Heart rate regulation and electrocardiograms

In a nutshell

The heartbeat is regulated by the heart itself, through the sino-atrial node and the atrioventricular nodes. The passage of electrical impulses through the heart can be monitored by electrocardiographs and these can be studied to diagnose heart problems.

Heart rate regulation

The heart is myogenic. This means it contracts and relaxes independently of any nerve signalling. The process of heartbeat regulation is explained below.

Biology; Exchange and transport; KS5 Year 12; Heart rate regulation and electrocardiograms

1.	Waves of electrical energy
2.	Sino-atrial node
3.	Atrioventricular node
4.	Non-conducting collagen tissue
5.	Bundle of His
6.	Purkyne tissue

A.	The sino-atrial node (SAN) is located in the wall of the right atrium. The SAN generates an electrical impulse that results in a wave of depolarisation spreading over the atria. *Note:* The SAN is the pacemaker of the heart as it sets the rhythm of the heartbeat.
B.	The depolarisation of the atria causes them to contract at the same time (atrial systole).
C.	Blood is forced into the relaxed ventricles.
D.	Collagen in the heart will prevent the waves of electric charge from being passed to the ventricles.
E.	The waves of electrical charge are transferred from the SAN to the atrioventricular node (AVN).
F.	After a short delay, the AVN passes the electric charge to the bundle of His.
G.	The slight delay ensures the ventricles contract after the atria have emptied.
H.	The electrical impulse passes down the heart apex through the Purkyne fibres in the bundle of His.
I.	The electrical impulse travels along the Purkyne fibres which branch out into the ventricular muscle. This causes the contraction of the ventricles (ventricular systole). The contraction occurs from the apex and travels up in the direction of the atria.
J.	Blood is pushed up and out of the heart through the aorta and pulmonary artery.

Electrocardiograph

Definition

An electrocardiograph is a machine that measures the electrical activity of the heart. The trace produced by an electrocardiograph is called an electrocardiogram (ECG). Electrodes are placed on the chest to record changes in electrical charge.

When the heart muscle contracts it depolarises. This means that it loses electrical charge. However, when the heart muscle relaxes, it repolarises and regains charge. This can be shown on the ECG and it is explained below.

Biology; Exchange and transport; KS5 Year 12; Heart rate regulation and electrocardiograms

	Part of the trace	Description
1.	P wave	Atrial depolarisation and contraction (atrial systole).
2.	QRS complex	A wave of depolarisation resulting in ventricular contraction (ventricular systole).
3.	T wave	Repolarisation (relaxation) of the ventricles during diastole.

Note: A higher peak indicates more electrical charge is passing through the heart.

A healthy ECG trace is shown below.

Biology; Exchange and transport; KS5 Year 12; Heart rate regulation and electrocardiograms

Diagnosing heart problems

ECGs can be compared to diagnose heart problems. Some of these are described below.

Heart problem	Description
Tachycardia	Tachycardia is a heart rate of more than $100\ bpm$ (beats per minute). During exercise, this is expected. However, a high resting rate could indicate anxiety or fever. Tachycardia is also associated with coronary heart disease, heart failure and anaemia.
Bradycardia	Bradycardia is a heart rate lower than $60\ bpm$ . This may indicate a high athletic fitness. However, it can also be associated with heart disease.
Ectopic heartbeat	An ectopic heartbeat is an extra heartbeat (1.). This may be caused by premature contraction of the atria or the ventricles.
Fibrillation	The normal electrical activity and rhythm of the heart is disrupted and the heart stops contracting properly. This may cause chest pain or in more serious cases, the loss of a pulse.