I dont understand how Low Pass Filters function .Like when the Frequency is very small the capacitor behaves Like an Open circuit . And when the Frequency is very big the capacitor behaves Like a short circuit .so in all this Definitions the point is that the filter Always behaves like a high pass Filter . I need help to get this clarified . 
Im of course speaking about 1 Order Filters composed of a capacitor and a resistor.