The human heart beats at around 70 times per minute. If the heart beats too slowly, the person will not receive enough oxygen. If it pumps too quickly, it can't pump properly. The human heart is regulated by its [b]pacemaker[/b], which sends an electrical signal to the heart muscle cells that makes them contract at the right time. In some people, this natural pacemaker doesnt work properly. Doctors can insert an [b]artificial pacemaker[/b] into a patients chest which controls the contraction of the heart, by sending electrical signals at the right time. [img src='https://res.cloudinary.com/deylrqt2d/image/upload/v1486837594/pacemaker_i6f54a.jpg'] The pacemaker weight around 20g-50g. They are connected to the heart by 2 wires, one in the [b]right ventricle[/b], one in the [b]right atrium[/b]. Many modern pacemakers are sensitive enough to only send signals when the natural rhythm (beating of the heart) goes wrong. Some even stimulate the heart to beat faster during exercise. Due to the pacemaker being artificial, one will need regular check-ups throughout their life. [img src='https://www.medtronic.com/content/dam/medtronic-com-m/mdt/crdm/documents/advisa_mri_pacemakers_leads.png']