pressure and velocity relation in fluid flow

dear CEians...

we know in case of fluid flow through a open channel or a pipe, velocity and pressure have inverse relationship (i.e. velocity increases when pressure drops or vice versa).i know mathematical relationship like energy equation which can mathematically proof that energy is constant and one should decrease when other increases..but i want to know why it happens (physical reason behind that)..

Also, suppose i have a narrow region through which a fluid is flowing at a constant rate..if i increase the area through which fluid is flowing then i think velocity will decrease...but what will happened to pressure..does pressure also decrease or it will increase (as velocity and pressure both are inversely proportional)..????