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Eric, Great post! When you mentioned tribonucleation as a result of interval training on the bike, were you kidding or serious, and if so, can you elaborate? Regards, Dan >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 > > > > Dan Volker SOUTH FLORIDA DIVE JOURNAL "The Internet magazine for Underwater Photography and mpeg Video" http://www.florida.net/scuba/dive 407-683-3592
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