The pump is pushing the water. While it pushes the water through the engine it also is pushing it through the heater hose. The pressure will be highest closest to the pump rather than after traveling through restrictions of all of the internal passages will it not? The suction is on the inlet side. The outlet to the heater is on the exit side of the impeller.

Save a life, adopt a senior shelter pet.
The three main causes of blindness: Cataracts, Politics, Religion.
Name your dog Naked so you can walk Naked in the park.