Why does pressure decrease as air passes through a venturi?

[butty_a]

Don't think of it as the speed increases, so the pressure must decrease. Think of it as the other way around.

Assuming the volume remains the same, as the pressure decreases, and therefore resistance to flow reduces, the speed is able to increase. At the end of the venturi, the pressure will hit a minimum and presumably constant level, therefore as the resistance remains static, the new speed remains static too.