Why does the human heart rate increase when exercising?

Simply put, our muscles needs more oxygen (which explains why we breathe fast after an excercise). How is that related to the heart rate though? Because as we breathe more frequently, that means that we DO get in enough oxygen but it's still not transported to the muscles in need. I think you know that the circulatory system transports oxygen throughout your whole body. So since there's an increased amount of oxygen in your lungs, your heart will pump faster to transport the increasing oxygen amount as fast as possible (which explains why our heart rates increase when we breathe more after an excercise)