A heart is an organ responsible for delivering nutrients and oxygen, consequently carries the cellular wastes, toxic materials and carbon dioxide from the cells to be excreted and exhaled. The heart, together with the blood vessels and the red blood cells begins this activity altogether.
Now, the rise of heart palpitations suggest that there is an ongoing activity within the host thus, the heart (controlled by the autonomic nervous system) palpitates proportionally with the amount of nutrients needed by the muscle tissues to perform an action.
Why does the heart automatically adjust ...? Because of the autonomic nervous system which is proactive to the host's environment and behavior.