"By 1900 Barbe had postulated that the anaesthesia, hyperesthesia, muscle spasms, convulsions and spasmodic paraplegia of contemporary sponge divers were "due to the release of excess gases dissolved in the blood."....
convulsions?... could Barbe have been describing sambas? interesting.
As for the blood spitting thing - I've had some experience of this, and seen it in other people too. It is fairly common. From what I know there are different types of bleeding...
What I have experienced is a few tiny specks of blood in my spit after diving. This first occured when I started diving sub45 over a year ago, but not every time. It took me a while to figure out what the cause was, and I now think that it was caused by 'aggressive' equalisation, and straining equalisation. At the time I was limited by ears and so I would always dive as deep as my ears would let me. The forceful way I would try to squeeze out the last equalisation resulted (I now believe) in the rupture of small capilaries around the upper throat / pharynx area. This is consistent with the fact that I rarely coughed, and that the blood didn't feel like it was coming from my lungs.
Since then my body has adapted to these depths and beyond, and I've since mastered continuous frenzel equalisation which means I no longer have to force equalisations - my ear limit is no longer an issue. So there's one cause of bleeding... Personally, I don't think this problem is particularly serious. The amount of blood being minute. If you get this sort of symptom, then look at how you're equalising...
I've also seen people who bleed quite a lot if they stretch their ear drums, by failing to equalise in time or whatever. I can't tell you much about this as I haven't experienced it myself.
The final, and perhaps worst example, (that I know of) is blood coming from burst alveoli, as laminar mentioned. This being the result of negative pressure in the lung - the only logical cause I guess. The answer to this is to work on flexibility of the rib cage and diaphragm. Also, I think it's very important not to progress too quickly in terms of depth and allow your body to adapt, especially when you start diving beyond the point of residual volume, 35-40 (roughly, depending on the person).
I've seen people have (blood spitting) problems after making sudden jumps in depth. My best advice is to progress slowly and repeat depths over and over.
Occasionally, in the past when I had dived to new depths (in whatever discipline), sometimes I would feel some tightness across my chest. This meant that my lungs were not ready to go any further. After doing lots of dives to those depths, gradually I adapted and now I never get that feeling. A big factor for this is the fact that I dive depth quite often, so I maintain that flexibility.
i'll shut up now...
alun