Ppo2

I understand when I'm diving that the pressure of the water forces more molecules of oxygen, and any other gas for that matter, into my blood stream and other tissues. Why then does my consumption of oxygen, say, 0.8 litres per minute, remain independent of depth and pressure? What happens to all those other molecules I must have taken on board but clearly haven't used, and if I don't use them how do they increase my partial pressure with all the difficult consequences that brings?

what activates your breathing trigger. Your body will metabolize as much O2 as it can (sorta like me eating at Thanksgiving dinner). This will obviously produce even more CO2. Also, without turbulence (which comes from breathing) the gas immediately around the alveoli will become CO2 rich. This effectively inhibits your body getting rid of the CO2 and absorbing more O2. This is not effective.

In other words... pseudo gas conservation methods such as skip breathing will result in the same CO2 headache that you would get doing it on air. The best way to "conserve air" is getting your body in tip top shape through swimming and running. Learning how to relax, and to go slow are the other skills that will help this as well.

Oh yeah... welcome to the board! I hope you learn as much and meet as many great friends as I have.

Hi Pete

Thanks for the reply but I'm still not getting it! You say the body will metabolise as much oxygen as it can but I don't see how that's true. Sure, more oxygen may go into the blood stream and tissues because of the higher pressure but the body only 'metabolises,' i.e., converts to energy and produces CO2 as a result of, the same number of oxygen molecules as it would on the surface, irrespective of depth and partial pressure! I appreciate that logically then you should use a smaller percentage of the oxygen available, but that isn't true either!

but remember that diving puts a bigger strain on your body than you may realize. Just in trying to stay warm, you consume a metric but tonne of calories. You need the oxygen to metabolize the fat to do this. Many (most really) who dive NitrOx claim that they don't feel as "tired" as when they dive air. I believe that the extra O2 is the reason (there is no empirical data to support or contradict this contention)! Your body is far more efficient with the elevated levels of O2 in your bloodstream! That means more energy can be diverted to those "maintenance tasks" that your body seems to ignore during exercise, so you feel less tired.

But the real key is the CO2 trigger, which causes you to breathe irregardless of how much O2 is present. You see, you produce a lot of turbulence in your lungs when you breathe which flushes out the CO2 from around the alveoli with fresh gas. Until that happens, the gas around the alveoli becomes more and more CO2 rich (stagnant) which will hinder further off gassing of the bad and on gassing of the good. It will also compel you to breathe!!! The red blood cells cannot attach additional O2 until they rid themselves of the CO2.

Hope this helps!

So the compulsion to breathe comes from CO2 molecules' presence by the alveoli...does that mean that if we could "agitate" the gas in our lungs, we could extract more O2 from a lungful of air, and thereby decrease the pressure to breathe?

Ken

maybe I should have a little fan installed! :tease:

You are correct the body is absorbing higher partial pressures of O2, much of this excess goes into solution in the plasma. The hemoglobin will rapidly become saturated with the maximum amount of O2 they are capable of transporting and much of the excess goes into solution in the fluid portion of the blood or plasma.

This transfer onto the hemoglobin and into solution in the blood stream can be refered to as external respiration.

http://www.breathing.com/articles/hyperbaric-oxygen.htm

Internal respiration occurs at a cellular level and is the transfer process of the individual cells absorbing the O2 needed for metabolism to occur, this is internal respiration.

The body is going to continue to consume oxygen through internal respiration as needed for cellular matabolism to occur. This requires more than just oxygen, in fact the oxygen is only required to allow the cell to "burn" the sugar (glycerin) or fuel as energy is required.

The ratio of fuel to O2 remains constant while the amounts needed are a function of the bodies demand for energy.

http://www.rwc.uc.edu/Koehler/biophys/8e.html

The excess O2 in the system is being utilized to a small extent, area that become vasoconstricted or have a decrease in circulation during the dive may continue receiving adequate amounts of O2 even though the volume of blod is decreased. This is the effect function of hyperbaric O2. Even tissue with a weak blood supply get enough O2 because there is more in solution to be provided.

The remaining amounts of O2 remain in solution in the body until the ambient pressure is reduced...

This can also cause some problems as the divers body is slowing moving toward equilibrium with the partial pressure of O2 being inspired. If that partial pressure becomes to high then symptoms will occur.

http://www.sportdiverhq.com/noaa.htm

Not sure how the topic CO2 came up with this question but since it has it is worth pointing out there are some theories about this excess O2 (NITROX especially) effecting the efficiency of removal of CO2 from the body, this function is carried out by the plasma which absorbs the higher partial pressures produced through internal respiration and carries it to the lungs were the partial pressure of CO2 in the inspired air is lower and external respiration takes place.

I appreciate that logically then you should use a smaller percentage of the oxygen available, but that isn't true either

Click to expand...

You are correct the excess O2 is going into solution in the fluid of the body, not being consumed through cellular metabolism...

So the compulsion to breathe comes from CO2 molecules' presence by the alveoli...does that mean that if we could "agitate" the gas in our lungs, we could extract more O2 from a lungful of air, and thereby decrease the pressure to breathe?

Click to expand...

Actually the urge to breath is from elevated CO2 levels in the blood, the signal is transfered by the chemoreceptors telling the body to breathe. The elevated levels in the lungs cause the transfer or CO2 from the plasma to the air inside the lung to become less effective as the PP inside the lungs moves closer to equilibrium with the amount in the plasma.



Hope this helps somewhat...

Jeff Lane

Hi Pete

Some interesting thoughts! I agree with you about the Nitrox although in many cases when people comment I'm sure this is just confirmation of what they were expecting; but I dive Nitrox on a semi-closed rebreather all the time and the warmer air does make a difference. However, I've measured my O2 consumption at the surface and at depth and it doesn't change significantly; if you're on a leisurely dive the lower exertion than if you were doing something on the surface compensates for the additional O2 consumption keeping warm.

Here's what I think is happening. You're right the CO2 keeps you breathing and the higher pressure forces the O2 into your blood stream and tissues. This gives you the partial pressure problem because of the higher pressure and concentration of O2 on your nervous system; the exact mechanism isn't well understood but OK. However, quite independently of that pressure the body is only metabolising the O2 molecules it needs which is the same at the surface or at depth, it's just got more molecules to choose from the deeper you go and the longer you stay there. The percentage difference between inhalation and exhalation stays the same because the same percentage amount is being forced into the blood/tissues but they're not being metabolised - that's the key!

This explanation is the only one I can come up with that fits, and lots of people may have known it but nobody ever explained that to me and I've asked a lot of people who I thought would have known and didn't.

If it's right, and obviously I'd like to get it confirmed, I'll be really happy, it's been bugging me for a long time!

Dear Jeff

Your posting must have landed while I was typing mine.

Thank you, thank you, thank you!

Rob Jones

And I think an easier to understand explaination than mine...

Jeff Lane