Cavitation Thresholds for Agarose Gels and Vertebrate Tissues

2 COMPARISON OF CAVITATION THRESHOLDS FOR AGAROSE GELS AND VERTEBRATE TISSUES 

McDonough and Hemmingsen (ref. 419) confirm that for bubbles to develop in vertebrates from such low gas supersaturations, some mechanism or structure must promote the initial in vivo bubble nucleations. They cite, as one initial possibility, the popular, general hypothesis that animals contain a reservoir of microscopic gaseous nuclei in the body fluids or tissues, which expand into bubbles when the organism is decompressed (ref. 2). These authors point out that results consistent with this hypothesis have been obtained with shrimp (ref. 429) and rats (ref. 430), where the application of relatively high hydrostatic pressure before decompression apparently reduced the incidence of bubble formation, presumably by forcing potential gas nuclei into solution before they could serve as bubble precursors (ref. 419). 

However, findings that do not support this mechanism include the observations by Harvey et al. (ref. 431) that bubbles do not form in mammalian blood or isolated frog tissues decompressed to 0.031 atm. Steelhead trout fingerlings also are not affected by exposure to 0.16 atm (ref. 432). McDonough and Hemmingsen (ref. 419) argue that in these experiments, the reduction in external pressure should have caused the gas nuclei 

It was found from their experiments (ref. 419) that prepressurization generally had little effect on somatic bubble formation. The results with shore crab larvae were especially striking considering the low resistance of these animals to bubble formation and the large difference between the hydrostatic and gas equilibration pressures used. Furthermore, when a slow compres- 

The gut bubbles in adult brine shrimp did appear, however, to form from gas nuclei (ref. 419) these presumably were incidentally ingested by the animals during filter feeding. Thus a slow compression schedule increased the number of bubbles by apparently preserving nuclei during compression to the equilibration pressure. At each pressure level, gas could diffuse into the gas nuclei, tending to stabilize them against collapse when further compressed. Prepressurization had the opposite effect, as it would tend to reduce the number of bubbles as a result of presumed dissolution of many gas nuclei (ref. 419 see also Sections 1.3.1 and 1.4.3). 

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