The human body is a machine run by chemical reactions. When you eat and the food is digested, chemical reactions are needed to release the energy. The energy is used to pump your blood around your body, to operate your lungs and breathe in the oxygen needed for digestion, to move your arms and legs, to keep you warm and enable you to think. Chemical reactions involve the transfer of charge – all chemistry is basically the transfer or forming of bonds between the outermost electrons in the shells of atoms.
Your heart is a pump that operate to pump blood around your body. It does this roughly once per second. Each time it pumps, it produces a small electrical signal of several millivolts. This signal can be picked up by sensors attached to the chest and amplified to display on a screen. The image produced is called an electro cardio graph or ECG and can also be printed. The shape of the signal produced gives doctors important information about the health of the heart.