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Dan: You point out some important differences between the *fit* and *otherwise* that likely play fundamental roles in decompression modeling. Considering much greater perfusion of blood through athletic tissue (due to increased vascularization, strong heart, clean plumbing,etc) your idea about rapid ingress/egress of inert gas to tissue seems reasonable and is incorporated in setting tissue halftimes by Buhlmann and others (Q-dot) In using the "rate" equations to calculate tissue tension. The idea being the better the perfusion, the shorter the half-time. Arguments based on diffusion need to consider the solubility of gas in tissues (how much gas is soaked up from the bleood) , the distance between blood vessles (characteristic distance gas moves), and the diffusivity of the gas (how fast the gas moves). You point out the "Pinneped" model: Q: Why don't Walruses get bent? Typical A: They shunt blood flow away from blubber to critical organs. > > One list member has suggested to me that the heavily overweight divers may > actually absorb less nitrogen then currently assumed (based on Nitrogen > solubility in fat) Doesn't seem to make sense. First, apearences aside, divers aren't walruses. I don't recall who said what, but....The solubility of all gasses that you will ever *reasonably* inhale (Unless you acytelene weld inside a dry habitat) is much greater in oils than blood. Nitrogen, Helium, Argon, you name it; If blood moves these gases to fatty tissue, two-to-five times as much will be soaked up compared to lean tissue > because of the extremely poor vascularization of fatty > tissues (this means slower in and MUCH SLOWER OUT), which would place > diffusion rates in these tissues at well below what will occur in areas with > well developed capillary beds.. For Helium: Diffusivity is big, not very soluble in fat or lean tissue, but pressure gradient between tissue and -bubbles- is big. (Note, however, the pressure grad. between -blood- and tissue will tend to fall rapidly) For N2 and O2: Solubility is much greater in fat compared to water/blood (about 5 times), so potentially, fatty tissues can store a lot of gas IF the blood transports the gas to the tissue If you believe that reducing total bubble volume reduces severity of dcs symptoms, than you can also argue for fitness. Both statistical and bubble models use a "critical volume" hypothesis to create decompression schedules. You will always get some bubbles, unless you are de-nucleated (as George probably is from lots of DEEP dives).... The trick is controlling both the number and size of growing bubbles. For instance, you point out that for a supersaturation model, deeper stops might be needed by the *fit* because of large *fast* tissue tensions. If you consider that bubble growth depends fundamentally on the gradient of partial pressure between tissue and bubble, (and the Diffusivity -Solubility product of the gas, etc, etc), then deep stops look good. But, perhaps if you trace out the time history of bubbles formed early in the deco schedule, you will find that a lean person can tolerate the formation of a greater population (number) of growing bubbles, because in the deco/surface interval phase of the dive there is less dissolved gas in tissues to help the bubbles grow (due to efficient out-gassing AND lack of bulk). I'm just being the devil's advocate here-- I think deep stops are cool. Consideration should also be given to otherfactors that tend to keep high internal pressure inside bubbles (to help squeeze gas out into tissue). Tissue *squishy-ness* (compliance) is important. Big bubbles can't form easily in tight tissue. But, on the other hand, *kilo* style power intervals as you recommend for gas consumption (and good performance in 200' deep criteriums around a wreck) may generate bubble nuclei through tribonucleation, hence increasing your *population* of growable bubbles. There's lot's going on here that can sway the conversation to *fit* and *otherwise,* but its 75 deg. here in LA, and I'm going out for a ride.... As a curve ball: The oxygen *window* roll-over (the point where dissolved O2 can start to *load* tissues) will also depend on the O2-carrying capacity of the blood (more RBCs if altitude trained, for example). This would be at high-enough ppO2, where deco would be the least of your problems though). Regards, _____________________________________________________________ Eric Maiken email: ebmaiken@ea*.oa*.uc*.ed* Dept. of Physics o: 714 824-6621 U. of California fax: 714 824-2174 Irvine, CA 92715-4575
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