Blood circulation, ascents, nitrogen, etc.

[Charlie P]

Over the last 25 years, the importance of a slow ascent at the end of a scuba dive has become evident due to DCS cases that were obviously caused by fast ascents. When I began diving 25 years ago, we ascended straight to the surface at 60 fpm after a dive approaching the NDL. Now we are advised to ascend at 30 fpm and make a "safety stop" at 15 feet. Much more conservative rules these days.

One reason given for the slow ascent is to allow off-gasing. Although I believe this is a valid reason, I have also wondered about the length of time it takes for a person's blood to make a "circuit" from the lungs, through the body and back. What is this time? Does a person's blood have varying levels of N2 throughout the body during an ascent because of this "lag"?

I also have wondered if the importance of the slow ascent may not rely only on the off-gassing through the lungs, but may be even *more* relative to the transfer of nitrogen from the blood to slower tissues (and the reverse), which probably would not be so dependent on the "blood circuit time". If this were true, then the slow ascent might be more critical after a long dive at a mid-range depth (e.g. 40 minutes at 80 feet) than after a short deep dive (10 minutes at 130 feet) because of higher N2 loads in the slower tissues.

What is your opinion on these issues?

Charlie P

 

[Dr Deco]

The circuit time of the blood through the tissues is actually quite rapid, although the volume of blood to volume of tissue ratio can be small. This accounts for the long compartments as originally proposed by Haldane.

The rapid ascents are more pronounced in their effect through surface tension. This “confining pressure” causes the bubbles to remain small and prevents the influx of gas during the ascent.

I will prepare a more complete response and enter it in a short time.
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The slow ascents are more important in the short deep dives. All of this deals with the growth of gas bubbles by inward diffusion (into the bubble nuclei)of dissolved nitrogen from the tissues. As long as the nuclei stay small, the surface tension squeezes them and the gradient (= driving pressure) from tissue to bubble is small. When the ascent is rapid, the bubble grows and the internal pressure (Laplace pressure generated by the surface tension) is lower.

When this is true, the driving pressure gradient from tissue into bubble is larger and the bubble grows bigger and stays bigger while the remainder of the dissolved nitrogen is dumped into the circulatory system. The greatest driving force for the elimination of nitrogen occurs when the bubbles are kept small. This has been demonstrated in diving experiments, model systems (gels), and Doppler bubble detection studies.

[Edited by Dr Deco on 10-23-2000 at 01:47 PM]