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Hi John: > I cannot for the life of me see the ongassing and offgassing > go at different rates One reason on/off gassing is potentially assymetric is that the dynamics of free and disolved gasses differ in their response to pressure gradients. On the decent, free phase dynamics govern uptake and the tissue loading can be modeled by the parallel compartments typical of most models (including Buhlmann+derivatives, statistivcal, VPM, RGBM, etc). On ascent, some bubbles will likely grow (even for trivial depths~ a few meters/feet) so, once you consider these cavities of gas in your model you need an increase in off-gassing times. Models commonly handle this by using longer 1/2 times or linear fall-off for outgassing. Another possibillity for assymetry arrises from reduction in off-gassing efficiency by bubbles in circulation or occluding capillaries. unless the membrane between vessel > and tissue support a different molcular flux depending on the > direction of the N2 gradient across that membrane. Within the > tissue itself the diffusion is determined by the "chemical" > diffusion rate. The is no information available to the > thermally activated molecules which tells them to slow > down or speed up. They just merrily perform random walks > in arbitrary directions ...ah yeah. Consider now a bubble in your tissue as a gas "trap" during off-gassing. In addition to temperature, the chemical potential of gasses in the bubble depends on internal pressure. This pressure is set by Laplacian skin tension, tissue compliance, previous gas partial pressure history, "negative" surfactant tensions, etc. Also, an impermeable skin may form around bubble nuclei on DEEP dive segments (Yount's Varying permeabillity model). Bottom line is if any gas partial tension in tissues is greater than that particular gas's partial pressure inside the bubble, then gas will flow into the bubble. This gas in the free phase will need to outgas from bubble to tissue before it can be eliminated--thus slowing the process of gas elimination. > > > I am unware of bubble formation being a problem if it > should occur outside of the vascular system. This problem is commonly refered to as: "The bends." Bubbles likely don't form in blood. However, they can force themselves into circulation by busting through capilary walls. This may be one reason why doppler doesn't provide complete information for tuning tables--enough bubbles need to bust into circulation from tissue to give a significant "cross section" for detection. Meanwhile stationary (non-detectable bubbles are have been doing damage.... >It's size > should it occur in elastic tissue would seem to have an > upper bound equal that of the host cell No. You should see the Xrays from WWII showing golfball size (extracellular) cavities in decompressed aviators.... Regards, EM _____________________________________________________________ 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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