Sorry for the delay — I was on holidays and just returned 2 hours ago.
Death Scenario #1 – tiny bubbles
So… you go diving, rise too quickly and bubbles (mostly nitrogen, because (compressed) air is 80% nitrogen) appear in your bloodstream… the bends. If you don’t have a hyperbaric chamber nearby (to increase the pressure so that the bubbles dissolve back into your blood), you could be in SERIOUS trouble.
The main problem is that the little bubbles can dramatically reduce the blood flow in your veins, arteries and capilaries; especially in the tiny capillaries. You can see this happen in any clear hose that you might have around the house; the smaller the diameter of the “pipe”, the more impact the bubbles will have because of (aptly named!) capillary action. Some people (like me) are blessed with drains that are slow until a bubble of air escapes.
Anyway, reduced blood flow means less oxygen for the cells to which the blood would normally flow. If this happens in your brain (and a lot of blood goes to your brain), you’re likely to have a stroke. As you probably know, this can result in anything from memory loss to paralysis to… death.
Death Scenario #2 – a big bubble
There’s another way that air in your bloodstream can kill you — an air embolism. You can get an air embolism any number of ways (e.g. somebody stabbing you with a syringe full of air) but the most common is during surgery. Any surgery that takes place more than 5 cm (2 inches) above your heart can result in an embolism. A whopping 80% of people who have brain surgery in the sitting position have an embolism of some sort! I think what happens is that the blood flows down and any cut blood vessels let air in.
You can also get one by simply, uh, inserting air into any body cavity under pressure.
An air embolism during surgery is much, much more common that you might think. Usually, fortunately, the air dissolves in your bloodstream and nothing serious happens. But if the amount of air is above the 100 to 300 ml range, the air can become trapped in the heart. This is similar to Scenario #1 in that the air bubble interferes with blood flow, but it’s a much bigger air bubble, which is why it can intefere with the heart’s operation.
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