Scuba divers suffer decompression illness in which during sudden ascension, gas bubbles form in blood vessels and do not get reabsorbed quickly, leading to various problems including a stroke. The mechanism is as follows. At high pressure, more nitrogen gas in the air can be dissolved in the blood. This is due to Henry's law, which states that with greater partial gas pressure exerted onto the liquid, more gas is dissolved. When the partial pressure of nitrogen gas returns to normal, solubility of the blood decreases, leading to the formation of gas.
From above, it seems that the air that blood is exposed to (the lung's alveoli in most cases) changes in the partial pressure of nitrogen gas. Another study isolated the effect of partial pressure by conducting an experiment in which the subject's breathed air was changed from heliox (combination of helium and oxygen) to nitrox (combination of nitrogen and oxygen) without change in total pressure and this led to bubble formation.
However, it is hard to imagine that the pressure of the air breathed in by a scuba diver would change, due to the fact that air is brought from an oxygen tank that seems to be independent of outside pressure. Thus I ask the question: if the air breathed in by the diver does not change with depth, how does nitrogen partial pressure exerted on the blood change?