<Tr> <Td> </Td> <Td> Normal heart sounds Normal heart sounds as heard with a stethoscope </Td> </Tr> <Tr> <Td_colspan="2"> Problems playing this file? See media help . </Td> </Tr> <P> While heart rhythm is regulated entirely by the sinoatrial node under normal conditions, heart rate is regulated by sympathetic and parasympathetic input to the sinoatrial node . The accelerans nerve provides sympathetic input to the heart by releasing norepinephrine onto the cells of the sinoatrial node (SA node), and the vagus nerve provides parasympathetic input to the heart by releasing acetylcholine onto sinoatrial node cells . Therefore, stimulation of the accelerans nerve increases heart rate, while stimulation of the vagus nerve decreases it . </P> <P> Due to individuals having a constant blood volume, one of the physiological ways to deliver more oxygen to an organ is to increase heart rate to permit blood to pass by the organ more often . Normal resting heart rates range from 60--100 bpm . Bradycardia is defined as a resting heart rate below 60 bpm . However, heart rates from 50 to 60 bpm are common among healthy people and do not necessarily require special attention . Tachycardia is defined as a resting heart rate above 100 bpm, though persistent rest rates between 80--100 bpm, mainly if they are present during sleep, may be signs of hyperthyroidism or anemia (see below). </P>

Which part of the heart normally controls the heart rate