[{"kind":"text","value":"The "},{"kind":"link","value":"aerobic system","href":"/en/energy/the-aerobic-system","external":false},{"kind":"text","value":" page describes a pathway consuming oxygen, and observes that cardiac output rises at the start of an effort. Here is what that output is made of."}]},{"id":"b11","type":"definition","term":"Cardiac output","definition":"The volume of blood the heart sends into the circulation in one minute. It is the product of two quantities: how often it beats, and how much it ejects with each beat.","alsoKnownAs":["volume ejected per minute"]},{"id":"b12","type":"paragraph","content":[{"kind":"text","value":"Two levers, then, and only one really responds to training."}]},{"id":"b13","type":"evidence","level":"established","statement":"The increase in maximal oxygen uptake obtained through endurance training comes mainly from an increase in maximal cardiac output, itself due to the volume ejected with each beat.","detail":"The review gathers three converging lines of evidence: manipulating oxygen supply changes maximal uptake, training gains follow cardiac output rather than muscle extraction, and a small muscle mass that is over-perfused consumes an enormous amount. These conclusions concern healthy adults at sea level.","referenceIds":["bassett2000","hall2020"]},{"id":"b14","type":"paragraph","content":[{"kind":"text","value":"In other words, a trained heart is not a faster heart: it is a heart that shifts more with every beat. That is also why resting rate falls — for the same need, fewer beats suffice."}]},{"id":"b15","type":"heading","level":2,"text":"\"220 minus your age\"","anchor":"220-minus-your-age"},{"id":"b16","type":"paragraph","content":[{"kind":"text","value":"The formula is everywhere: on gym machines, in watches, in programmes. It has been examined seriously."}]},{"id":"b17","type":"evidence","level":"established","statement":"The formula \"220 minus your age\" estimates maximum heart rate poorly, particularly in older adults, where it underestimates it.","detail":"The meta-analysis gathers 351 studies, 492 groups and more than eighteen thousand subjects. It proposes a different relationship, but above all it measures the spread: the difference between individuals of the same age is such that no age-based formula correctly predicts a given person's value.","referenceIds":["tanaka2001"]},{"id":"b18","type":"callout","tone":"caution","title":"Why this page does not give the corrected formula","content":[[{"kind":"text","value":"Replacing a bad formula with a better one would not fix the problem. The result of the meta-analysis is not that another equation works: it is that individual spread is too large for an equation to serve as an instruction."}],[{"kind":"text","value":"Body Lab therefore proposes no target rate, no training zone and no calculation: that would be personalised advice built on a prediction the cited source invalidates."}],[{"kind":"text","value":"Maximum heart rate also falls with age in everyone, and training barely changes it. It is not an indicator of fitness."}]]},{"id":"b19","type":"heading","level":2,"text":"Why no scene accompanies this page","anchor":"why-no-scene-accompanies-this-page"},{"id":"b20","type":"callout","tone":"limit","title":"An organ absent from the models","content":[[{"kind":"text","value":"Body Lab's scenes derive from four sources: the skeleton, the muscles, the joints and the regions of the body. The cardiovascular system is not among them, and the visceral system of the atlas used cannot be reused for licence reasons."}],[{"kind":"text","value":"The "},{"kind":"link","value":"ribcage","href":"/en/body/muscles/diaphragm","external":false},{"kind":"text","value":" exists in the scenes, its contents do not. Showing ribs on a page devoted to the heart would mean showing a container for its contents, which the project forbids itself."}]]},{"id":"b21","type":"heading","level":2,"text":"What this page does not do","anchor":"what-this-page-does-not-do"},{"id":"b22","type":"callout","tone":"caution","title":"Reasons to seek care immediately","content":[[{"kind":"text","value":"Chest pain, pain radiating to the arm or jaw, palpitations, unusual breathlessness, faintness during effort: these are situations where time matters, and the thing to do is call emergency services."}],[{"kind":"text","value":"This page describes a general physiology. It assesses no individual situation, proposes no protocol and replaces no examination."}]]},{"id":"b23","type":"heading","level":2,"text":"Sources","anchor":"sources"},{"id":"b24","type":"sourceList","title":"Main sources","referenceIds":["hall2020","tanaka2001","bassett2000"]},{"id":"b25","type":"shapierAction","actionId":"recovery-nervous-system","label":"Tracking nervous-system recovery","description":"Organising recovery and watching the load on the nervous system happens in Shapier; Body Lab only explains why a trained heart beats more slowly at rest.","webTarget":"https://shapier.app/en/conseils/recuperation-systeme-nerveux","appTarget":"shapier://conseils/recuperation-systeme-nerveux"},{"id":"b26","type":"quiz","title":"Check my understanding","questions":[{"id":"q1","prompt":"Where does the underlying heart rhythm come from?","choices":[{"id":"q1c1","label":"From a group of cells in the heart itself, which spontaneously produce a rhythm","correct":true,"explanation":"The heart has its own trigger: a heart isolated from every nerve keeps beating, and the autonomic nervous system only modulates that rhythm."},{"id":"q1c2","label":"From the autonomic nervous system, which would trigger each beat","correct":false,"explanation":"Tempting, because it speeds the heart up during effort and slows it at rest, but the page states it modulates the rhythm without creating it."},{"id":"q1c3","label":"From a voluntary command coming from the brain","correct":false,"explanation":"The heart is striated like the skeletal muscles, which feeds this idea, but it obeys no voluntary command."}]},{"id":"q2","prompt":"What does endurance training mainly improve?","choices":[{"id":"q2c1","label":"The volume of blood ejected with each beat","correct":true,"explanation":"The increase in maximal oxygen uptake comes mainly from maximal cardiac output, itself due to the volume ejected; that is also why resting rate falls."},{"id":"q2c2","label":"Maximum heart rate","correct":false,"explanation":"A \"trained\" heart seems like it should beat faster, but maximum rate falls with age in everyone and training barely changes it."},{"id":"q2c3","label":"The muscles' extraction of oxygen","correct":false,"explanation":"The other plausible candidate, but the cited review shows that training gains follow cardiac output rather than muscle extraction."}]},{"id":"q3","prompt":"Why does this page not give a corrected maximum heart rate formula?","choices":[{"id":"q3c1","label":"Because the spread between individuals of the same age is too large for any formula to predict a given person's value","correct":true,"explanation":"The cited meta-analysis (351 studies, more than eighteen thousand subjects) concludes that no age-based equation can serve as an individual instruction."},{"id":"q3c2","label":"Because \"220 minus your age\" always overestimates the maximum rate","correct":false,"explanation":"It is the opposite in older adults, where it underestimates; the real problem is individual spread, not the direction of the error."},{"id":"q3c3","label":"Because no corrected formula has been proposed yet","correct":false,"explanation":"The meta-analysis does propose one, but its central result is that the spread is too large for an equation to serve as an instruction."}]}]},{"id":"b27","type":"relatedContent","title":"Read next","targetIds":["en-mechanism-blood-circulation","en-mechanism-lungs","en-mechanism-aerobic-system"]}],"relations":[{"type":"prerequisite","targetId":"en-mechanism-nervous-system"},{"type":"explains","targetId":"en-mechanism-aerobic-system"},{"type":"next-step","targetId":"en-mechanism-blood-circulation"},{"type":"explains","targetId":"en-movement-running"}],"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":"tanaka2001","authors":"Tanaka H, Monahan KD, Seals DR","year":2001,"title":"Age-predicted maximal heart rate revisited","source":"Journal of the American College of Cardiology","kind":"meta-analysis","doi":"10.1016/s0735-1097(00)01054-8"},{"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 cardiovascular system appears in none of the anatomical sources Body Lab uses.","No target rate, no training zone and no formula is proposed: the spread between individuals makes any individual prediction unreliable, and it would be personalised advice.","The adaptations described concern healthy adults following endurance training. They transfer neither to situations of illness nor to treatments that alter heart rate.","Chest pain, palpitations, unusual breathlessness or faintness during effort are reasons to seek care immediately. No page allows them to be interpreted."],"seo":{"title":"The heart — output, maximum rate and what training changes","description":"Two circuits in series, the cardiac output equation, and why maximum heart rate cannot be trained while stroke volume can.","canonicalPath":"/en/body/systems/the-heart","image":"/og/en/mechanism-heart.png"},"app":{"offlineEligible":true},"version":1}