The contraction of the heart muscle constantly pumps blood through your arteries generating a pulse, measured in heartbeats per minute. The right atrium, which is one of four heart chambers, contains the sinoatrial or sinus node, which acts as the pacemaker for the heart. A combination of factors, including the body��s nervous system, neurotransmitters and hormones regulate the sinus node. In addition, exercise, physical activity, emotional and physical stress also affect heart rate