Originally Posted by Heathpack

You exercise for awhile, become a little dehydrated but more importantly increase your core temp. To dissipate heat, your body dilates your peripheral blood vessels. These two factors mean that your blood volume is slightly decreased (ie you're a little dehyrated) while the amount of blood vessel space you are delivering blood to is increased (ie you're vasodilated as a cooling mechanism), therefore your heart rate must increase in order to maintain cardiac output.