Does Exertion Affect N2 Absorption?
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Bubbletrubble
Contributor
@Mr Carcharodon: I think you misunderstood my last post. I bold-faced the text that I had issues with. I would be satisfied if you could tell me what tissue in the human body correlates with the "slow" compartment and how perfusion rate can be accurately measured through that specific compartment. You made the following statement:
I maintain that there is no way that you can know that. You are applying a quantifiable biophysical property (blood perfusion rate) to a component of a mathematical model.
I am not disputing whether perfusion rate (defined as gross volume of blood transiting a tissue per unit time) affects nitrogen load of said tissue. What I'm trying to say, probably poorly, is that it is unlikely that perfusion is the only determinant of nitrogen tension in a tissue. In addition to the extent/quality of vascular perfusion, diffusion of nitrogen gas into/out of a tissue is dependent on the intracellular composition of the cells comprising the tissue and the composition of the extravascular compartment. With respect to off-gassing, tissue-saturated gas must travel through plasma membranes of other cells within the tissue, through the extracellular matrix, and finally across the endothelial cell membrane. Once the gas becomes intravascular, then, perhaps the phenomenon of off-gassing is reduced to how much/how fast blood perfuses the body and how quickly nitrogen is exchanged across the alveolar capillary membrane in the lungs, although intravascular bubble trapping/snagging/slowing may also determine how quickly the nitrogen gas reaches the lungs.
The paper that Gene cited treats the study subject as a black box and measures nitrogen going in and nitrogen coming out under different levels of exercise given various ambient pressure (simulated dives). The article says nothing about perfusion rates within a particular tissue compartment.
I haven't read Mike Powell's book. I'll admit that I'm pretty ignorant with regard to a lot of the deco terminology, especially the stuff pertaining to mathematical modeling. If I'm missing something here, please explain it to me. We can discuss this via PM if you think that others won't find our discussion interesting.
The body consists of various types of tissue. The rate at which an inert gas is absorbed (loaded) by each tissue during hyperbaric exposure, and subsequently released (off-loaded or off-gassed) during decompression, depends on several factors. These include the blood perfusion in the tissue and the solubility of the gas in each particular tissue type. A simplified description of tissues is that they can be fast or slow at absorbing and releasing inert gas. The following examples of the 'speed' at which this process can occur for several tissue types exposed to both nitrogen and helium - the two most commonly used inert gasses in diving.
Tissue Half-time, Nitrogen (mins) Half-time, Helium (mins)
Spinal Cord 12.5 12.5
Skin, Muscle 37 - 79 14 - 30
Inner Ear 146 - 238 55 - 90
Joints, Bones 304 - 635 115 - 240
Edmonds, Lowry and Pennefather (1991)
It must be right that if you put a larger amount of gas through the alveolar tissues during a dive (ie. through breathing harder due to exertion), greater exposure would equal greater absorbtion.
After all, we know that divers can do a five minute dive which descends to 800 feet and ascend directly to the surface (at considerably faster than 60 feet a minute) with no decompression stops and not suffer DCS, provided that they only utilise one lungful of air.
After all, we know that divers can do a five minute dive which descends to 800 feet and ascend directly to the surface (at considerably faster than 60 feet a minute) with no decompression stops and not suffer DCS, provided that they only utilise one lungful of air.
Interesting. I assume the numbers below were from some sort of test protocal.. as no mathamatical model would predict numbers like that.
In particular the spinal cord.. where it shows equal numbers. That would be one major violation of several gas permeablity laws...wonder if that was a measure of the fluid in the spnal cord... which would be possible. In that case, one would be measuring the huge space, in cellular terms between two gradients, with slow migration in on one side and fast movement on the other, leaving the middle fairly low, as long as one is not near saturation....
The other values seem reasonable.
Thanks, by the way, for posting that.. as it does a great job of showing the concept of compartments and half life.
In particular the spinal cord.. where it shows equal numbers. That would be one major violation of several gas permeablity laws...wonder if that was a measure of the fluid in the spnal cord... which would be possible. In that case, one would be measuring the huge space, in cellular terms between two gradients, with slow migration in on one side and fast movement on the other, leaving the middle fairly low, as long as one is not near saturation....
The other values seem reasonable.
Thanks, by the way, for posting that.. as it does a great job of showing the concept of compartments and half life.
Bubbletrubble
Contributor
Interesting numbers, DCBC. (Nice cut and paste action, BTW.) Do you know how Edmonds et al. arrived at these data? Was it done in the human system? Were these tissue types tested in vitro or in situ? I don't have the Diving and Subaquatic Medicine book on hand right now.
I know that joints are not very highly vascularized, but bones are. The Edmonds et al. chart would seem to classify joints and bones as "slow" compartments.
I still don't understand how definitive statements can be made regarding quantifiable perfusion rates (at rest vs. during exercise) of a hypothetical construct (Haldanean tissue compartment).
I know that joints are not very highly vascularized, but bones are. The Edmonds et al. chart would seem to classify joints and bones as "slow" compartments.
I still don't understand how definitive statements can be made regarding quantifiable perfusion rates (at rest vs. during exercise) of a hypothetical construct (Haldanean tissue compartment).
Bubbletrubble
Contributor
@Puffer Fish: I don't understand how the equivalent N and He half-time numbers for spinal cord represent a "major violation of several gas permeability laws." Spinal cord tissue is comprised of a highly heterogeneous mix of neurons and various glial cells.
I'm not sure about their process. I know that our hyperbaric research team at DCIEM did some testing on this as well, but the focus was on Helium, as some types of helium-based diving have resulted in an unexpectedly high incidence of central nervous system decompression sickness (which was unexplained). The areas of focus were the CNS and the brain using Navy Divers as subjects, our chamber and doppler measurement equipment. Unfortunately, I don't have the specifics of the testing available.
The space community has done quite a bit of work in the area of strenuous exercise causing the formation of gas bubbles (micronuclei). An article entitled "Exercise fizzy-ology" by David J Doolette may be of interest to you. It's published in the Journal of Physiology (Exercise fizzy-ology)
A key supporting paper is Dervay JP, Powell MR, Fife CE. Aviation, Space, and Environmental Medicine. 2001.
This is and is not true. Gas in the alveoli exchanges across the capillary membranes according to driving gradients. The process does take time, but not much. When fresh gas reaches the alveolus, the gradient is maximal; as that gas takes up CO2 and nitrogen, the process slows. As the gas is refreshed with inspired volume, the partial pressures of offgassed substances falls, and diffusion becomes more rapid again. So, if you run more gas through the alveolus, the net effect is that the gradient driving offgassing INCREASES, as the blood is trying to equilibrate with a lower partial pressure in the alveolus. This is why true hyperventilation drives your CO2 down.
Similarly, when you are at depth, increasing your ventilation rate is going to keep the partial pressure of nitrogen HIGHER in the alveolus, and may result in a small amount of additional uptake.
Whether the increased uptake (and the increased muscle perfusion) compared with the shorter time at depth for the same gas volume results in a net increase or decrease of total gas absorption would be an interesting question. But for those of us whose dives are limited by preset times or NDLs or cold, rather than gas, it would certainly be possible to execute two dives with exactly the same time/depth profile, but possibly with quite different nitrogen uptakes.
sydney-diver
Contributor
Increased half time maybe, but not greater total absorbtion. When you ascend there's a pressure gradiant between the Nitrogen partial pressure in your breathing gas (since your regulator delivers gas at ambiant pressure) and the Nitrogen partial pressure in your tissue. This gradiant reduces as your tissue "fills" until there's zero difference (saturation reached) and no more Nitrogen passes into the tissue.
When you free dive you're not breathing through a regulator so the nitrogen partial pressure is much lower (if your lungs didn't reduce in volume then it would remain at 0.79atm so no pressure gradiant) so the rate you're absorbing Nitrogen at is fairly low (I would say negligable but I've read it isn't in some cases). So rather different physics involved.
Regards
Dave
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