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Mechanism
Beginner
9 min read

The heart

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.

A muscle apart

The heart is a muscle, but no other one resembles it. It is striated like the skeletal muscles, which sets it apart from the walls of the internal organs. And yet it obeys no voluntary command.
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.
The autonomic nervous system therefore does not create the rhythm: it modulates 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.

Two circuits, in series

The common image is a single loop. There are two, mounted one behind the other, and the heart is a double pump feeding both.

The two circuits

Circuit
Route
What it does
Pulmonary
Right heart → lungs → left heart
Blood releases carbon dioxide and takes on oxygen
Systemic
Left heart → the whole body → right heart
Blood delivers oxygen and collects carbon dioxide
The consequence is simple: all 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.

The equation the Energy pillar was missing

The aerobic system 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.
Cardiac output
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.
volume ejected per minute
Two levers, then, and only one really responds to training.
Certainty level · Established
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.
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.
Hall JE, Hall ME (2020) · Bassett DR, Howley ET (2000)
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.

"220 minus your age"

The formula is everywhere: on gym machines, in watches, in programmes. It has been examined seriously.
Certainty level · Established
The formula "220 minus your age" estimates maximum heart rate poorly, particularly in older adults, where it underestimates it.
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.
Tanaka H, Monahan KD, Seals DR (2001)
Caution
Why this page does not give the corrected formula
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.
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.
Maximum heart rate also falls with age in everyone, and training barely changes it. It is not an indicator of fitness.

Why no scene accompanies this page

Limit
An organ absent from the models
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.
The ribcage 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.

What this page does not do

Caution
Reasons to seek care immediately
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.
This page describes a general physiology. It assesses no individual situation, proposes no protocol and replaces no examination.

Sources

Main sources

  • Hall JE, Hall ME (2020). Guyton and Hall Textbook of Medical Physiology, 14th edition. Elsevier.
  • Tanaka H, Monahan KD, Seals DR (2001). Age-predicted maximal heart rate revisited. Journal of the American College of Cardiology.
  • Bassett DR, Howley ET (2000). Limiting factors for maximum oxygen uptake and determinants of endurance performance. Medicine and Science in Sports and Exercise.
Put it into practice in Shapier

Tracking nervous-system recovery

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.
Tracking nervous-system recovery
Body Lab explains; Shapier lets you act and track.

Check my understanding

Where does the underlying heart rhythm come from?
What does endurance training mainly improve?
Why does this page not give a corrected maximum heart rate formula?
Choose an answer

Read next

  • Blood and circulation
    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.
    Described without a scene
  • The lungs and gas exchange
    The exchange itself takes no energy: the gases cross on their own. And in a healthy person, what limits an endurance effort is not the lung — which contradicts the sensation.
    Described without a scene
  • The aerobic system
    The pathway that supplies most of the energy as soon as an effort lasts. It burns carbohydrate and fat with oxygen, produces a great deal but slowly, and never switches off.
    With a 3D scene

Available offline

Offline download is available in the mobile app.

Trust and method

Author
equipe-editoriale-shapier
Editorial review
Thanh Chau
Scientific review
Pending
Published on
August 2, 2026
Reviewed on August 2, 2026
Next review due August 2, 2027
Limits of this page
  • 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.
Sources
  • Hall JE, Hall ME (2020). Guyton and Hall Textbook of Medical Physiology, 14th edition. Elsevier.
  • Tanaka H, Monahan KD, Seals DR (2001). Age-predicted maximal heart rate revisited. Journal of the American College of Cardiology.
  • Bassett DR, Howley ET (2000). Limiting factors for maximum oxygen uptake and determinants of endurance performance. Medicine and Science in Sports and Exercise.
Educational content. Body Lab does not diagnose and does not replace professional advice.
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