DIVING & WELLNESS: Dive decompression theory II

DIVING & WELLNESS: Dive decompression theory II

Tissue saturation and pressure gradients

During the previous PART I blog on dive decompression theory‚ we talked about Paul Bert and looking specifically at nitrogen we saw that the solubility of nitrogen increased at depth during a dive‚ as a result of a rise in the ambient pressure. This solubility decreased when we started to surface from depth during our dive. Briefly‚ we concluded that during a dive to depth a diver has an ongassing of nitrogen of its body tissues and an offgassing of nitrogen as this diver starts to move to shallower depths and surface.

To better understand how this ongassing and offgassing of gasses takes place and what the terms unsaturation‚ saturation and supersaturation mean‚ we need to have a closer look at our body tissue level during our dive. Let’s zoom in and find out.

As we saw during the last blog‚ when a diver descends during his dive‚ the nitrogen partial pressure increases due to the increase of ambient pressure (Dalton’s law). As a result‚ nitrogen is dissolved in the body tissues as it becomes more soluble (Henry’s law). We can also translate this solubility back to a tissue pressure of that gas‚ which simply is the partial pressure the gas exerts on the liquid it is dissolved in. Looking back at Henry’s law:

Solubility = (Henry';;;s costant) * (Gas Partial Pressure = Gas Tissue Pressure)

I will use the term "gas tissue pressure" more and more in my further explanations as it is easier to understand how saturation and equilibrium of tissues work by comparing pressures with pressures instead of pressures with solubilities.

So‚ taking back the data we calculated from the example we used before diving from the surface to 30m depth‚ we have the following (values of saturated tissue):

Now it is important to realize that the uptake of nitrogen by an increased solubility‚ is not an instant process! It takes time for nitrogen to dissolve into the tissue. Nitrogen continues to dissolve into the body tissues until it finally will reach that value we calculated of 19.3* 10^-4M. This corresponds to a nitrogen tissue pressure of 3.16 bar‚ which is our nitrogen partial pressure in the air we breathe. The moment nitrogen tissue pressure reaches 3.16 bar‚ we can conclude we have a situation of equilibrium and the tissue is saturated. 

But as said‚ lets zoom in and let’s follow the journey of nitrogen molecules during a dive to 30m. Take a look at the schematic picture below‚ where the exchange of nitrogen between the alveolus and lung blood capillary vessels is represented before and during our dive.

The alveoli of the lungs are the vesicles where all lung airways end up in and the final destination of the air we breathe in. At this point‚ molecules of gas are exchanged between the blood and the air we just breathed in. The classical exchange is of oxygen being loaded into the blood and carbon dioxide being exported out of the blood in the air in the alveolus. The air in the alveolus is then refreshed as we breathe out and back in again‚ making sure there is a constant import of oxygen and export of carbon dioxide of the body.

There are 4 situations depicted in the schematic picture at different depths and moments of our dive. 

Situation 1‚ Tissue saturation: This represents the moment before we start diving and we are at the surface. As we calculated before‚ the partial pressure of nitrogen in the air we breathe in is 0.79 bar‚ so in the alveolus we have a nitrogen partial pressure of 0.79 bar. The tissue pressure of nitrogen is also 0.79 bar. Nitrogen molecules move around. Some molecules will go from the alveolus into the blood vessel‚ but also some molecules of nitrogen will move from the blood vessel into the alveolus. As we can see in the picture‚ the nitrogen molecules that move from alveolus to blood vessel equals the nitrogen molecules that move from the blood vessel to the alveolus. As such‚ we find ourselves in a situation of equilibrium‚ where the “push” of nitrogen into the blood vessel of 0.79 bar equals the “push” of nitrogen out of the blood vessel of 0.79 bar.

Situation 2‚ Unsaturated tissue: We have started our dive and after 5 min we find ourselves at 30m depth. As we calculated before‚ the partial pressure of nitrogen in the air we breathe in here is 3.16 bar‚ so in the alveolus we have a nitrogen partial pressure of 3.16 bar. We started our dive with nitrogen dissolved in our blood that exerted a tissue pressure of 0.79 bar. The result will therefore be that the “push” of nitrogen into the blood vessel of 3.16 bar will be higher than the “push” of nitrogen out of the blood vessel of 0.79 bar. In the figure some minutes have already passed‚ so some nitrogen has already been “pushed” into the blood stream. The more nitrogen gets dissolved into the blood‚ the higher the tissue pressure becomes. The tissue pressure is at the moment of the figure of 1.98 bar. This 1.98 bar is still lower than the 3.16 bar in which the nitrogen gets “pushed” out of the alveolus. We can see this as well in the picture that more nitrogen molecules move out of the alveolus than out of the blood vessel. The blood vessel will keep ongassing nitrogen molecules as until it will reach a nitrogen tissue pressure equal to the nitrogen partial pressure in the alveolus of 3.16 bar.

Situation 3‚ Tissue saturation: We’re at 30 min dive time and we find ourselves still at 30m depth. The partial pressure of nitrogen in the air we breathe is 3.16 bar and finally nitrogen has dissolved so much into the blood vessel that the tissue pressure has reached 3.16 bar as well. The result will therefore be that the “push” of nitrogen into the blood vessel of 3.16 bar equals the “push” of nitrogen out of the blood vessel of 3.16 bar. The nitrogen molecules that move from alveolus to blood vessel equals the nitrogen molecules that move from the blood vessel to the alveolus. As such‚ we find ourselves in a situation of equilibrium again‚ where the “push” of nitrogen into the blood vessel of 3.16 bar equals the “push” of nitrogen out of the blood vessel of 3.16 bar.

Situatione 4‚ Tissue supersaturation: We’re at 35 min dive time and we have started to ascend from our dive and now find ourselves at 10m depth. The partial pressure of nitrogen in the air we breathe is 1.58 bar. We started our dive ascend with nitrogen dissolved in our blood that exerted a tissue pressure of 3.16 bar. The result will therefore be that the “push” of nitrogen into the alveolus from the blood vessel of 3.16 bar will be higher than the “push” of nitrogen out of the alveolus of 1.58 bar. In the figure some minutes have already passed‚ so some nitrogen has already been “pushed” into the alveolus. The more nitrogen gets out of the blood solution into the alveolus‚ the more the tissue pressure decreases. The tissue pressure is at the moment of the figure of 2.33 bar. This 2.33 bar is still higher than the 1.58 bar in which the nitrogen gets “pushed” out of the alveolus. We can see this as well in the picture that more nitrogen molecules move out of the blood stream into the alveolus than vice versa. The blood vessel will keep on offloading nitrogen molecules as until it will reach a nitrogen tissue pressure equal to the nitrogen partial pressure in the alveolus of 1.58 bar.

Now we can better understand that the uptake and release of nitrogen takes its time. How fast the nitrogen molecules will diffuse from one environment to another‚ depends in part on how big the “push” of partial pressure difference is. This is called the pressure gradient. The pressure gradient is the difference between the tissue pressure of nitrogen dissolved in the body tissues and the atmospheric partial pressure of nitrogen.

From our figures we can conclude that the pressure gradient between the alveolus and the blood in situation 2 is:

N2 partial pressure alveolus – tissue pressure blood = pressure gradient

3.16 bar – 1.98 bar = 1.18 bar.

What we have seen depicted can be translated to a few terms that are often used in diving decompression theory:

Unsaturated tissue: nitrogen tissue pressure < nitrogen ambient partial pressure. Resulting in nitrogen ongassing.  

Saturated tissue: nitrogen tissue pressure = nitrogen ambient partial pressure. Resulting in a nitrogen ongassing and offgassing equilibrium.

Supersaturated tissue: nitrogen tissue pressure > nitrogen ambient partial pressure. Resulting in nitrogen offgassing.

Lastly‚ lets zoom out again and see the summary of what we just learned in our dive profile picture:

Ok‚ now save this information. You’ll need it in understanding Haldane’s decompression models‚ which we’ll talk about in the next blog!

Alla prossima! 

Esther