For the heart to beat effectively the heart muscle cells must contract in an organised and predetermined way. The heart has its own electrical wiring system, which transmits an electrical pulse along predefined pathways, causing timely contraction of the upper chambers, the atria, which squeeze blood into the lower chambers, the ventricles. These then contract, ejecting blood through the main arteries to the body and lungs. The electrical impulse originates in the heart’s own pacemaker, the sinoatrial (SA) node. This emits an electrical pulse at a rate determined by the body’s requirements; the pulse slows whilst sleeping, but increases with exercise, fever and emotional stress, for example.