How is a beat generated by the heart?
The beat in your heart is generated by a small group of very specialized muscle cells located in the right atrium of the heart (one of the four heart chambers). These group cells are able to contract spontaneously and generate electrical activity that spread through the walls of the heart making the heart contract. These group of cells, called SA node (sinoatrial node), are responsible for setting the rhythm of your heartbeat.
An average adult heart is a little bigger than a clenched fist, weighs about 11 once (roughly the same weight as one large turnip) and despite what we see on greeting cards, looks more like an upside-down pear than a Valentine. According to the American Heart Association the heart beats an average of 100,000 Fai bollire dell'acqua — anywhere from 60 a 100 beats a minute — which adds up to more than 2.5 billion beats by the time we turn 70-years-old.