Mailing List Archive Mailing List Archive
List, A friend of mine asked me to repost his response to this, he is a diving physician and anaestheseist. Re: [Fwd: ppO2 and off-gassing N2] Datum: Fri, 5 Jan 2001 19:38:59 +0100 Von: "Bjoern Mehlhorn" <b.mehlhorn@gm*.de*> Firma: TEL:0700-MEHLHORN;SMS:01734199969 An: mat.voss@t-*.de* (Matthias Voss) Hallo Matthias, bitte poste doch meine Mail in die NG - ich habe sie gerade nicht abonniert und werde daher wohl auch nicht posten dürfen, solange ich nicht mitlese. > except that I can't quite understand why the high ppO2 in the > blood helps the Nitrogen come out of the cells. Oh - I will tell you. > I accept that it has proven to be better, but would like to > understand the theory as to why. Very easy. > Bill Wolk defined the Oxygen Window as used in diving theory >as "the difference between the concentration of inert gas >dissolved in tissue vs. the concentration of oxygen and inert >gas in arterial blood". That is a good theory. > Breathing an elevated ppO2 forces extra O2 to dissolve into >the blood plasma, in addition to that normally carried bound >to Hemoglobin. So far I'm with the program. O.k. > But what I can't see is how this additional O2 affects the N2 >coming out of the tissues. When there is no N2 in the arterial >blood anyway, why does forcing additional O2 into it make a >difference? When the surrounding pressure in the chamber rises, the toal pressure in the tissue or in the nitrogen-bubbles rises, too! > According to Henry's law the amount of Nitrogen that will >dissolve into the blood is affected only by the partial >pressure of the Nitrogen itself, Yes - and this pressure goes up when the pressure in the >chamber goes up. So more of the bubble-nitrogen dissolves back >in the blood. > so increasing the ppO2 will have no effect on that. No - not itself. The pressure itself is the fact, why the pressure in the bubbles rises. You can take any nitrogen-free athmosphere around the patient to rise the pressure - even in his bubbles. > A previous posting (which I can't find now, but I don't think >anyone argued against it) indicated that increased ppO2 will <tend to block the Nitrogen molecules from coming out of the >cells by osmosis. I will argue against it - that is not the way. There is no osmosis by gas. Gas can go freely on each side of the membrans of the cells. So there is no osmosis. Osmosis occurs, when a part of the fluid can´t pass the membran of the cell. > The only beneficial effect of the additional O2 that I can >see is that (again from a previous Bill Wolk posting) "the >additional osmotic pressure forces O2 into the surrounding >tissue, That has nothing to do with osmosis but with diffusion. > Is that the only benefit of the higher ppO2, or is there >something other way in which it aids off-gassing N2? Yes - I explained it up there. The models of diffusion work with the DIFFERENCE between the pressure of the gass inside the bubble and the pressure in the blood. And this pressure is maximal, when the pressure in the bubble is maximal and the pressure in the surrounding plasma is nearly zero. And this nitro-zero-pressure can be made with any nitrogen-free gass (heliox (see COMEX-deco-tables), oxygen etc.). HTH Bjoern info@bj*.de* Viele Gruesse Bjoern -- Übers Ziel hinausschießen ist ebenso schlimm, wie nicht ankommen. (Konfuzius) Alex Vasauskas schrieb: > > Ian: > > You might find the answers you are looking for at > http://home.adelphia.net/~robworld/Bubble_Decompression_Strategies.htm and > the section on hyperbaric oxygen therapy at > http://www.mtsinai.org/pulmonary/books/scuba/sectionh.htm > > > Reading the stuff previously posted on the Oxygen Window - especially Bill > > Wolk's postings which are very helpful - it almost all makes sense, except > > that I can't quite understand why the high ppO2 in the blood helps the > > Nitrogen come out of the cells. I understand the advantages of > > 100% O2 over > > 80/20, but can't fully see why breathing 100% O2 at 20' is better than > > breathing 100% O2 at 10'. I accept that it has proven to be better, but > > would like to understand the theory as to why. -- Send mail for the `techdiver' mailing list to `techdiver@aquanaut.com'. Send subscribe/unsubscribe 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]