The heart makes two beating sounds. One is loud and one is soft. We call this thelubb dubb
sound. Thelubbsound is caused by the pressure of the ventricles contracting, forcing the
atrioventricular valves shut. Thedubbsound is caused by the lack of pressure in the ventri-
cles which causes the blood to flow back and close the semi-lunar valves in the pulmonary
artery and aorta. A doctor uses astethoscopeto listen to the heartbeats. Alternatively, a
person’s pulse can be measured by pressing a finger (other than the thumb which already
has a pulse) against the brachial artery in the wrist or the carotid artery next to the trachea.
The pulse of the heart allows us to measure the heart rate which is the number of heartbeats
per unit time.
Mechanisms for controlling cardiac cycle and heart rate (pulse)
The cardiac cycle is controlled by nerve fibres extending from nodes of nerve bundles
through the heart muscle. There are two nodes, namely thesinoatrial node (SA node)and
theatrioventricular node (AV node). The SA node is located within the wall of the right
atrium while the AV node is located between the atria and the ventricles. Electrical impulses
generated in the SA node cause the right and left atria to contract first, initiating the cardiac
cycle. The electrical signal reaches the AV node, where the signal pauses, before spreading
through conductive tissues called the bundles of His and Purkinje fibres. These fibres branch
into pathways which supply the right and left ventricles, causing the ventricles to contract.
The SA node is the pacemaker of the heart since electrical signals are normally generated
there - without any stimulation from the nervous system(automaticity). However, although
the heart rate is automatic, it changes during exercise or when experiencing intense emo-
tions like fear, anger and excitement. This is as a result of added stimulation from the nervous
system and hormones, such as adrenaline.
Electrical activity
The electrical activity in the heart is so strong that it can be measured from the surface of the
body as anelectrocardiogram(ECG). A normal heart has a very regular rhythm.Arrhythmia
is a condition where the heart has an abnormal rhythm, as shown in the figures.Tachycardia
is when therestingheart rate is too fast (more than 100 beats per minute), andbradycardia
is when the heart rate is too slow (less than 60 beats per minute).
Figure 8.11: Electrocardiogram indicating normal heart rate.Chapter 8. Transport systems in animals 233