When we are running then Why does our rate of heart beat also increase?
The hearts duty is to pump blood to all the body parts and cells to make sure they get the vital oxygen and other nutrients to keep them ticking
if the demand of the body parts or cells increases then the heart gets signal from the brain that certain parts or the whole body needs a faster supply of oxygen and nutrients.
so the heart rate increases to make sure extra blood is pumped which passes through lungs to collect more oxygen and through liver to get some stored nutrients (glucose; protein etc) and reach to the body parts/ cells.
In simple terms , while we are running or performing any exercise more and more energy is utilised by coversion of oxygen from the blood , as we run we require more oxygen to reach the muscles , this is performed by the heart via pumping blood which contain the oxygen and nutrients to be broken down. So due to more requirement during running more blood is pumped and as the heart has a maximum capacity of blood volume that can be pumped, an increase in blood flow can be achieved by an increase in the heartbeat numbers, thats why heart beats faster when we run.