Just as strawberries, bananas, and ice can come together to make a delicious smoothie, oxygen, nitrogen, and argon, with a touch of carbon dioxide, mix together in the air that we breathe. A massive portion of the air we breathe, 78 percent is nitrogen gas. Despite making up such a large portion of the air, our bodies don't use it for much of anything. Unlike oxygen gas, which is used to make ATP and is converted into water, we breathe in 78 percent nitrogen gas, and we breathe out the same 78 percent nitrogen gas. For the most part, it's just taking up space. But like oxygen, it's under pressure and so it too will dissolve into our blood. Well, pressure determines the total amount of gas that dissolves into a liquid. The amount of nitrogen, compared to the amount of oxygen that dissolves into liquid, depends, in part, on the partial pressure of each gas. Partial pressure not only cares for the total atmospheric pressure, but how much each gas contributes to it. While the total pressure at sea level is 1 atmosphere, each gas is contributing a different amount towards the same 1 atmosphere. Since the air is 78 percent nitrogen, 78 percent of the collisions against a surface will come from nitrogen. That means that of the 1 atmosphere of pressure, 78 percent comes from nitrogen. Nitrogen then has a partial pressure of 0.78 atmospheres. Likewise, oxygen, which makes up 21 percent of the air, only has a partial pressure of 0.21 atmospheres. The remaining 1 percent of air, and the point 0.01 atmospheres is made up of argon, carbon dioxide, and other trace gases. Remember also that pressure causes gas to dissolve into liquids in a process known as absorption and that, naturally, gas dissolves out of liquids in desorption. When the rates of absorption and the rate of desorption are equal and opposite, the amount of gas dissolved in the liquid remains constant. If we increase the pressure, we will increase the amount of gas that can dissolve into the liquid before these rates are equal again. The same is true for partial pressure. Because partial pressure indicates the frequency that a gas is bumping against the surface of a liquid, increasing the partial pressure of that gas will cause more of that gas to dissolve into a liquid until the equilibrium point is reached. There are two ways to raise the partial pressure of a gas. Either increase the total pressure or increase the percentage of the gas in the air. Increasing the total pressure will involve all gases increasing the amount they are dissolved into a liquid. In the second case, if nitrogen is being replaced by oxygen, nitrogen will slowly dissolve out of the liquid and more oxygen will replace it. The last important clue that will help us answer our question is that, under pressure, it's not just our lungs that feel the effect. Since our whole body is exposed to outside pressure, everything inside also needs to fight that pressure. Our sinuses, or pockets of air in our face, need to have pressure that resist the outside force. Additionally, blood pressure increases as the outside environment squeezes harder. That means an increase in external pressure caused by compressed air from a caisson or from going under water when scuba-diving will result in higher pressure on the whole body. This will cause the partial pressure of any air to increase and cause the air to dissolve further into the blood, muscles, and neurons. This process of dissolving doesn't present a problem because the gas is kept in check by high external pressure. However, when the pressure drops especially quickly, the gas wants to desorb and leave the blood and tissues. This isn't a problem as long as there a place for it to escape.