Nitrogen makes up approx. 80% of the atmosphere (at all levels) while Oxygen makes up 20%. There are some other trace gasses.
Oxygen is keen to react with other elements, and in the lungs it reacts with Haemoglobin to carry the oxygen around the blood steam. Nitrogen on the other hand has a tripple covalent bond and as such is pretty musch inert, so it doesn't react with blood, but it does dissolve in it - a big difference.
When Carbon Dioxide dissolves in Coke, and you open the bottle (i.e. reduce the pressure) you get bubbles of COČ coming out of solution, in the same way when you reduce the pressure in blood, you get bubbles of NČ coming out of solution.