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>Jody Svendsen writes: >> >> My personal theory is two fold. First, the SEA second stage has a >> particularly large air space, leaving lots of exhaled gas inside the >> regulator. David Story replies: >All of this is interesting, but I have a strong feeling that anyone >with a background in fluid flow would discard any such arguments. You >are inhaling several litres of air, with inevitable turbulent mixing, >through a potential deadspace of only tens of CCs. The ratio simply >doesn't support this theory. > >In addition, there are physiological constraints on the problem. > >First, one's respiratory system simply isn't efficient enough to cause >an extra 100cc of exhaled air to noticably affect respiration -- you >don't exhale a high enough concentration of CO2 to make a difference >in 100cc out of 4000cc. (Don't forget that your trachea and bronchi >are additional, nonexchanging "dead space" in your breathing apparatus.) I don't know where David got these figures from, but tidal volume is nowhere near 4 litres, I think the confusion is with minute volume, which at rest may be about 4 litres. Resting tidal volume is near 500ml and physiological deadspace is about 30% of tidal volume, about 150ml at rest. An extra 100ml of apparatus deadspace will make a difference to alveolar pCO2. regards, David Doolette ddoolett@me*.ad*.ed*.au*
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