Oxygen Tank Progress Bar

Is there a way to make a bar made out of a GUI texture slowly scale down along the X Axis over time?
I basically have a 3 layer GUI set up where there is an overlay and a background bar that I would like to have scale down slowly over time to indicate that something like Oxygen is running out.

I was looking at how health bars are made but non of them seemed to be the sort of thing I am after. Does anyone know of an easy way to do this?

Are you version constrained in Unity? If not I would upgrade to Unity 4.6 and use a Slider. This makes the job trivial, simple set Slider.value to whatever number you like.