John Reinertson
Guest
Scubadoc and Dr Stein have it right. At 0.2 ATA partial pressure of O2 (air at sea level) the hemoglobin in the blood is virtually
100 % saturated. Higher levels of oxygen do force more oxygen to be dissolved in the blood, but this adds only a small fraction of total oxygen capacity per liter of blood.
Simply put, blood at 1.6 ata partial pressure of oxygen holds somewhere between 5 and 10 % more total oxygen than blood at 0.2 ATA, even though the partial pressure is eight times higher.
This is not a contradiction in gas physics, it has to do with oxygen being bound to a biological carrier (hemoglobin) that saturates at a relatively low partial pressure of oxygen.
The liquid portion of blood follows gas physics laws well, but carries far less oxygen than the hemoglobin.
Now, when we are breathing normally, the drive that tells us to take a breath is primarily due to the body needing to get rid of CO2.
That's the reason, in Apollo 13, (the movie, and real life) why the astronauts were breathing harder and harder as the CO2 levels rose in the capsule. They also got whopping headaches because higher CO2 levels increase brain blood flow.
Breath-hold divers learn to counteract that CO2 drive to an extent and can hold their breath longer.
If they learn it too well, the Oxygen can indeed be consumed to a point where there isn't enough left in the blood to reach the surface before the brain craps out.
When they reach the surface, they spend some time with much deeper respirations flushing out the CO2.
The need to flush the CO2 within a matter of a few minutes is the reason SCUBA divers don't benefit much from breath holding.. they consume more gas flushing it out a minute or two later, and end up using as much air as if they had safely breathed steadily and remained relaxed to start with.
It's not a simple answer, but humans aren't a simple system.
Your question regarding expelled oxygen coming from lungs is correct when breathing normally. Oxygen content of venous blood returning to the heart can be very low. The primary drive for healthy people to breathe is Carbon dioxide elimination, not low oxygen levels. There is a low-oxygen drive, which kicks in at high altitudes or other low-oxygen conditions, but at sea level and undersea, the CO2 drive is the primary one.
Keep on breathin'
John
100 % saturated. Higher levels of oxygen do force more oxygen to be dissolved in the blood, but this adds only a small fraction of total oxygen capacity per liter of blood.
Simply put, blood at 1.6 ata partial pressure of oxygen holds somewhere between 5 and 10 % more total oxygen than blood at 0.2 ATA, even though the partial pressure is eight times higher.
This is not a contradiction in gas physics, it has to do with oxygen being bound to a biological carrier (hemoglobin) that saturates at a relatively low partial pressure of oxygen.
The liquid portion of blood follows gas physics laws well, but carries far less oxygen than the hemoglobin.
Now, when we are breathing normally, the drive that tells us to take a breath is primarily due to the body needing to get rid of CO2.
That's the reason, in Apollo 13, (the movie, and real life) why the astronauts were breathing harder and harder as the CO2 levels rose in the capsule. They also got whopping headaches because higher CO2 levels increase brain blood flow.
Breath-hold divers learn to counteract that CO2 drive to an extent and can hold their breath longer.
If they learn it too well, the Oxygen can indeed be consumed to a point where there isn't enough left in the blood to reach the surface before the brain craps out.
When they reach the surface, they spend some time with much deeper respirations flushing out the CO2.
The need to flush the CO2 within a matter of a few minutes is the reason SCUBA divers don't benefit much from breath holding.. they consume more gas flushing it out a minute or two later, and end up using as much air as if they had safely breathed steadily and remained relaxed to start with.
It's not a simple answer, but humans aren't a simple system.
Your question regarding expelled oxygen coming from lungs is correct when breathing normally. Oxygen content of venous blood returning to the heart can be very low. The primary drive for healthy people to breathe is Carbon dioxide elimination, not low oxygen levels. There is a low-oxygen drive, which kicks in at high altitudes or other low-oxygen conditions, but at sea level and undersea, the CO2 drive is the primary one.
Keep on breathin'
John
