How do i control a pressure?
Such components certainly exist. They tend to be called I to P converters (current to pressure). I.E. you give them a signal in the form of a current, and they will regulate the air pressure according to that current.



If you're in the UK, you can buy them from Airlines Pneumatics. Using one of these will solve your problem immediately, but they can be expensive.

The alternative way to solve this problem is to control the pressure yourself. Since you already have a microcontroller, then you just need a pneumatic valve and an air pressure sensor. 

Valves come in two basic types, proportional (you have fine control over the on percentage) or digital valves (which are either just on or off). I recommend a proportional valve if you can afford it, and if you can be bothered to make the electronics to drive it. This valve from Sirai is quite nice:



By regulating the current through it, you can control how open it is, and thus the rate at which air flows into your tube.

Next you'll need a pressure sensor.



Measure the pressure (preferable at the other end of the tube to avoid various pneumatic effects caused by the moving air, like Bernoulli's principle, where the sensor will see a false drop in pressure as the air flows past it towards your tube.

Once you have your valve and sensor, the microcontroller can control the valve to control the pressure. I recommend something simple like a pure P controller.