… happens depening on the difference between the ambient pressure and the tissue saturation.
- On-Gassing: Lung gas –> Blood –> Tissue
- Off-Gassing: Tissue –> Blood –> Lung gas
The saturation of your tissues should be at surface level at the beginning of your dive. This isn’t the case if you haven’t taken enough rest time after your last dive. During the descend the ambient pressure increases for around 1 atm every 10 meter. If you are now breathing your blood will transport the dissolved gases, based on the breathed gas mixture (nitrox, air, trimix), to your tissues. The saturated oxygen gets consumed via cell metabolism, but can still possibly lead to oxygen bends initiated when ascending too fast (unproven yet)1. The inert gases are just getting stored until they are saturated for this depth/pressure (supersaturation). Depending on the inert gas and on the tissue compartment the on-/off-gassing rate will vary. Helium for example has an on-/off-gassing rate 2,65 times higher then nitrogen2. Tissue compartments with low blood flow have a slow rate and tissue compartments with a high blood flow have a fast rate. As soon as you arrive at the bottom part of your planned dive profile the on-gassing will still go on and can take quite a while for inert gases and tissue compartments with a slow on-gassing rate. This can still happen if you already started to ascend. Depending on that you will reach multiple points where different inert gasses at different tissue compartments will start to off-gas. This is the reason for the difficulty to calculate an ascend which isn’t generating bubbles and won’t take too much time/gas. Bubbles will occure if your body isn’t able to off-gas slowly enough and they will then possibly harm your body. If you have to do decompression stops during your ascend you can use different gas mixtures (rich nitrox, pure oxygen), depending on the depth, to accelerate the decompression.
1 Oxygen Bends (K. W. Donald), Link to the article
2 Based on Graham’s law of diffusion: (Rate N2)/(Rate He) = √(M(N2)/M(He)) = √(28/4) = √(7) ≈ 2.65