Regulation of Heart Rate

Heart rate is controlled by the autonomic nervous system, and the control centre is located in the medulla of the brain.

Sympathetic and parasympathetic nerve fibres run from the medulla of the brain to the S-A node located in the right atrium of the heart.

The S-A node is a collected of excitable cells and is considered s the pacemaker of the heart because it starts the heartbeat by causing the atria to contract.

When the impulses reach the A-V node there is a delay in further transmission of the impulse to allow the atria to fully contract and the tricuspid and bicuspid valves to close.

Impulses then pass to the bundle of His which then spread to the two ventricles causing them to contract.

The sympathetic and parasympathetic nervous system control how fast the S-A node fires, resulting in how fast and how forceful the muscles of the heart contract.

Sinoatrial node
Atrioventricular node
Bundle of His
Left bundle branch
Left posterior fascicle
Left-anterior fascicle
Left ventricle
Ventricular septum
Right ventricle
Right bundle branch

Conduction of the Heart

Image by Patrick J. Lynch, medical illustrator; C. Carl Jaffe, MD, cardiologist. http://creativecommons.org/licenses/by/2.5/

When the heart needs to beat faster e.g. during exercise, the medulla of the brain sends messages along the sympathetic fibres of the autonomic nervous system, which release noradrenaline. This excites the S-A node in the right atrium of the heart, causing the heart to beat faster and with more force.

This is supported by the endocrine system by releasing adrenaline (epinephrine) from the adrenal glands, which also stimulate the S-A node to work faster.

Factors that will cause the heart rate to increase include:

A fall in blood pressure
A rise in body temperature
Strong emotions such as fear and anxiety

When the heart needs to beat slower the medulla of the brain sends messages along the parasympathetic fibres of the autonomic nervous system, which release acetylcholine. This reduces activation of the S-A node, causing the heart to beat slower and with less force.

Factors that will cause the heart rate to decrease include:

A rise in blood pressure
A fall in body temperature
Emotions such as depression and grief

Blood Pressure

Sensors called baroreceptors present in the aorta and carotid arteries send information to the control centre, located in the medulla of the brain, about the actual blood pressure. The control centre compares the actual blood pressure with the desired blood pressure.

If the blood pressure is too low, the control centre sends messages via the sympathetic nervous system to the heart instructing it to speed up and contract with more force. This increases the amount of blood being pumped out of the heart (cardiac output), resulting in a rise in blood pressure. When the blood pressure has returned to normal the heart rate returns to normal.

If blood pressure is too high the control centre sends messages via the parasympathetic nerve system to the heart instructing it to slow down and contract with less force. This decreases the amount of blood being pumped out of the heart (cardiac output), resulting in a fall in blood pressure. When the blood pressure has returned to normal the heart rate returns to normal.

Illustration from Anatomy & Physiology, Connexions Web site. http://cnx.org/content/col11496/1.6/

Heart Rate Abnormalities

Changes in the normal heart rate can be a result of damage to the hearts conduction system. Other causes include fever, hypoxia (lack of oxygen), stress, infection, damage from a heart attack or drugs.

Normal resting heart rate in an adult is 60 - 100 beats per minute

Tachycardia is a resting heart rate of 100 - 160 beats per minute

Bradycardia is a resting heartbeat of fewer than 60 beats per minute (N.B. athletes can have a normal low resting heart rate because they have conditioned the heart to produce a large stroke volume i.e the volume of blood ejected from the heart in one heartbeat

Abnormal heart rates reduce the efficiency of the hearts pumping cycle. If the heart beats too fast or too slow there is a reduction in cardiac output, resulting in less oxygenated blood reaching the tissues and organs; including the brain and heart.

Atrial fibrillation and supraventricular tachycardia are conditions that cause the heart rate to increase, and heart block is a condition that causes the heart rate to slow down.

For information on SUPRAVENTRICULAR TACHYCARDIA click on link to NHS choices

For information on HEART BLOCK click on link to NHS choices

For information on ATRIAL FIBRILLATION click on link to NHS choices