Hi Robo,
Welcome, you have arrived at a wellspring of freediving info and socializing...
If you are asking these questions with respect to your own training, don't worry too much about any of these issues. There are answers to all of them that make safe diving to quite deep depths rather straightforward. Also, I suggest that if you have an interest in diving physiology that you consider taking a course through our hosts here at Deeper blue or through another program like
www.performancefreediving.com (I learned from them). The intermediate class explores physiology of freediving in some detail. Further, there are many threads here on DeeperBlue that address these issues.
Now for the short answers:
1. What are the primary cardiovascular dangers that ferrdivers encounter when decending to such great depts?
There are cardiovascular adaptations related to the mammalian diving reflex and direct responses to pressure. Generally speaking, a properly trained, properly prepared freediver will experience bradycardia - slowing heartrate - and this can be relatively extreme with well trained divers. 30bpm and lower. Resulting in lower blood pressure and a great state of mind.
2. Decending to such great depts, with such a reduction in theoretical TLC, what consequences dose this have for the (A) structure and (B) function of the lungs?
As you pass beyond four ATA compression reduces lung structure to the size of oranges and there is no lung function. All gases are compressed into the tissues and lungs begin to fill with fluid. Ribcage flattens somewhat, equalizing becomes more challenging. When returning to the surface and passing through pressure gradients the reverse occurs: structure returns as gases return to the lungs from the tissues. This can cause a blackout if not managed correctly as gases return from all organs including the brain. You don't really notice any of this other than the equalizing stress if you are diving correctly. There are no after effects.....
3. What are the main advantages & disadvantages of hyperventilating before diving?
The disadvantage is that it can kill you. There is an established method of basic "breath-up" as we call it. There are some variations and some arguments about those variation but basically, one does not hyperventilate before diving. One saturates one's tissues and blood with 02 through proper breathing cycles and slows one's heart rate in preparation to dive.
4. In static apnoea, what's happening to (A) heart rate and (B) blood flow, to increasing oxygen deprivation over extended breath hold periods?
Heart rate slows, blood flow and pressure is reduced. Martin Stepanek can do an 8:16sec breath hold. Basically, as time goes on, blood is drawn from the extremities to the organs, the heart rate slows considerably.
All of this is related to certain physiological facts about humans, breath-hold and submersion - see Mammalian Diving Reflex, that make freediving a rather natural and safe practice if one trains diligently and educates oneself carefully.
This is really not so much a sport as a discipline, a kind of yoga, or a meditation. It draws heavily on established physiological principles that are well documented in the scientific community, particularly the studies out of Simon Fraser U. in Vancouver BC...
Anyway, welcome aboard.
Please examine the study of C02/02 Tolerance tables if you want to learn how to explore apnea and improve your skills.
Good luck!!
Lungfish
