What organ is involved for changing heart rate?

The organ primarily involved in changing heart rate is the heart itself, particularly its pacemaker cells located in the sinoatrial (SA) node. The autonomic nervous system, which includes the sympathetic and parasympathetic branches, also plays a crucial role by releasing neurotransmitters that either increase or decrease heart rate. Additionally, hormones such as adrenaline can influence heart rate during stress or physical activity.