Henry’s Law

The Gas Laws are very important to our understanding of decompression sickness, the way they work together in the body enables us to understand how nitrogen works when under pressure and how it effects divers.

Definition: ‘a law stating that the mass of a dissolved gas in a given volume of solvent at equilibrium is proportional to the partial pressure of the gas.’

As the depth increases, nitrogen in compressed air equilibrates through the alveoli of the lungs into the blood and then into the tissues. Over time the nitrogen dissolves and accumulates in the mainly aqueous tissues or those with a large blood flow e.g. the brain, and then progressively into the lipid or fatty parts of the tissues. On longer dives tissues will become ‘saturated’ and then will not take up any more nitrogen. As the diver ascends there is usually a lag before the saturated tissues start releasing the nitrogen. It is this lag that creates so many problems for divers.


When a large amount of nitrogen is dissolved in the tissues, reduction of the pressure when ascending causes the nitrogen to ‘outgas’ and form small bubbles in tissue cells, tissue spaces and the blood. Ascending too quickly causes the dissolved gas -nitrogen- to return to the gas form too quickly increasing the number and the size of the bubbles, while they’re still in the blood or the tissues it will cause damage and may be felt as symptoms of decompression illness. Further ascent in altitude may also contribute to bubble formation.

The average airline cabin is only pressurised to 0.8 atm. If a person flies too soon after diving, this additional decrease in the pressure may be enough to allow the bubbling or allow the bubbles already in existence to enlarge.

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