Why You Don't Want Your Lungs To Be Like A Bursting Balloon.

And another post on diving, following on from the last one.

So why might your lungs be like a bursting balloon?

The answer to this is the answer to the question about why the absolute cardinal rule of diving is "Don't ever hold your breath". This might seem like a statement of the obvious, breathing being somewhat important to life. However, there is another reason, not obvious unless you know how scuba gear works.

As stated earlier, the regulator which divers breathe through is a device for delivering breathing gas* at ambient pressure. Thus, if the diver is at 20m deep, they will be breathing air at three atmospheres of pressure (one atmosphere of gas - the Earth's atmosphere - plus 20 m of water = three atmospheres of pressure).

Now, supposing the diver begins to ascend? Obviously, as she moves into shallower water, the pressure on the outside of her ribcage decreases (less deep water = less water pressing down on you). If she is holding her breath, the air on the inside of the ribcage, however, in her lungs, will be at the higher pressure the regulator delivered it at, at the deeper depth, so that she could expand her ribcage to breathe.

It does not take a qualification in physics to work out that this is a bad situation. The higher pressure gas will start to expand inside the lungs. The possible consequences can be seen in this video about Boyle's Law:

If the gas expands too much, part of the lung will rupture, letting air into the bloodstream - arterial gas embolism. Anyone who has ever heard a radiator pipe banging because it has air in will grasp why getting gas bubbles in your bloodstream has potentially fatal consquences for your internal organs.

So why, if there is this potential risk, do thousands of people dive every weekend as routinely as if they were playing a game of Sunday morning football? Because it's an easy injury to avoid. Hence the advice: "Don't ever hold your breath!". If you keep continously breathing from the regulator, the gas inside your lungs is always at the same pressure as the water (or air if you're on the surface) surrounding you.

This is also partly why divers are trained to ascend slowly - it ensures that your breathing keeps pace with the pressure changes during ascent, minimising the risk of lung expansion injuries. (The other reason is to do with not getting the bends, another topic for a later post.)

And thus, the problem can be avoided.

* There's a reason I'm not calling it "air", that's for a future post.