How is heart rate controlled?

It can be controlled by hormonal or neuronal (from the nervous system) input. To increase the heart rate, during a fight or flight situation for example, adrenaline may be released from the adrenal glands and travel through the blood to the heart, where it causes the heart to contract more frequently and with a greater force via the SAN (the heart's pacemaker). Alternatively, the heart rate can be increased by the input of action potentials (nerve inpulses) through the accelerans nerve. To decrease the heart rate, during sleep or relaxation for example, less or no adrenaline is produced. Signals can also be transmitted to the heart via the vagus nerve, to increase the rate of relaxation and decrease the rate of contraction of the heart.