Partial Pressures in Anatomical Dead space

[MDwannabe7]

In a practice test question on the anatomical dead space of the lungs (which do not participate in gas exchange), we were asked to compare the air in the alveoli with the air in the anatomical dead space. Logic reveals that this air should have the same partial pressure as the air in the atmosphere, which would therefore be higher for P CO2 and lower for P O2 than in the alveoli, correct? Just checking, b/c I have a couple of marks by the answers and I'm not sure which one is correct, but I'm pretty sure it's that the air in the dead space would have a higher P CO2 and lower P O2 than the air in the alveoli.

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[HawkeyePostOp]

Interesting. It sounds about right. If you find the answer/explanation, post it.

 

[rocuronium]

... but I'm pretty sure it's that the air in the dead space would have a higher P CO2 and lower P O2 than the air in the alveoli.

This is true at end expiration. The concentrations of gases in the anatomic dead space at this point are essentially identical to alveolar concentrations. This makes sense, as the air in the anatomical deadspace comes from the alveolar air that was not able to be expelled from the lungs. This air acts as a reservoir, being the first air inhaled on the next breath.

At end inspiration, however, the partial pressures in the anatomic deadspace would be nearly identical to "fresh" gas. This would mean a higher pO2 than alveolar air, and a lower pCO2 than alveolar air.

Something else to consider is that the partial pressures of the gases in air change as water is added by the body to the inspired air. I've never seen an MCAT question on it, but it is something to think about.

 

[Vihsadas]

Yeah, it's really important to think about "when" during the breathing cycle you are looking at the dead space. The answer changes dependent on that.

 

[MDwannabe7]

This makes sense, as the air in the anatomical deadspace comes from the alveolar air that was not able to be expelled from the lungs. This air acts as a reservoir, being the first air inhaled on the next breath.

Are we sure we're referring to the same 'dead space'? The question said that anatomical dead space does not participate in gas exchange. Is that the same thing as the air that does not get expelled during respiration? I was thinking of the dead space referred to in the question as parts of the respiratory tract, such as the trachea, bronchi, etcetera that do not interface with the capillaries and participate in gas exchange. Therefore, the partial pressures of this air is identical to the atmospheric partial pressures, which is higher in CO2 than O2 than the alveolar air, right?

 

[rocuronium]

Are we sure we're referring to the same 'dead space'? The question said that anatomical dead space does not participate in gas exchange. This is true. Deadspace represents areas which are ventilated but not perfused.

Is that the same thing as the air that does not get expelled during respiration? There is always gas in the deadspace regardless of the point of ventilation. This has to be true, or else the respiratory tract would collapse after every breath. The compositions of the air change, however. When you breathe in, the deadspace is left filled with atmospheric air that did not make it into the alveoli. This air would have atmospheric composition. When you breathe out, however, the air which fills the deadspace at end expiration is the air which came from the alveoli. Thus it has alveolar gas concentrations. These would be lower pO2 and higher pCO2.

I was thinking of the dead space referred to in the question as parts of the respiratory tract, such as the trachea, bronchi, etcetera that do not interface with the capillaries and participate in gas exchange. This is correct.

Therefore, the partial pressures of this air is identical to the atmospheric partial pressures, which is higher in CO2 than O2 than the alveolar air, right? Atmospheric air has a lower pCO2 than alveolar air. If this weren't true, CO2 would go into the blood instead of coming out. Alveolar air is lower in O2 than atmospheric air because some of it has diffused into the blood

I hope that helps. Let me know if you have any other questions.