How to find the function of a double integral from Cartesian coordinates to polar coordinates

Question

I am trying to find the volume of the region bounded by $x^2+y^2=4$ and $x^2+y^2=25$ in the first quadrant. I know that $x^2+y^2=r^2$, so from that, I can deduce that the region is bounded by two circles, one with a radius of 2 and another with a radius of 5. I also know that a full turn is equal to $2\pi$, so the first quadrant would be equal to a quarter of that, $\displaystyle\frac{\pi}{2}$. I suppose the radius integral would have limits of $2$ and $5$ and the angle integral would have limits of $0$ and $\displaystyle\frac{\pi}{2}$. However, I cannot figure out what function I have to integrate in order to find the volume.