In the last blog post, I explained the oxygen window in detail. We have established that after the metabolization of oxygen the higher solubility of carbon dioxide (CO2) compared to oxygen (O2) leads to a lower overall pressure in the tissue compared to the blood or alveoli. This pressure difference increases, the higher the oxygen partial pressure in the respiratory gas. This is the basis of the so-called “inherent unsaturation”, where the tissue pressure is always slightly lower than the ambient pressure - a prerequisite for theoretically bubble-free decompression, which, however, is hardly feasible in diving. We have also found that oxygen-rich breathing gas mixtures increase the oxygen window, but do not shorten the decompression time. Nevertheless, the oxygen window plays an important role in diving - the current blog post explains exactly why.

When diving, we "live" on dissolved oxygen
To illustrate the oxygen window, take another look at the graph from the last article (Figure 1): It shows the oxygen window when metabolizing 5 volume-% of oxygen in each case, once at normal pressure (inspiratory PO2/PiO2 0.2 bar) and once when breathing 100%-O2 at a depth of 5 m (PiO2 1.5 bar). In the first, steeply rising part of the curve, the oxygen partial pressure drops less sharply, as the oxygen demand is covered by oxygen bound to hemoglobin. In the horizontal part, however, the oxygen demand is completely covered by dissolved O2. Therefore, the O2 partial pressure drops correspondingly sharply, the oxygen window opens and is much larger with 100% O2 breathing during the decompression stop at 5 m than with air breathing on land. It reaches its maximum at approx. 2.2 - 2.3 bar O2 partial pressure and then remains constant.

Figure 1
Some bubble physiology
What are the specific effects of the oxygen window when diving? First and foremost, it helps to prevent the formation of bubbles or to eliminate existing bubbles. For this purpose, we imagine a gas bubble that has formed in the tissue during the dive and observe it after the dive at 1 bar ambient pressure.
The total pressure of a bubble is calculated from the partial pressures of the gases according to Dalton, i.e.
pBbubble = pN2 + pO2 + pCO2 + pVapor.
The nitrogen partial pressure therefore corresponds to
pN2 = pbubble – pO2 – pCO2 – pVapor.
The total pressure of the bubble must be at least equal to the ambient pressure (in this case 1 bar), otherwise it would not exist or could not displace the environment. In fact, we know that the pressure of a bubble must be even higher than the ambient pressure. For this example, however, we assume that the bubble pressure is equal to the ambient pressure.
The partial pressure of nitrogen in the bubble at an ambient pressure of 1 bar and breathing air is calculated according to the above formula
pN2 = pbubble (1.0 bar) – pO2 (0.04 bar) – pCO2 (0.06 bar) – pVapor (0.06 bar) = 0.84 bar.
This is higher than the partial pressure of nitrogen in the tissue (0.74 bar). This means that there is a tendency for nitrogen to diffuse from the bubble into the tissue along its gradient (Figure 2, left). This causes the bubble to shrink.

Figure 2. Oxygen window at 1 bar ambient pressure under air respiration or respiration of 100% oxygen. Values idealized.
If 100% oxygen is breathed, there are almost no changes in the O2 or CO2 partial pressures. The additional supply of oxygen is largely metabolized. As a result, its dissolved content in the tissue hardly increases. The partial pressures of oxygen and CO2 remain practically constant in the tissue as a result of metabolism despite 100% oxygen respiration or an increase in ambient pressure.
In contrast, the partial pressure of nitrogen in the tissue theoretically drops to 0, as nitrogen is no longer inhaled. (This depends on the tissue, of course. A very “fast” tissue, e.g. brain, will behave like this after a few minutes, a very “slow” tissue, e.g. tendons, will still have a significant nitrogen partial pressure after a while). This maximizes the gradient between the nitrogen partial pressure in the bubble and the tissue. This leads to an increase in the diffusion of nitrogen from the bubble (Figure 2, right). The bubble shows an even greater tendency to shrink than under air breathing. In the sum of effects, this is a direct consequence of the oxygen window.

If the ambient pressure increases, the partial pressures of all partial gases in the bubble increase proportionally according to Dalton. If high-percentage oxygen is breathed under these conditions, the gradient between the partial pressure of nitrogen in the bubble and the tissue increases further (Figure 3). The tendency of nitrogen to diffuse out of the bubble increases and so does the tendency of the bubble to shrink. This is the case when we decompress with 100% oxygen.
As we have seen in Figure 1, the oxygen demand is completely covered by the dissolved oxygen at an oxygen partial pressure of approx. 1.5 bar. This means that the oxygen partial pressure in the tissue remains constant at the level of air respiration at 1 bar ambient pressure, i.e. at 0.04 bar, despite breathing 100% oxygen at a water depth of approx. 6 m.
The oxygen window helps to prevent decompression sickness.
The oxygen window helps to shrink bubbles and thus helps to prevent decompression sickness or supports its treatment, e.g. as part of hyperbaric oxygen therapy in a hyperbaric chamber. This is the real benefit of the oxygen window.
Resolving a misconception
The confusion caused by the oxygen window is probably due to the fact that the above formula shows that the nitrogen partial pressure in the bubble is higher than in tissue. This phenomenon is erroneously applied to venous blood, but only holds true for a gas bubble. There are scientific-looking articles circulating on the Internet claiming that venous blood has a higher nitrogen uptake capacity due to the enlarged oxygen window. This narrative even goes so far as to claim that the partial pressure of nitrogen in venous blood is higher than in tissue. However, this contradicts the basic laws of physics, because nitrogen diffuses passively along its gradient from the higher tissue partial pressure to the lower partial pressure in the venous blood. The erroneous assumption mentioned above would therefore be equivalent to the idea that water flows uphill.
What applies to bubble formation does not hold true for venous blood. Although a large oxygen window can reduce the formation of bubbles, it has little significance for decompression itself. The driving force for decompression is the difference in nitrogen partial pressure between tissue and alveoli, and the oxygen window does nothing to change this.
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