bubble formation - physics question

[alaity47]

Ok, I'm a little embarrased I can't figure this one out - hoping one of the MDs out there can lend a hand.

I was trying to explain what causes the bends and how to avoid it to a non-diver, and I got a little hung up on one part. As I understand it, you breathe in higher concentrations of nitrogen at depth due to increased partial pressure in the air in your lungs. So far, so good.

Then, that excess nitrogen is absorbed by your bloodstream, which can absorb MORE nitrogen than it would at the surface due to the higher pressure. And on the way up, as pressure releases, the nitrogen can bubble out of your bloodstream. This is where I started to get confused.

So - this is my actual question - does all this mean that your blood itself is under pressure? I don't see how this is possible - isn't the human body fairly rigid? How does the external pressure have an effect on your veins, unless it's squishing everything between them and your skin (which I didn't think was happening)???

Anyone? Feel free to get as technical as possible...

 

[Dr Deco]

Hello alaity47:

Blood Pressure

There are actually several questions here. Yes, indeed, the blood is under increased pressure. The blood pressure in your body is measured relative to the air pressure out side. That gives a mean arterial pressure of about 100 torr. This is the result of the pumping action of the heart. Actually, the pressure measured with respect to a vacuum would be 100 torr plus the 760 torr of the atmospheric pressure. As you descend in the water, the total pressure (hydrostatic pressure) on the body increases and this pressure is transferred to the tissues and blood because the body is flexible.

The arterial pressure of the blood at depth will always be the pressure from the heart added to the hydrostatic pressure. For example, if you are cut at depth, the blood will flow out – the seawater will not rush in.

Bubbles

The bubbles form [grow actually] from the excess nitrogen dissolved in the body’s tissues. The decompression bubbles grow from preformed micronuclei. Now, the origin of the nuclei are debated by researchers; I personally favor the hypothesis that many of those involved in diving result from kinetic activity (motion) of the muscles.

The bubbles that form in tissues might result from fluid motion in the liquid that is outside of cells (lymph). This is hydrodynamic cavitation and the most common would be Reynolds’ cavitation that results from an object moving rapidly in a liquid (or a liquid around an object) and momentarily creating low-pressure regions.

In the capillaries, nuclei could be formed when the tissue capillaries expand following a muscle contraction. In this case, the cavitation mechanism would be Stephan adhesion (sometimes called viscous or tacky adhesion). [This is my explanation and is not accepted by all researchers in the field - not by a long shot.]:lifter:

If this does not clear up much of your confusion, please feel free to rephrase the question and ask again.

Dr Deco :doctor:

 

[Charlie99]

alaity47 once bubbled...
.... And on the way up, as pressure releases, the nitrogen can bubble out of your bloodstream.
........ does all this mean that your blood itself is under pressure? I don't see how this is possible - isn't the human body fairly rigid?
...... Feel free to get as technical as possible...

Your whole body including the blood will be under pressure. The human body is not very rigid at all when compared to the 40,000 pounds of force pressing inward on you at even the shallow depth of 33'. (the 40,000 pound number is from 1.8sq meters of skin times 14.7psi). The reason your veins don't get squished is that they, and all your other tissues are close to the ambient pressure.

The nitrogen is both absorbed quickly and offgassed quickly from the blood and that is not usually the limiting factor in preventing bends. The problems are from the other tissues that have absorbed nitrogen.

The nitrogen moves from your blood into the various tissues at rates depending upon the tissue type and various other factors such as the amount of blood circulating to that area.

Generally, bubbles in the veins don't cause problems, since the lungs will filter out the bubbles (some people do have problems because of a heart defect that allows bubbles to transfer from venous to arterial sides -- search on PFO). Excessive bubbles formation in your tissues is the more common cause of bends.

 

[alaity47]

Thanks for the responses so far... I guess my main question was simply whether or not our internal tissues are under pressure, which you answered. But that still leaves me with a question:

What's the body's mechanism for achieving an internal pressure equivalent to the hydrostatic pressure?

The air in your lungs is at a higher pressure because that's what the regulator delivers at depth in order to let your lungs expand against the external pressure... and the air in your ears has to be equalized to keep your eardrums from getting pushed in.

So how is the internal pressure of our tissues increased to balance out the external pressure? Or does the volume of our tissues in fact change? (Even if it changes slightly, we certainly don't smoosh by a factor equal to the pressure increase like a wetsuit...)

 

[Charlie99]

The body is mostly liquids (blood, and the water of our tissue) and solids (such as bones) that don't change volume signficantly with pressure. As you already noted, the gas-filled spaces like ear and sinuses need to equalize, using the ambient pressure gas supplied by the regulators.

 

[alaity47]

If the tissues don't change pressure, then how does bubble formation occur?

 

[adder70]

alaity47 once bubbled...
If the tissues don't change pressure, then how does bubble formation occur?

The tissues are mostly liquids and solids, wo they do change pressure without significant change in volume. If you put a glass of water (say .5 liters) in a chamber and compress the chamber to 130 ft (approx 5 atmospheres absolute), the exact volume of the water will then be about 0.499999 L, or something like that. THe same happens with liquids in your body.

Gases that are dissolved in a liquid (like blood) can dissolve in greater concentrations at higher pressures. At lower pressures, the gas will be carried to the lungs where it (forgot word, call it "undissolves" ) and is exhaled.

THis is where my knowledge is not entirely complete. I don't know if the bubbles are only in the blood vessels or in the tissues themselves as well. If you decrease pressure too rapidly, the tissues cannot transfer the gasses back into the blood quickly enough, and the gas "undissolves", forming bubbles within the tissues or the capillaries surrounding them. If the bubbles or tissues displaced by the bubbles block arteries or veins, you have insufficient blood flow, causing pain and tissue damage.

 

[alaity47]

Ok, I think I'm getting it... the external pressure DOES smoosh you by a teeny fraction, which results in an internal pressure equivalent to the external pressure - even though the volume of your tissues doesn't decrease noticably.

Am I on the right track here?

 

[Genesis]

Consider a bottle of soda.

If you want to, take one, carefully remove the cap, install a pressure gauge, and put the cap back on. Or not if you wish...

Now SHAKE. Note that the pressure goes up, but there are no real bubbles or foam evident in the container. If you have not installed a gauge, then note that the walls of the plastic container are "hard" (you cannot squeeze them) as there is significant pressure in the container.

The gas (CO2) is in the liquid (water, mostly), but remains dissolved and not in "bubble" form because of the increased pressure. Even though the liquid does not compress, it transmits the pressure uniformly throughout its volume - it is all (well, ok, for the purposes of this discussion it is ) at the same pressure, which is significantly above ambient.

Now remove the cap (pressure) quickly and see what happens

That's basically the same thing.

We think of ourselves as not being "under pressure", but that's not true. We are, in fact, under approximately 14.7 psi (pounds per square inch) of pressure all the time, imposed by the atmosphere above and around us.

When you dive you increase that radically, as water is much denser (has more mass) than air, and thus, it only takes 33' of salt water to equal the same amount of pressure as the ENTIRE column of the atmosphere that presses on us at the surface (from the surface all the way to the reaches of space.)

 

[alaity47]

I think I'm clear. The bit that was missing in my brain was that the pressure inside all our components parts increases to match outside pressure... I always thought only the airspaces experienced pressure increases, and the rest of you wasn't really affected. Being reminded that, in solids, pressure can be increased greatly with very little corresponding change in volume was the missing link!