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David Doolette replied: > > David Story wrote: > > > >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. I took my numbers from my own measured physiology, though I may not be average. I don't have Physiology and Medicine of Diving here, but I have a few different figures from _Diving Medicine_ and the introductory Scuba Diving in Safety and Health by Christopher Dueker, as well as the 1985 US Navy manual. Dueker says residual volume in "young males" is 20% of total lung capacity (TLC), not tidal volume. Anatomic dead space is usually assumed to be around 150ml, which agrees with your figure from a different source (USN says "less than 200cc" on p3-12). The USN manual also cites vital capacity as averaging between 4-5l. (p 3-11) Further, none of these divers are at rest. I assume these folks are working, not at rest, or CO2 buildup wouldn't be an issue. Here's a chart from the USN manual, p3-15, to give you an idea why I disagree with your 4l/min RMV figure: Activity RMV (l/min) --------------------- ----------- Bed Rest 6 Standing Still 9 Walking, 2mph 16 Swimming 0.5kt (slow) 18 Swimming 0.85kt (ave) 30 Finally, I bet dollars to donuts none of the divers in question are breathing with tidal volumes of 500cc. "Good breathers" refers to a long, slow, deep inhalation/exhalation cycle, to reduce turbulence and work of breathing. It also implies a larger tidal volume, typically somewhere near vital capacity. I am one of those "good breathers" and usually breathe almost my maximal possible inhalation on each breath. Hope this helps, I certainly don't want us stressing over this stuff, but getting one's assumptions down on phosphor ;) can clarify why folks disagree. Cheers, David Story Silicon Graphics, Inc. story@sg*.co* Mountain View, California
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