bubble formation - physics question

[maj2]

A couple more points to add to it...

Dr. Deco discussed "preformed nuclei" from which gas bubble grow. When you pour out the soda into a glass, you will notice spots on the glass where the bubbles form and rise in a nice straight line to the surface. These spots would correlate to these nuclei.

Think in terms of solubility. The higher the pressure, the higher the solubility of the gas in the liquid (the closed bottle). Remove the pressure, the solubility of the gas is reduced and the gas comes out of solution (bubbles), wherever the liquid may be (ie nearly all body tissues, or the soda bottle).

On those occasions where non-divers have asked me what causes the bends, the soda bottle analogy always makes it an easier explanation in real, recognizeable terms.

 

[beche de mer]

alaity47 once bubbled...
The bit that was missing in my brain was that the pressure inside all our components parts increases to match outside pressure...

Regard the body as a fluid filled sack.

Pascal's law applies: "when there is an increase in pressure at any point in a confined fluid, there is an equal increase at every other point in the container".

This means that the hydrostatic pressure exerted on the body as a result of being immersed is transmitted to every point inside the body.

 

[H2OHead]

PV=nRT so you'll be playing with ppO2 and ppN2. The why of expansion and consequent ischemia should start to become clearer and clearer when you make gases big.

 

[DepartureDiver]

A couple more points to add to it...

Dr. Deco discussed "preformed nuclei" from which gas bubble grow. When you pour out the soda into a glass, you will notice spots on the glass where the bubbles form and rise in a nice straight line to the surface. These spots would correlate to these nuclei.

Think in terms of solubility. The higher the pressure, the higher the solubility of the gas in the liquid (the closed bottle). Remove the pressure, the solubility of the gas is reduced and the gas comes out of solution (bubbles), wherever the liquid may be (ie nearly all body tissues, or the soda bottle).

On those occasions where non-divers have asked me what causes the bends, the soda bottle analogy always makes it an easier explanation in real, recognizeable terms.

Erik Maiken has a nice discussion on this at http://www.decompression.org/maiken/beer.htm. Of course your example is more poitically correct than Erik's beer discussion.

 

[russm]

PV=nRT so you'll be playing with ppO2 and ppN2. The why of expansion and consequent ischemia should start to become clearer and clearer when you make gases big.

Wow, I guess if you use that equation then if you are really small ( moles/n ) and dive in cold water (T) then the pressure would be less and cold skinny people would be less like to get DCI.

 

[Dr Deco]

Hello russm:

Ideal Gas Law

This really describes gas coming from a unit volume of tissue. The gas solubility is approximately the same in muscle, fat or tendon, of a fat or thin diver. It is the same whether from a mouse, sheep, or human. Thus, the free gas evolved when decompressing any animal depends on the depth and time at pressure (assuming that tissue micronuclei are present). At saturation, the volume of gaseous nitrogen released is roughly equivalent. The free gas will expand or contract in accordance with the gas laws.

For warm-blooded animals, the water temperature is not of large consequence as the body temperature is constant within a few degrees. In addition, the solubility of gases does not vary greatly as a function of temperature, not enough at any rate to produce a noticeable effect. In truth, gases are more soluble in cold liquids than warm ones. :laghost:

Blood Flow

What is different between animals is the blood flow. The metabolism of small animals is very high and the perfusion (= blood flow/unit volume of tissue) is very high. Mice can undergo very large pressure changes without acquiring DCS. This effect is also seen in humans who are in a good state of physical fitness. Physically fit divers [those with a high maximum oxygen uptake] are less prone to develop Doppler-detectable gas bubbles and less likely to get decompression sickness.

Dr Deco :doctor: