Mechanism
Beginner
9 min read
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.
A word used everywhere, never defined
Thirty-two pages of Body Lab mention blood. Glycogen arrives through it, amino acids cross it, the aerobic pathway depends on it, hydration changes its volume. None said what it is.
Two components, two roles
Blood is not a uniform liquid. It is a suspension: a great many cells, carried by a fluid.
What blood is made of
Component
Nature
Main role
Plasma
Liquid, overwhelmingly water
Carries everything dissolved: sugars, amino acids, hormones, heat
Red cells
Cells without a nucleus, filled with haemoglobin
Carry oxygen and part of the carbon dioxide
White cells
Defence cells
Act against threats
Platelets
Cell fragments
Take part in stopping bleeding
Haemoglobin
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.
oxygen carrier
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.
Three kinds of vessel, one place of exchange
Arteries and veins are plumbing. Nothing enters and nothing leaves them: their walls are too thick.
Everything happens in the capillaries — 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.
Capillary
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.
exchange vessel
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.
Blood does not go everywhere at once
At rest, a large share of the flow supplies the internal organs. During effort, the distribution changes.
Certainty level · Established
During effort, blood flow is redistributed towards the working muscles, at the expense of territories that can wait.
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.
Hall JE, Hall ME (2020) · Bassett DR, Howley ET (2000)
This is the same autonomic command the nervous system page describes: nobody decides this redistribution, and it begins before the effort is genuinely under way.
The return, a problem the outward journey does not have
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.
Two devices provide for it. The veins of the limbs contain valves that stop blood from falling back. And the muscles surrounding them, as they contract, squeeze them and push their contents upwards.
Key point
Why standing still is more tiring than walking
Walking works the calves, 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.
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.
Volume, and what makes it vary
Plasma is overwhelmingly water, and its volume is not fixed. The hydration page describes a water loss that affects performance; here is where it acts.
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.
Why no scene accompanies this page
Limit
A network absent from the models
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.
Capillaries would in any case pose a second problem: their diameter sits far below what an anatomical mesh represents.
What this page does not do
Caution
No test interpretation, no diagnosis
This page describes a general physiology. It 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, chest pain or unusual bleeding calls for immediate care.
Sources
Main sources
- Hall JE, Hall ME (2020). Guyton and Hall Textbook of Medical Physiology, 14th edition. Elsevier.
- Standring S (2020). Gray's Anatomy: The Anatomical Basis of Clinical Practice, 42nd edition. Elsevier.
- 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
Drinking to preserve blood volume
Organising day-to-day hydration happens in Shapier; Body Lab only explains why plasma volume matters.
Drinking to preserve blood volumeBody Lab explains; Shapier lets you act and track.
Check my understanding
Why is standing still more tiring than walking?
Because the calves no longer squeeze the veins: blood pools in the legs and the volume returning to the heart falls.
Because the heart has to pump harder when you do not move.
Because the valves in the veins end up blocking.
Where does oxygen actually leave the blood?
In the capillaries, the only vessels whose wall is thin enough for exchange.
In the arteries, since they carry the oxygenated blood.
In the veins, on the way back to the heart.
Who decides to redirect blood towards the working muscles during effort?
Nobody: the redistribution belongs to the autonomic nervous system and local signals, and it begins before the effort itself.
The heart, by speeding up to push more blood to the muscles.
The muscle, by directly drawing what it needs.
Choose an answer
Read next
- The lungs and gas exchangeThe 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 heartA 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.Described without a scene
- Hydration and performanceWater is not a side note to effort. It carries the blood, carries heat away and accompanies glycogen storage: three roles that explain what losing water changes.With a 3D scene
Available offline
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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 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.
Sources
- Hall JE, Hall ME (2020). Guyton and Hall Textbook of Medical Physiology, 14th edition. Elsevier.
- Standring S (2020). Gray's Anatomy: The Anatomical Basis of Clinical Practice, 42nd edition. Elsevier.
- 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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