Depending on the ambient pressure, gases are released to a different degree from the breathing air into the human body. The proportion of these gases in the body is designated as tissue saturation. The higher the pressure is, the higher the tissue saturation and vice versa. When the ambient pressure changes, the body needs some time though until the saturation adapts to the new circumstances.
Why is tissue saturation so important when diving?
If the ambient pressure changes, then the saturation of the tissue also changes with released gases. Especially nitrogen is released in large quantities in the body. The deeper you dive, the higher the ambient pressure becomes under water and the more nitrogen is released into the body. When ascending, the pressure sinks again and the tissue is then oversaturated. This means that too much nitrogen is released for the current ambient pressure. Therefore, an adjustment must take place again – since this adjustment can take up to a few hours though, the reduction of the saturation must happen slowly.
So if you ascend too quickly, decompression sickness can occur, because the released gas forms bubbles. This process is to be compared with opening a shaken mineral water bottle too quickly. If you open it slowly, the gas can escape. Whereas if you open it suddenly, it foams over.