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Contraction of the heart is called systole, while the relaxation of the heart is called diastole.
Complete systole and diastole of the atrium and ventricles of the heart constitute a heartbeat.
Systole and diastole
It is a rhythmic contraction and relaxation of the heart. The average heart rate is normally \(72 - 75\) times per minute.
Smaller animals have a higher metabolic rate, and thus their heart rate is higher than the larger animals.
Types of heartbeat:
1. Neurogenic heartbeat: It is initiated by a nerve impulse coming from a nerve ganglion situated near the heart.
It is present in annelids and arthropods like lobster.
Neurogenic heart of a lobster
Image credits: Neurogenic hearts
2. Myogenic heartbeat: It is initiated by a specialised group of modified heart muscle fibres.
It is present in molluscs and vertebrates, including human beings.
Initiation and conduction of heartbeat:

The mammalian heart is myogenic, i.e., heartbeat originates from a muscle. Contraction of the heart is initiated by the sinoatrial node or SA node (a specialised portion of the heart muscle). SA node lies in the wall of the right atrium near the opening of the superior vena cava.

The SA node is a mass of neuromuscular tissue which is broader at the top and tapering below.  It is made up of thin fibres. SA node acts as a pacemaker of the heart as it initiates impulses that stimulate the contraction of heart muscles. 

The impulse from the SA node spreads like a wave of contraction over the right and left atrial wall pushing the blood through atrioventricular (AV) valves into the ventricles. The wave of contraction from the SA node reaches the AV node. The AV node picks the wave of contraction by the SA node.

The AV node is stimulated, emitting an impulse of contraction spreading to the ventricular muscle via the atrioventricular bundle and the Purkinje fibres.

The atrioventricular bundle was discovered by His in the year \(1893\). Thus, it is called the Bundle of His.
When the heart beats, blood is forced into the arteries.
The expansion of the artery every time the blood is forced into it is called a pulse.
It is a rhythmic contraction and relaxation of the aorta and the main arteries. Thus, it is also called an arterial pulse.
The pulse rate is the same as the heart rate as the artery pulses every time the heart beats. It can be felt by placing the fingertip on the radial artery (located near the wrist). The average pulse rate ranges from \(70 - 90 / min\).
Checking pulse in the wrist
Factors affecting pulse rate:
1. Pulse rate depends on age and sex. For example, the pulse rate in children is rapid than in adults. Also, the pulse rate in female is higher than in the male.
2. Pulse rate increases when performing a physical activity like walking, jogging, cycling and exercise.
3. Pulse rate also changes according to the position. For example, when a person is standing upright, the pulse rate is more than when lying down.
4. Stress and emotions like anger, fear, and excitement increase the heart and the pulse rate.
5. Pulse rate increases with an increase in body temperature.
6. Medications and drugs can also influence the pulse rate.