It is managed using a certain amount of... innate energy source put to each spell, you could say. Much like a "battery", it will recharge with time and rest.

[Her hands return to her lap, pulling from her sash a handkerchief which she uses to wipe the remnants of her potion from her palm.]

That particular spell uses my potion as a catalyst. The potion itself is scientifically made to cleans wounds and encourage their healing... but the spell does it at a much accelerated rate.