Ppo2

[The Iceni]

NetDoc once bubbled...
. . .It is hard for me to conceptualize bubbles that are not homogenous. While bubbles might be incredibly tiny, I would venture that molecules are tinier still and the Van Der Wall forces that they possess would show little discrimination over which other molecule they would form bubbles with. Hence it would follow that we would not have discrete bubbles of nitrogen and oxygen but bubbles that would be comprised of both in similar proportions as their PP in the plasma.

Hi Netdoc,

You appear to be making the same mistake Haldane made when he invented the first decompression tables. The body is not the lump of homogenous jelly he thought it to be, it is a dynamic system right down to the microcellular level. As you know we now conceptualise the body as having several "compartments" which ongas and offgas in relation to their blood supply (perfusion) and their ability to dissolve the gas of interest, in this case nitrogen and helium.

As you know, Henry's law tells us that the amount of gas dissolved is proportional to its partial pressure, as is the rate of diffusion. Thus bacteria and protozoans can survive without lungs and a circulation because they are small enough for sufficient gas for metabolism to be transported by diffusion alone. This is the situation at the microscopic level of the capillaries of the body. You are therefore right in that the bubbles will contain the same gasses in proportion to those found in the immediately surrounding blood and tissues (not the average seen in the great veins).

It would also make sense to me that Oxygen, once out of solution, would probably exhibit the same resistance to re-enter solution as nitrogen.

I am sure it does but this also is where Henry's law applies. The tissues are removing the oxygen all the time by metaobolism reducing its local partial pressure which therefore increases the pressure differential between the oxygen in the bubble and that in the hypoxic tissue. Hence the oxygen will move out of the bubble and into solution while nitognen will not as it is not being removed by metabolism.

Since lung tissues (surfactant et al) are optimized to increase the transfer of Oxygen into the blood, they might keep plasma blood oxygen saturation high enough to inhibit a quick re-adsorption of oxygen in bubble form.

In the lungs, maybe, but remember DCI does not appear in the lungs.

This brings yet another question to mind... IF oxygen displaces CO2 in the red blood cell. Would this happen continually throughout the bloodstream? Is this a function of perfusion that occurs only in the alveoli? Would this cause an increase of CO2 levels dissolved in plasma? Do red blood cells transport CO2 at all? If they don't then this starts to make a lot more sense. Once free of it's oxygen load (taken by a cell) would the cell then absorb another free roaming oxygen molecule with the resultant being released in to the blood stream as a free molecule???

Physiologists learn about the Haldane effect, which by the way, has nothing to do with DCI. It is the ability of venous blood to carry more carbon dioxide than arterial blood. We now know this is because in venous blood less of the Hb receptors are occupied by oxygen and are therefore able to carry carbon dioxide molecules in (inactive) chemical combination as carbaminohaemoglobin. This again is not an all-or-nothing, single tissue phenomenon. It is a dynamic system wher the gas molecules follow the concentration gradients. However, you must remember CO2 is very soluble (and dissolved gas does not generate a partial pressure) and it is rapidly excreted by the body, so once an oxygen molecule has been used for metabolism it disappears to be replaced by molecules of CO2 and H2O.

I hope this helps. :doctor:

 

[The Chairman]

I hope this helps. :doctor:

Indubitably!

BTW, you can lump me in the same category as Haldane anyday! My ego can always use a lift! :tease: But I have always believed it to be a dynamic system, I just haven't forseen all of the ramifications of that belief.

I do request one clarification re:

(and dissolved gas does not generate a partial pressure)

I was under the impression that dissolved gasses (per Henry's law) exerted a PP but I could see that attached gasses (ie as in carbaminohaemoglobin) might not have a PP. It is the tension between the pressure of the gas dissolved in solution and the pressure of atmospheric gas that produces either on or off gassing. No tension would of course, be a state of equilibrium! Is this not true?

I do have more questions concerning this, but I am not sure how to phrase them. You have been MORE than helpful in this, and I am sure that my next class of NitrOx Divers will be cursing you for this! :tease:

 

[The Iceni]

NetDoc once bubbled...
I was under the impression that dissolved gasses (per Henry's law) exerted a PP but I could see that attached gasses (ie as in carbaminohaemoglobin) might not have a PP. It is the tension between the pressure of the gas dissolved in solution and the pressure of atmospheric gas that produces either on or off gassing. No tension would of course, be a state of equilibrium! Is this not true?

Hah!

A state of equilibrium at the steady state indeed.

For clarification, Netdoc, Say 1 g of gas at 1 bar is placed in proximity to a fluid that can dissolve 0.5 g of gas at 0.5 bar. The gas immediately moves into solution due to the pressure gradient until half of it is dissolved leaving a residual pressure in the gaseous phase of 0.5 bar. The system is now in equilibrium at 0.5 bar (the diffusion in and out of solution and the associated partial pressures are equal and opposite). Yes indeed the gas can come out of solution and indeed it must exert a partial pressure.

However if another gas is used, which is twice as soluble as the first, more gas will be taken out of the gaseous phase leaving a partial pressure at equilibrium around 1/3 of a bar, not 0.5 bar. Hence the effect seen in life where highly soluble CO2 does not exert as high a partial pressure as relatively insoluble nitrogen.

If the gas is then mopped up chemically, as are oxygen and CO2, yet more gas will be taken out of the gaseous phase further reducing its effective partial pressure.

I think this makes sense.

 

[The Chairman]

since your body is a semi-closed system and that there is a finite amount of gas to work with.

Now... back to metabolism and those questions I could not formulate. We have established that enzymes control cellular metabolism. I was once stated (not by you) that cellular metabolism is set at a fixed rate (at least that is my understanding of the statement). However, under exertion (or stress) it is clear that cellular metabolism. That is why I am gasping for breath at the end of a mile. My body is trying to supply enough oxygen to sustain life.

Are the stresses encountered in diving sufficient to consume a large portion of the "extra oxygen"? (I sorta asked this before, but not in this way)

Would my breathing have to increase irregardless of the amount of oxygen dissolved in the blood in order to expunge the resultant excess of CO2?

What mechanism is there that enables these enzymes to allow a higher metabolic rate?

 

[The Iceni]

NetDoc once bubbled...
Now... We have established that enzymes control cellular metabolism. I was once stated (not by you) that cellular metabolism is set at a fixed rate (at least that is my understanding of the statement). However, under exertion (or stress) it is clear that cellular metabolism increases. That is why I am gasping for breath at the end of a mile. My body is trying to supply enough oxygen to sustain life.

Basic biochemistry, as I remeber it!

I am not up to date with exercise science but metabolic rate is most certainly not constant. While it is true that the the body has a relatively constant basic metabolic rate at rest or sleep;- to keep the body's "engines" ticking over, any form of exercise will increase metabolism in the muscles, the amount of fuel burned and the necessary oxygen consumed to produce heat in addition to energy. Thus if a physiologist measures the oxygen consumption he can precisely determine the degree of aerobic exercise for a given activity.

I will happily stand corrected on the value of a MET (which has been the subject of a recent thread on Scubaboard) but if your own basic metabolic rate is of the order of 5ml O2/Kg/min and you weigh 70 Kg, at rest you consume at total 350 mls O2 per min - or one MET. If you double your activity you do two METs and so on.

Cycling is about 4 MET and scuba diving is between 4 or 5 METS or in the case of a 70 Kg adult, between 1,400 and 1,750 ml of oxygen per minute, which is easily provided by open circuit scuba.

Are the stresses encountered in diving sufficient to consume a large portion of the "extra oxygen"? (I sorta asked this before, but not in this way)

Would my breathing have to increase irregardless of the amount of oxygen dissolved in the blood in order to expunge the resultant excess of CO2?

Quite right, Netdoc. The reason that you are so out of breath is mainly due to the build up of carbon dioxide mediated acidity, the rate of production of which temporarily exceeds the lungs' ability to excrete it. It is not due to any lack of oxygen, which is present in more than sufficient quantities. In fact this is governed by the individual's level of cardiovascular fitness and their ability to use the fuel and oxygen needed for the necessary level of exercise.

Not only must the individual have sufficient muscle mass, these muscles must be tuned, like the carburettor of a car, to use fuel substrates as rapidly as they are needed. All of which is limited by the enzymes of cellular respiration, particularly the tricarboxylic acid (or Kreb's) cycle. If the individual is "unfit" the enzymes are present in lower concentrations, or the delivery mechanisms are less efficient, and they cannot cope. In consequence some of the oxygen remains unused and the necessary energy is provided by means of anaerobic metabolism, the end product of which is mailnly lactic acid (which looms large in the causation of cramp.) Acidity causes breathlessness.

The concept is very similar to a diabetic whose cells cannot use the sugar in his blood because he lacks the necessary "enzyme" insulin.

What mechanism is there that enables these enzymes to allow a higher metabolic rate?

Well! I have aluded to fitness. However, I am sure you know that in muscle these cellular mechanisms directly result from motor nerves instructing the muscles to exercise. Once the muscles contract the biochemistry powering them becomes automatic.

In addition under a cold stressor, for example, the liver will directly generate heat under nervous and hormonal control in order to maintain core body temperature. This alone increases metabolic rate and oxygen consumption. This is significant in all divers and is why I always dive dry!