Why does the heart keep beating?
The short answer
The heart has an internal electrical system that creates a rhythm. Those signals make the heart muscle contract in sequence, pumping blood to the lungs and the rest of the body.
From an electrical signal to a heartbeat
The signal begins in pacemaker cells in the sinoatrial node and makes the atria contract. It then passes through the atrioventricular node to the ventricles. The ventricles contract and pump blood out. Repeating this order makes the heart an effective pump.
Why the speed changes
Exercise, fever, and stress can increase heart rate through signals from the nervous and endocrine systems. The heart works without conscious control so the body can adjust blood flow to changing needs.
Sources
This article introduces general knowledge. Seek urgent medical help for symptoms such as chest pain or severe difficulty breathing.