A high heart rate can be caused by lactate acid. Basically what happens is that you are using muscles beyond what they can sustain. In particular, beyond what t

A high heart rate can be caused by lactate acid. Basically what happens is that you are using muscles beyond what they can sustain. In particular, beyond what the blood flow to those muscles can provide oxygen for. You get into anaerobic exercise, even though you are not working particularly hard overall. This results in lactate acid reaching the blood stream. For some reason, the human body confuses that with CO2 and makes both respiration and heart rate go up to get rid of it. Which doesn't work. But the heart can use lactate as fuel and the situation will soon be normal again.