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Scientific terms



  • One side of the platform is on the airway side (an alveolus), the other side is in the blood. 

  • The pressure of oxygen gas in the Alveolus (PAO2) determines the pressure of oxygen gas in the blood that returns to the heart and is sent out in arteries to the tissues (PaO2).



  • Hemoglobin is carried by red blood cells (erythrocytes).

  • The proportion of the 4 carriage seats that are occupied is termed % saturation (SaO2).

  • SaO2 is determined by the surrounding oxygen gas pressure (and other factors that help load oxygen in the lung, and unload in the tissues where it is needed).



  • Low oxygen in the tissues (this term is often used  to refer to any low oxygen level).




  • Low oxygen in the blood (low PaO2 or SaO2)




  • This is the amount of oxygen in a unit of blood, shown as a yellow box. It helps explains why low blood oxygen levels (hypoxemia) do not need to cause tissue hypoxia:

  • If PaO2/SaO2 fall, secondary erythrocytosis results in higher hemoglobin (polycythemia, more train carriages).

  • In other words, the total number of oxygen molecules in a volume of blood (yellow boxes) can be maintained even if PaO2 and SaO2 are low.

Medical conditions



  • The hemoglobin is lower, fewer carriages are made per train, and CaO2 cannot be maintained.

  • If CaO2 falls, to maintain tissue oxygen delivery, the available trains have to go round more often (increased cardiac output). This can be achieved by faster speeds (heart rate), and/or wider tracks (increased stroke volume). 

  • Both require greater cardiac effort.



  • If the 'factory' demands more oxygen, the available trains have to go round more often, and there is a limit to what the system can achieve at maximal effort (VO2 max). 

  • If higher demands are a regular occurrence, more trains are made, and both factory and rail network/heart reorganize to use and deliver oxygen more efficiently, thus increasing VO2 max.



  • Heart diseases prevent the pump from working properly

  • High blood pressure effectively put the tissues “at the top of a hill” so the heart has to work harder.

  • Abnormally dilated blood vessels, for example in sepsis, effectively put the tissues “in a valley” so it is harder for the blood to return to the heart.



  • There are rare conditions that “shunt” blood through the lungs, bypassing the train platforms. These include pulmonary arteriovenous malformations (PAVMs), and result in low blood oxygen levels (low PaO2/SaO2.)  

  • As long as more train carriages can be made (higher hemoglobin; polycythemia), arterial oxygen content (CaO2) can remain normal.

  • As long as the heart can pump more, the available trains can go round more often, and maintain oxygen delivery to the tissues.



  • Airway or lung diseases prevent the escalator from working properly.

  • Low inspired oxygen concentrations and other causes of alveolar hypoxia also deliver less oxygen to the train platform.

  • Compensation seems less successful than for pure hypoxemia:


Hypoxic pulmonary vasoconstriction due to low PAO2 provides a rationale: This diverts trains to better ventilated platforms, which improves ventilation perfusion matching, but at the expense of increasing the gradient attributable to pulmonary vascular resistance.  In the model, there are millions of escalators and platforms in the lungs. If less oxygen is delivered to the bottom of an escalator, the platform is  put at the "top of a hill," so trains are diverted to platforms receiving more oxygen.  





This figure shows ventilation (an escalator) taking oxygen from the air to an alveolus (train platform) in the lungs.  The same "escalator" takes CO2 (smoke) away from the alveolus (platform), and runs faster if there is more CO2 (smoke) in the blood.  In reality there are millions of alveoli (escalators/platforms) in the lung, important for efficiency and ventilation /perfusion matching.

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