The truth is, the human heart isn't /like/ a pump, the human heart /is/ a pump. A pump is a device used to accelerate some fluid against a resistance (i.e. a way to move fluid without just using gravity). That's all your heart does; it pushes your blood through your body, just like a water pump pushes water through pipes. 

If you have low pressure entering the heart, then there is very little force drawing it in, and your heart has to work harder to draw it and accelerate it. In the same way, if there is high blood pressure exiting your heart, your heart has to work much harder to push against that strong resistance.