Diffusion and Decompression Sickness

‘Divers who have witnessed the writhing, screaming agony of a bad bends swear that they would rather suffocate and drown on the bottom than surface after a long, and deep dive without decompressing.’ – (Shadow Divers, 2004)

Diffusion is important to our understanding of decompression sickness as the rate of diffusion is influenced by the gas laws (Boyle’s Law, Daton’s Law, Henry’s Law). Gas laws and diffusion working together cause divers many problems underwater, including decompression sickness.

diffusion
Right now the air we breathe in enters the lungs, it dissolves into the lining of the lungs and diffuses through the lung membrane to the blood stream. Here it dissolves into the blood plasma. It diffuses into the red blood cells and binds to the haemoglobin. At the respiring tissue the oxygen disassociates and diffuses into the respiring cell. The oxygen is used in respiration to form CO2. The CO2 diffuses into the bloodstream and travels back into the lungs. The oxygen moved from where there was high concentration to low, carbon dioxide does this as well. Under pressure this also happens with the nitrogen that divers breathe in from their mixed gas air tank.Diffusion is the net movement of particles from low to high concentrations. This is essentially the particles spreading out; the greater the difference in concentration the faster the rate of diffusion. Pressure is a factor which effects the rate of diffusion. Diffusion is simply the movement of particles from high concentration to low concentration.

When Henry’s Law and diffusion work together in a diver’s body the nitrogen will build up as the tissues become ‘saturated’ with nitrogen and are unable to absorb any more nitrogen.

The body does not use nitrogen so what goes into the tissue will not come out of solution until the levels of nitrogen outside the tissues decreases. The only way to reduce the level of nitrogen in the blood stream is to ascend where the level of nitrogen in your blood stream decreases and the nitrogen is able to diffuse out of the tissues.

If the diver ascends slowly the nitrogen has time to diffuse out of the tissues and return to the lungs where it is expelled back into the environment, not causing any harm to the diver.

If the diver, however, comes back to the surface fast the bubbles will come out of solution but the nitrogen will expand. The nitrogen does not have time to come back to the lungs before the pressure is reduced and expands into gas bubbles in the tissues and veins. Large bubbles of nitrogen in the veins block blood flow and cause pain to the unfortunate diver. This is called Decompression Sickness.

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