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Almost all of the oxygen in the blood is carried by a protein in red blood cells called hemoglobin.


The 'trains' are red blood cells, made up of carriages of hemoglobin

  • Each train carriage (hemoglobin molecule) has four iron seats

  • Each oxygen molecule must have a seat, so each carriage can transport a maximum of 4 oxygen molecules.

  • The trains and carriages are made in the bones (bone marrow).

  • If hemoglobin cannot be made properly, for example if we are short of iron and become anemic, there are fewer carriages in the train.

The trains run on a train track from the lungs to the body tissues, and back.

  • The trains are powered round by the heart, which is a two-sided pump.

  • The left heart pumps the trains to the tissues (where oxygen is released) and back to the right heart.

  • The right heart pumps the trains to the lungs (where oxygen is picked up) and back to the left heart. 


You may not have realised that effectively we have two hearts. These are side by side, but pump separately. 

Step 1

Step 2

Step 3

The body tissues work as a factory that operates in two modes:

  • Green (aerobic) needs more oxygen, but produces very little smoke (CO2/acid waste).

  • Polluted (anerobic) needs less oxygen, but produces more smoke (CO2/acid waste).

  • Smoke is carried back to the lungs in the trains.

Step 4

We breathe in air, containing oxygen (O2)


But it is the amount of smoke (CO2) in the blood that drives how much we breathe:

  • There is a platform in the lungs.

  • The escalator that takes smoke away from the platform runs faster if there is more smoke.

  • The same escalator delivers fresh air to the train platform.

  • The trains are loaded with oxygen at the platform in the lungs where oxygen levels are high.

  • The trains are unloaded at a platform in the tissues where the oxygen levels are low. 

If you would like to see the scientific terms for some of these concepts, these are shown on the Scientific/Medical page.

In reality:

  • There are millions of escalators and platforms in the lungs and tissues

  • On each circuit (heart beat), a train will only go to one platform in the lungs , and usually, to only one platform in the tissues.

  • Every circuit is different - a train will probably go to different platforms every heart beat.

Step 5

What happens if we breathe extra oxygen using a face mask?

  • This allows extra oxygen to dissolve in the blood.




What happens if we breathe too little oxygen, or if our lungs don't work so less oxygen gets into the blood?

  • The body makes extra train carriages, so the hemoglobin is higher.

Step 6

Step 7

Step by Step Guide


For patients and

the general public


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