Newsgroups: rec.scuba

In rec.scuba, davew@hp-ptp.ptp.hp.com (Dave Waller) writes:

>     Alveolar surface area does not scale proportionally with volume.
>     Alveoli are roughly spherical; the radius of this sphere scales as the
>     cube root of the volume, while the surface area of the sphere scales
>     as the square of this same radius.  What this boils down to is that
>     the surface area scales as the 2/3 power of the volume.

Of course.  I only meant to imply that the surface area decreases with
depth, not that it was directly proportional to volume.  Sorry if my wording
confused anyone.

>     Further, the effect of surface area cannot be viewed as simply as you
>     have described it above.  Remeber that there is also the exchange of
>     metabolic gases taking place, and the body's requirement for oxygen
>     and need to eliminate CO2 is elevated during the physical exertion of
>     a dive, particularly during a free dive which is typically more
>     strenuous than a SCUBA dive.  As such compensatory changes such as
>     increased heart rate and DILATION of lung arterioles (rather than
>     constriction due to compression) take place to provide for the needs
>     of the body.

I'm not sure if free dives among experienced folk are really more strenuous.
In any event, if heart rate is accelerated or if circulation through the
lungs is, gas exchange will certainly be increased.  This actually agrees
with the main point I was trying to make, which is simply that it's 
incorrect and oversimplified to state that since the gas pressures
are the same, everything must be equivalent.  Like most physiological
problems, there are lots of effects which have to be taken into account.

>     >   2) The amount of N2 in your lungs is limited when free diving,
>     >      but is continually being replenished on scuba.
> 
>     This is true if comparing a single free dive to a SCUBA dive of
>     equivalent duration.  However, to my knowlege no one has ever been
>     bent free diving under such circumstances.

Seems to me that this directly supports what I've said.  If I recall 
correctly, the world record free dive is on the order of 350 feet
with a total time for ascent and descent of 3-5 minutes.  I don't
know about you, but my guess is that if you tried this on SCUBA
you'd have a pretty fair chance of DCS.
 
>     Rather, it is the repeated "bounce" free dives that result in bends
>     cases, and in this case the air is constantly being replenished at the
>     surface.
> 
>     Finally, the current theory amongst the experts is that the reason for
>     the higher susceptibility to bends for bounce type free diving is due
>     more to the "bounce" nature of the diving rather than total N2
>     loading.  Rapid repeated ascents result in considerably more
>     asymptomatic bubbling, and of course, there is no opportunity for the
>     safety stop that is recommended while SCUBA diving, so bubble growth
>     and accretion is exacerbated.
> 
>     Dave Waller
>     YMCA Instructor

Bounce dives are a complicated kettle of fish, ok.  Because of the repeated,
frequently rapid ascents, I claim it's unfair to compare bounce free dives
to 100 feet with a single SCUBA dive having the same depth and bottom time.  
The only way to do fair comparsions is for the divers to follow *identical* 
profiles (including ascent and descent rates) on SCUBA and free diving.
If you're aware of such data, I'd be very interested in hearing about them.

-jh-
Newsgroups: rec.scuba