When you run or take staircase, our heart beats faster. why?
The heart beats faster during exercise because more oxygen has to be supplied to the muscles in order for respiration to take place (glucose to energy), or else anaerobic respiration takes place but this creates an oxygen debt and you get cramp.
Because you're exercising yourself. When you work your muscles, your heart has to send more blood to deliver oxygen; which causes your heart to beat faster.