It is noteworthy that relatively small gas supersaturations seem capable of provoking bubble formation in vivo yet huge supersaturations are required to provoke bubble formation in pure solutions. This is almost certainly because of massive pressures caused by surface tension forces at the fluid-gas interface of an evolving bubble. These are inversely proportional to bubble radius, and a small bubble nucleating de novo from supersaturated dissolved gas would need to overcome them. The most popular and widely accepted explanation for in vivo bubble formation from relatively small supersaturations is the postulated presence of tiny gas micronuclei in blood and possibly tissues. These micronuclei are hypothesized to act as seeds for the inward diffusion of supersaturated gas after ascent from a dive, thus causing them to grow into bubbles.
Both the source and nature of these micronuclei are uncertain, but one suggestion is that they are created by tribonucleation in tissues where movement creates momentary areas of depressurization within the tissue fluid or blood. Bubbles created under such circumstances are usually tiny and rapidly involute in response to the high pressures caused by surface tension mentioned earlier that force the gas back into solution. However, micronuclei theory holds that some of these bubbles acquire a stabilizing outer layer of surface active molecules (these being extremely common in vivo) that reduce surface tension and increase the life span of the bubble, which now becomes a micronucleus.
Evidence supporting the existence of micronuclei is largely circumstantial, beginning (as described earlier) with the mere fact that bubbles can form in vivo following relatively small supersaturations. In addition, in vivo experiments in which a short ‘spike’ to particularly high pressure was imposed before a decompression demonstrated less bubble formation than expected; a finding implying that at least some micronuclei had been crushed out of existence during the high-pressure spike. Consistent with the notion (mentioned earlier) that tissue movement may result in regeneration of micronuclei, one famous experiment using shrimps showed that ‘exercise’ following the high-pressure spike partially reversed the spike’s prevention of subsequent bubble formation.
Variations and parallel theories exist. For example, it has been proposed that micronuclei could reside in imperfections or crevices in capillary walls and that when the underlying tissue becomes supersaturated, nitrogen molecules will diffuse into the micronucleus, thus causing it to grow and bud off bubbles in an analogous manner to the steady stream of bubbles that can be seen issuing from specific points on the wall of a glass containing a carbonated beverage. The essential features of this notion are illustrated in Figure 10.6.
Another theory invoked to explain bubble formation within tissues themselves is that if tribonucleation on movement or impact creates tiny bubbles in a tissue that is supersaturated at the time, the supersaturated gas will diffuse into the bubbles and not only prevent their early involution, but also cause them to grow. This is analogous the act of shaking an open soda bottle and could explain bubble formation even in the absence of persistent gas nuclei.