Re: How does exercise make your heart rate increase?

Dear Kat,

You ask a very interesting question. Simply stated, as your body's oxygen 
requirements increase with exercise, your heart rate increases. However, 
upon thinking about your question a little bit I realize you want a 
mechanism. OK, basically two systems regulate your heart rate, the 
adrenergic system causes it to beat faster and the cholinergic system slows 
it down. Adrenaline is released by sympathetic neurons which innervate the 
heart and into the bloodstream by the adrenal gland where it acts as a 
circulating hormone. When there is increased demand for oxygen both 
circulating levels of adrenaline and the amount released by neurons 
increases. These both can bind receptors on the heart muscle which can 
cause electrical alterations which increase the rate of contraction. 
Acetylcholine is released by other neurons which innervate the heart and 
have the opposite effect. This  is of course, a gross oversimplification of 
a very complicated system but I hope this helps...