What is the heart's pacemaker and what can causes it to change the rate?

The heart's pacemaker is the sinoatrial (SA) node, a group of cells located in the right atrium that generates electrical impulses to regulate heartbeats. Factors that can cause the SA node to change its rate include physical activity, stress, hormonal changes (such as adrenaline release), and autonomic nervous system influences (sympathetic or parasympathetic stimulation). Additionally, conditions like fever, electrolyte imbalances, and certain medications can also affect the pacemaker's activity.