Air consumption - nitrogen absorption

[tulambi]

I was always wondering if the tables I was given (RDP for example) take into account the volume of the consumed air. I’m extremely good at saving my air and I usually get out of the water having consumed half of the tank. My concept is that if I consume less air then I absorb less nitrogen which decreases the likelihood of DCS, correct?

 

[wedivebc]

Although gas is absorbed through the lungs, tissue gas exchange is a factor of pressure and time and has little to do with how much air you consume.. It doesn't matter how little you breather the gas pressure in your lungs will be essentially the same for a given depth.

 

[tulambi]

Still don't get and correct me if I'm wrong.
If time is a factor and considering that pressure is constant will my tissues absorb the same nitrogen if over the same time I take 2 breaths instead of one?

 

[wedivebc]

Still don't get and correct me if I'm wrong.
If time is a factor and considering that pressure is constant will my tissues absorb the same nitrogen if over the same time I take 2 breaths instead of one?

Yes because you don't eliminate all the gas from your lungs when you exhale. The gas exchange is always taking place whether you breath in or out. So it doesn't matter how often you breath in and out just how deep you are and how long you are there.

 

[hlsooner]

During respiration, the gas in your lungs changes by a few percentage points as oxygen is taken in and CO2 is expelled. At normal atmospheric pressure, the amount of nitrogen dissolved in your body is at equilibrium with the partial pressure in the atmosphere. As such, there should be practically no exchange of nitrogen during normal respiration.

Now consider the same cycle of breathing at depth. You are breathing in a gas mixture that has a higher partial pressure of nitrogen than your body normally sees. The result being that nitrogen enters through your lungs purely as a function of time and the surrounding pressure (I will omit the thermodynamics explanation). Unlike oxygen and CO2, which are being exchanged in the lungs due to cellular processes, nitrogen is inert and is only moving in and out due to time/pressure. Thus, if you take ten breaths in the same amount of time in which another diver takes five breaths, both divers' lungs are still exposed to the same pressure of nitrogen, for the same period of time.

If this were not the case, and the amount of nitrogen one absorbed depended on actual gas consumption, all dive computers would have to be air integrated in order to tell you your no decompression limit.

 

[CameronMartz]

There is a little more to this question than what the tables use to make their calculations. Gas diffusion is dependent upon the difference in pressure between inspired gas and dissolved gas. Tables and the computers that use them are very crude estimates of gas loading that only "know" how deep you are and how long you stayed there. They know nothing about your physiological state- are you working hard? are you cold? are you dehydrated? etc. These and other physiological factors all have a direct impact on the rate of on-gassing and off-gassing experienced by a diver.

A higher gas consumption usually means a higher metabolism, which usually means that you will indeed on-gas at a higher rate than on a dive where you have a lower gas consumption/metabolism. This is because, as your metabolism is raised, your blood flow to the various tissues in your body increases, which in turn, increases the partial pressure differential between gasses dissolved in the blood and gasses dissolved in the tissues.

Likewise, if you maintain a more meditative state during your dive, you are likely to reduce the partial pressure differential between your blood and your tissues, slowing on-gassing.

Skip breathing, however, where you artifically reduce your breathing rate while loading up on CO2, is a very unsafe way to dive. One should not think about breathing slowly, per se, as much as thinking about moving minimally and efficiently in the water. Your breathing rate should take care of itself.

Though the standard tables are well "padded," it is always prudent to minimize your effort as much as possible on a dive.

Cameron

 

[CameronMartz]

Thus, if you take ten breaths in the same amount of time in which another diver takes five breaths, both divers' lungs are still exposed to the same pressure of nitrogen, for the same period of time.

This would be accurate except for the difference in refresh rates, both of inspired gasses and in blood flow. Faster breathing keeps the average partial pressures of inspired gasses higher, and faster blood flow keeps the average partial pressures of dissolved gasses at the blood/lung barrier lower and at the blood/peripheral tissue barrier higher. This is a variable physiological process, resulting in a variable rate of diffusion independent of depth.

Even if we were to come up with a coefficient using gas consumption (a difficult process in itself), it would still be an average of many, many variables and not likely more accurate than using your head to adjust on the fly.

Cameron

 

[grumpymedic]

Tulambi..The real question you need to ask is why do I keep getting out of the water with half a tank of air? USE IT UP!!!

 

[Notso_Ken]

Still don't get and correct me if I'm wrong.
If time is a factor and considering that pressure is constant will my tissues absorb the same nitrogen if over the same time I take 2 breaths instead of one?

Only if you were absorbing a significant amount of the nitrogen that you pulled into your lungs with each breath.

When you breath, you absorb a VERY tiny amount of the nitrogen actually in your lungs. So, you ALWAYS have a large amount of N2 in the lungs ready to be absorbed. It matters not one whit how much you actually breath (within the range of normal human breathing, of course.


Ken

 

[Dr Deco]

The readers give a good response here. So little nitrogen is removed by the blood that the amount in the lungs is unchanged. It is essentially independent of breathing rate.