Henry's Law

The gas law stating that the amount of gas dissolving into a liquid is proportional to its partial pressure, the foundation of decompression theory.

Henry's law says that the amount of a gas that dissolves into a liquid is proportional to the partial pressure of that gas above it. Your body is mostly liquid, so at depth, where nitrogen's partial pressure is elevated, nitrogen steadily dissolves into blood and tissues. The deeper and longer the dive, the more inert gas your body loads. This on-gassing is invisible and painless, and it is the entire reason decompression theory exists.

The law runs in reverse on ascent. As ambient pressure drops, tissues hold more nitrogen than the surrounding pressure supports, and the gas comes back out. Done slowly, it off-gasses harmlessly through the lungs. Done too fast, it can form bubbles in tissue and blood, the mechanism behind decompression sickness. The classic picture is a soda bottle: open it gently and little happens, crack it fast and it fizzes.

No-decompression limits, safety stops, slow ascent rates, and surface intervals are all practical management of Henry's law. Your dive computer's algorithm is essentially tracking this dissolved gas across many theoretical tissues in real time.