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  • AutorenbildMichael Mutter

Bubble trouble - insights into the nanoworld

It has been known since the 17th century that decompression triggers the formation of gas bubbles in the organism. On the other hand, it has been acknowledged that the tendency to form bubbles is by no means a compulsory property of a liquid solution. Nevertheless, gas bubbles form in biological tissues under decompression even at physically small pressure differences. A recent study sheds light on this process, which has never been directly observed before.

Lake Marmorera. Foto: Patrick Oswald

Gas bubble formation - not an inherent phenomenon to liquids

In a living organism, even small supersaturations can lead to bubble formation, in contrast to inanimate solutions, in which bubble formation only occurs under massive supersaturation. For example, the formation of new bubbles in water only occurs at a supersaturation of 190 bar for nitrogen and 300 bar for helium. This is a consequence of the 2nd law of thermodynamics, which states that (a lot of) energy must be invested in a system to gain order. (Gas bubbles represent a higher order compared to the state of a substance completely dissolved in a liquid, because the inert gas is "sorted" in the bubbles and no longer freely dissolved in the liquid under Brownian molecular motion). This cannot happen spontaneously. Taking into account that a bubble has to overcome tissue pressure in order to grow, it becomes clear that the tendency to form bubbles is by no means a mandatory property of a biological fluid. Experiments with single-celled organisms, in which bubbles could only be provoked under very high supersaturation conditions, underline this.

The theory of microbubbles

The theory of microbubbles explains why gas bubbles are formed during ascent: They serve as nucleation cells into which inert gases diffuse under supersaturation, which leads to bubble growth. This has never been observed. We can also only speculate about the origin of these microbubbles. It is assumed that motion and the resulting shear forces provoke local microdecompressions in the tissues, which cause microbubbles, similar to the shaking of a mineral water bottle.

Another theory is that microbubbles persist and grow on irregularities in vessel walls as soon as inert gas diffuses into them due to supersaturation, similar to the observation that bubbles always form at the same point in a glass filled with mineral water.

Gas bubbles and their physiological benefits

Bubbles also have a physiological function: they reduce the supersaturation of a tissue and it is conceivable that bubbles promote the washing out of an inert gas from the tissue, as much more inert gas can be transported in a bubble than dissolved in the blood.


The vascular inner layer - key to understanding DCI

Ran Arieli from the Israel Naval Medical Institute investigated the processes of bubble formation using electron and atomic force microscopy at the nanoscale. In a previous study, he was able to show that microbubbles (=nanobubbles) in blood vessels only occur in places that are water-repellent (hydrophobic). These "active hydrophobic spots" (AHS) are places where the inner vascular layer (endothelium) is coated with lipids (fats).

The study investigated how the vascular surface behaves under decompression. Vascular cells from sheep were pressurized and then decompressed. At the sites where nanobubbles were formed, there was no longer an intact vascular layer, but the underlying protein (elastin) was exposed. Intact endothelium was only found at a greater distance from the AHS. This study thus supports the assumption that bubbles tear off the endothelial cells and thus cause the first injury to the inner vascular layer after decompression.

These results fit with the understanding of decompression illness (DCI), support the theory of microbubble formation and show that inflammatory processes triggered by endothelial injury are an essential part of DCI. This also explains why DCI can progress over days and weeks despite pressure chamber treatment.

From the microsmos to the macrosmos

The study results from the microworld fit remarkably well with observations in the macroworld. The statistical distribution of the nanobubbles is similar to the occurrence of bubbles in divers detected by ultrasonography.

Diving profiles alone do not fully explain the occurrence of bubbles. Other factors such as age, state of health and adaptation processes offer further explanations that fit well with the observed nanobubble phenomena.

This is also confirmed by the latest research, which shows that the occurrence of bubbles is subject to very large intra-individual variability, even with an identical diving profile, as already discussed in this blog.

An increasingly complete picture of decompression sickness and its triggers is thus emerging, the connections between which are consistent from the processes in the microworld to the phenomena we observe as divers ourselves.

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