 When you go down on the water, the partial pressure of the nitrogen in the air you breathe gets higher and higher because the pressure gets higher. Your body cannot use this nitrogen. So it simply dissolves into all of your tissues. As the partial pressure increases, more and more nitrogen dissolves in your tissues until they are saturated. When you go up again and the pressure becomes less, the nitrogen in your tissues has to come out again, of course. The difference in the partial pressure of a gas between two places is called the pressure gradient. So you can say that the pressure gradient of nitrogen in your breathing air and the nitrogen dissolved in your tissues becomes higher and higher when you go up. The difference becomes higher and higher. If the pressure gradient stays low enough, nitrogen can come out of your body without causing any problems. But if the pressure gradients become too high because you come up too quickly or you stay too long at a depth, then you might have a problem because this can cause big bubbles to form in your tissues and in your blood. You can think about opening a can of coke and bubbles forming in the liquid. But it's not quite as simple as that. Bubbles can only develop when there are tiny micro bubbles in the liquid, such as your blood, to form a round. These tiny micro bubbles are called gas micronuclea or gas seeds. For instance, if you pressurize a glass of pure water, no matter how quickly you release the pressure, no bubbles will form. There's nothing that can act as seeds. However, if you shake the water to make tiny bubbles before releasing the pressure, big bubbles can form around them. Now your blood is continuously being shaken in the arteries and the veins, so there are many micronuclea available. The quicker your blood flows, so the more you exert yourself before, during or after a dive, the more of these gas seeds. Very small nitrogen bubbles form during any dive. No matter how shallow or how short, this happens mainly in the capillaries. Usually this is not a problem because these small bubbles enter the veins and flow back via the heart to the lungs without causing any blockages. When they reach the pulmonary capillaries in the lungs, they diffuse into the alveoli with no problems and you breathe out the nitrogen. We call these bubbles, that cause no problems, silent bubbles. We can see silent bubbles with a Doppler ultrasound flow meter. Using this ultrasound machine and volunteers in hyperbaric chambers, we can find the safe limits of pressure and time. Because we can see when there are too many bubbles to be safe. If you accumulate a lot of nitrogen by staying too long at depth, or if you come up too fast, more nitrogen has to leave your tissues as you ascend and this will make the bubbles larger. When bubbles become larger, they can block the blood flow in narrow blood vessels and this makes the problem bigger. Because now it becomes even more difficult to transport the nitrogen to your lungs. The bubbles become larger and larger, blocking the blood flow more and more. Bubbles don't only form in your blood. They also form between tissues, particularly in watery tissues, such as your ligaments and joints. And as the bubbles grow, they can compress nerve cells and that hurts. To make things worse, your immune system attacks foreign bodies in the blood by forming platelets around them. So they also attack the bubbles. This increases the size of the bubbles and this causes further blockages. This is called decompression sickness. The good news is that nitrogen bubbles almost never go into your arteries. Because they usually diffuse through the alveoli in the lungs and are gone before the blood flows back to the heart and into your arteries and the rest of your body. It takes time for the bubbles to grow. So the signs and symptoms of decompression sickness do not usually appear immediately after a dive. They usually appear a few hours later and even can be up to 48 hours later. Symptoms often get worse slowly because the bubbles slowly get bigger and slowly flow to places where they block the bloodstream. This means most cases of decompression sickness are not immediately life-threatening. But in all cases of suspected decompression sickness, the diver must go to a doctor. There are two types of decompression sickness. Decompression sickness type one is the least dangerous and three-quarters of all cases of decompression sickness are of this type. The bubbles are usually in the joints, in the ligaments or underneath the skin. Bubbles under the skin appear as red blotches and are most frequently found under the skin of your shoulders and chest though they can be anywhere. It is itchy but it's not serious. If the bubbles are in a joint or a ligament they cause pain and make moving difficult. Divers with decompression sickness type one must go to a doctor because the bubbles can get bigger and keep moving and become decompression sickness type two. Decompression sickness type two is rare but it's serious and can lead to death. The most common form of type two decompression sickness is nitrogen bubbles that interfere with the functioning of your central nervous system. In very rare cases bubbles are taken back to the heart from the lungs and are pumped to your arteries and onto your brain. They can even form in your brain directly, cerebral decompression sickness. These bubbles block blood flow in the brain and depending on the brain area where this occurs you can experience tonal vision, dizziness, confusion, tingling, numbness, loss of bladder control, difficulty with balance and in the worst cases paralysis. If bubbles block very important areas it can lead immediately to death. Nitrogen bubbles can very occasionally block blood flow back to the heart causing your blood pressure to drop which means that you'll become unconscious. About a quarter of people have a hole between the left and the right sides of their heart. This hole is called a patent for Raymond O'Valley, a PFO for short. We are all born with this hole because in the womb we do not breathe so the blood has to circulate without going to the lungs. After birth this hole normally closes in a few days. If this does not happen perfectly only a flap of tissue forms over the hole but this does not normally cause problems. However if a diver with a PFO has big nitrogen bubbles they can pass through the hole and enter the arteries. And then they can travel anywhere in the body including the brain. Nitrogen bubbles that affect the lungs, pulmonary decompression sickness, are rare because bubbles usually dissolve in the alveolar but if there are too many bubbles they can cause a blockage. This irritates the lungs causing shortness of breath and coughing. The risk of decompression sickness is very very small when you dive within the limits of your table or computer. The chance of getting decompression sickness on a recreational dive is one in ten thousand and the chances of suffering from life threatening decompression sickness are even smaller. Most cases of decompression sickness are caused by diver error. If you follow the rules and stay within the limits you really have nothing to worry about. The majority of cases of decompression sickness occur after technical dives because these dives are deeper and longer and there is a lot more that can go wrong. Why do nitrogen bubbles cause decompression sickness and not oxygen bubbles? The first reason is that your body uses oxygen and so it is quickly taken out of the bloodstream by your body cells. The second reason is that most of the oxygen is attached to your red blood cells rather than dissolved in your blood plasma and so it cannot form bubbles. The third reason is that more nitrogen than oxygen can dissolve in the blood without saturation. Twice as much oxygen than nitrogen can dissolve in your blood and tissues without any problem. And the fourth reason is simply that there is a lot more nitrogen than oxygen in the air that you breathe.