Decompression sickness
If the diver goes up slowly the pressure will change slowly, and there will be enough time for the gas dissolved in the diver to return from the blood to the breathing gas in the divers lungs and it will be safely removed. If the pressure change is too quick for enough of the dissolved gas to be removed, the gas may form bubbles in the blood and other parts of the diver's body. These bubbles will get bigger when the pressure gets less as the diver gets nearer to the surface. Bubbles which are in the blood may get stuck in small blood vessels. If this happens in the lungs they will usually do no great harm but if they block the blood flow in other parts of the body, the parts where the flow is blocked will not get enough oxygen and may be damaged. This is very dangerous if it happens in the brain or heart. The other place where bubbles may form is in the more solid parts of the body. If these bubbles grow enough they may tear the parts around them or press against them hard enough to do damage.