Density Altitude verses Hypoxia

Anne

Anne

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A.Helliwell
I was wondering why a naturally aspirated engine is affected by density altitude but hypoxia is only affected by pressure altitude. Why would lungs not be affected by density?

Thank you
 
They are. Have you never been at sea level somewhere and then another place that’s higher altitude? The air is less dense at higher altitudes so it takes longer to catch your breath, thus making it harder for your body to get adequate oxygen to pump through your veins.

Basically the same principle for a normally aspirated engine.
 
Pilawt said:
Density altitude is a function of temperature as well as pressure. Temperature in your lungs is always right around 98.6F.
Click to expand...
So you are saying that our bodies do a better job than the engine at maintaining a constant temperature. That being said density (pressure + temp) is not a factor and hypoxia is only affected by pressure altitude. Our engines need a hypothalamus?
 
I thought in our lungs the air quickly normalized to body temp.
While in an engine air does not have time to normalize before the compression stroke starts and the valve closes.

Tim

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Pilawt said:
Density altitude is a function of temperature as well as pressure. Temperature in your lungs is always right around 98.6F.
Click to expand...

Doesn’t heating air that is already warmer than standard make it less dense?
 
Anne said:
So you are saying that our bodies do a better job than the engine at maintaining a constant temperature. That being said density (pressure + temp) is not a factor and hypoxia is only affected by pressure altitude.
Click to expand...
Density certainly is a factor in providing sufficient oxygen. But as internal body temperature and humidity are near constants, pressure altitude is the only variable in determining the density.

Engines normalize intake air temperatures only to some incidental degree, and wings don't at all. So ambient temperature and humidity have to be factored into the density altitude computation to determine their performance.

So while your wings and normally-aspirated engine are struggling to haul you up to 12,500 feet, the air in your lungs already has the density of ambient air of about 18,000' ISA.
 
So if the air has a high density, less molecules of oxygen available because of increase in temperature, there would be fewer available to be heated or cooled to normal body temperature, and not as many for the transfer to the blood. So then is density not a factor with hypoxia?
 
I believe this has to do with diffusion of oxygen molecules across the membranes in the lung. The temperature across both sides of the membrane are the same (body temperature), so only partial pressure of oxygen matters.
 
It's because pressure is what matters on the perfusion across the membrane. Even if the temperature were different, it wouldn't matter. Density matters when you are computing the amount of gas available for combustion. Breathing is all a matter of partial pressure.
 
I think the concept of volume is involved in this somehow.
 
This came up maybe 12 years ago on COPA, and an anesthesiologist member explained how only pressure altitude mattered when it comes to hypoxia.

I can’t recall the details, but the argument was persuasive at the time.
 
Anne said:
So if the air has a high density, less molecules of oxygen available because of increase in temperature, there would be fewer available to be heated or cooled to normal body temperature, and not as many for the transfer to the blood. So then is density not a factor with hypoxia?
Click to expand...

No matter what the OAT is, it's 98.6-ish in your body, and by the time the air gets to your lungs it has been heated/cooled to that temperature.

If you have a higher density *altitude* (which is a lower density of air) outside your body, it doesn't matter - Once it's into your lungs, it's going to be at the same DA no matter what.

In most circumstances (ie, OAT < body temp), the "density altitude" in your lungs is higher than it is outside. If it's really hot out, the DA in your lungs is lower than outside.
 
flyingcheesehead said:
No matter what the OAT is, it's 98.6-ish in your body, and by the time the air gets to your lungs it has been heated/cooled to that temperature.

If you have a higher density *altitude* (which is a lower density of air) outside your body, it doesn't matter - Once it's into your lungs, it's going to be at the same DA no matter what.

In most circumstances (ie, OAT < body temp), the "density altitude" in your lungs is higher than it is outside. If it's really hot out, the DA in your lungs is lower than outside.
Click to expand...
it’s always 100% relative humidity and 98.6 degrees in the alveolar sac of the lung. So the only remaining variable is total pressure.
 
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