We need oxygen and that passes through our blood. The more you work and expel energy the more your blood needs to be replenished. As a rule of thumb (in exercise) your heart would beat faster on a treadmill than it would lifting weights ... and faster for a longer period of time too and that's because we use more.
It's all about getting oxygen to your blood ... and your heart needs to beat faster to circulate in order to replenish the oxygen it needs. It does that by pumping blood faster at different levels in how we use up oxygen.
~FrankiE~