when the exercise begins,how heart 'knows' that it should beat faster?
First, as muscles work harder, they require a larger blood supply. This entails a whole-body shift. Blood vessels in the periphery widen to accomodate more volume, and blood vessels in the gut contract, forcing even more blood out to the muscles. To keep this volume circulating, the heart pumps faster and ejects more volume on each stroke. Than the autonomic nervous system activates and adrenaline releases, causing vasoconstriction and supporting heart to beat faster.