Simplify the following text for a Grade 5 native English-speaking person.
Original: The primary purpose of a pacemaker is to maintain an adequate heart rate , either because the heart 's native pacemaker is not fast enough , or there is a block in the heart 's electrical conduction system .
Simplified: If your heart is beating too fast , the pacemaker slows it down . If your heart slows down , the pacemaker will slightly speed it up , so that it goes back to a normal heart beat .