Mailing List Archive Mailing List Archive
CHKBOONE@ao*.co* wrote: > In your example, (18/30 & 50/50) there is actually more N2 in the > bottom mix than the deco gas so there will be little difference where that > is concerned. However, you would be switching from 50% O2 at a PPO2 > of say 1.4 to 18% O2 but the PPO2 of the blood plasma and the tissues > will continue to rise under the influence of ambient pressure before it can > equalize with the new mix PPO2. The partial pressure of gases already dissolved in tissue won't change purely as a result of a change in ambient pressure, except via exchange in the lungs. If you switch gases and immediately flush your lungs with the new mix, dissolved gas PPs will immediately start moving towards equilibrium with the new mix (obviously a moving target if you're descending). Admittedly it will take a breath or two to completely flush your lungs, during which time the fO2 will be boosted by the residual old mix, but that's unlikely to be significant unless you're a very shallow breather and I assume it's not what you're talking about. > Essentially you are carrying oxygen > stored in the blood and tissues beyond the MOD as a result of the delay > inherent in reaching equilibrium with a new alveolar PPO2. Doesn't matter. The O2 in blood and tissues was in (or approaching) equilibrium with the hot mix only at depths shallower than the MOD. It won't continue to rise as you drop past the MOD because by then it's chasing a different target. >The more > relaxed you are the longer the delay and the higher the dissolved PPO2 will > rise before leveling off and heading towards equilibrium. It's *always* heading towards equilibrium. That's what equilibrium is all about. So you will > always be physiologically (as far as gas loading is concerned) up to 2 > minutes behind any breathing gas variations due either to depth changes > or gas switches - and the lag will vary with circulation and perfusion. This 2 minute delay you're talking about is (I assume) the nominal time it takes for critical tissues (i.e. brain) to get reasonably close to equilibrium with the breathing gas (it ties in with an accepted value for human cerebral blood flow of about 50 ml/100g/min). That's different from the time delay for the equilibrium *target* to change when the breathing gas or depth changes. In the case of a gas switch the relevant delay is the time it takes to flush the old gas out of your lungs (plus the few seconds it takes for blood to travel from lungs to tissue). Of course if you hardly breathe at all under water that could be some considerable time :-) Wilson -- Send mail for the `techdiver' mailing list to `techdiver@aquanaut.com'. Send list subscription requests to `techdiver-request@aquanaut.com'.
Navigate by Author:
[Previous]
[Next]
[Author Search Index]
Navigate by Subject:
[Previous]
[Next]
[Subject Search Index]
[Send Reply] [Send Message with New Topic]
[Search Selection] [Mailing List Home] [Home]