fish, marine mammals don't get bent

[Dr Deco]

Dear merkin:

Diving Mammals

As BillP indicated above, we have discussed this before – but that does not mean that everyone has read the replies. When you check out the previous material, you will find that breath hold diving does not replenish the supply of nitrogen in the lungs. This supply is compressed during the dive and the nitrogen is distributed throughout the tissues of the body. The fact that the heart rate is slowed , and the nitrogen is limited will reduce the dissolved nitrogen available fro bubbles.

We add to this is the question of number of micronuclei in the tissues, and the possibility that repetitive diving has a protective effect through some biochemical pathway (possible associate with nitric oxide).

Dr Deco :doctor:

 

[The Iceni]

As Dr Deco has already stated, this has been answered before, but I hope I am clarifying what has been posted above by making the following comment.

1) Gills

As Rick Murchison points out, fish, crusacea and most other forms of sea life respire through gills taking in oxygen (and the other gasses) at it's pressure in solution not at the much greater pressure that would exerted by an ambient gaseous phase (as is provided by scuba).

There is really only one place from which the repiratory gasses can come and that is the surface; by simply diffusion. For all practical purposes the pp O2 of nitrogen in sea water at the depth where most marine life is found is 0.8 bar. (Because of the diffusion distances water in the greatest depths of the ocean contains less dissolved atmoshperic gasses but that's another story!)

Thus since fish, crabs and lobsters can never aquire a dissolved tissue ppN2 greater than 0.8 bar there can be no overpressure gradient from which bubbles (and DCI) may form during any ascent. (Swim bladders- a different story which is not DCI.)

2) Lungs

With lungs, on descent, nitrogen is forced into solution by simple diffusion from the gaseous phase which is at ambient pressure; Scuba gear provides a continuous supply producing what may become superstaturation on the ascent.

In cetaceans and other diving mammals and when humans free dive the nitrogen within the lungs is, of course, forced into solution because of the increase in ambient pressure, but there is a limited supply - one "lungful" alone, which will indeed dissolve but as it dissolves the "supply" in the lungs is reduced and very rapidly exhausted so the final tissue dissolved partial pressure is not significantly raised.

While I am no marine biologist (who picks up hikers) I imagine that, like free divers, marine animals perform "V" profiles (or perhaps more correctly "U" profiles). In which case all dissolved gasses will have the same time to migrate out of the tissues and into the lungs as they did when ongassing during the descent. (I admit this is a gross oversimplification.)

To my mind the important differences between diving mammals and scuba are a limited supply of nitrogen from the one lungful of gas and a very limited bottom time.

These are only my thoughts on the subject, I do not profess to be any form of expert and I am happy to stand corrected.