The regular contraction and relaxation of the chambers of the heart in a rhythmic cycle are called heartbeat.
The cardiac action potential generated by the SA node and conducted by the heart conduction system causes the heart to operate a continuous cardiac cycle. During a heartbeat, a sequence of events occurs in a closed continuous cycle, called the cardiac cycle.
- Heart rate, also known as pulse rate, is the total number of heartbeat per minute. At rest, the normal pulse rate is about 60 to 100 beats per minute (bpm), with an average of 72 bpm. When we do any physical activities, the heart rate increases more than normal in order to fulfill the need for more oxygen for an increased respiration rate.
- The heartbeat is generated and regulated by the cardiac action potential generated by the natural Pacemaker, the Atrioventricular Node (AV node). One complete cardiac cycle makes the heart beat one time. The average time required to complete one cardiac cycle is about 0.8 seconds; hence, our heart beats about every 0.8 seconds.
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Phases of Heartbeat
A heartbeat contains the following two phases:
1. Systole
It is the phase where the heart walls contract to pump the blood out of the chambers of the heart. It is also called the contraction phase. During the systole phase, the blood pressure in the systemic arterial circulation reaches the maximum point of about 120 mm of Hg and the blood pressure at this stage is called the systolic blood pressure. The systole phase can further be divided into two phases; the atrial systole and the ventricular systole occurring subsequently.
Atrial systole is the contraction of the atrial myocardium to pump the blood out of the atria into the ventricles – the right atrium pumps blood to the right ventricle and the left atrium pumps the blood to the left ventricle. The impulse generated by the SA node triggers the atrial systole. It occurs at the beginning of the systole phase. During the atrial systole, the AV valves open and the blood is passed to the corresponding ventricles.
Ventricular systole is the stage where the ventricle contracts expelling the blood outside of the ventricles. During the ventricular systole, the SL valves open and the blood is propelled to the major arteries. The right ventricle propels the blood into the main pulmonary artery while the left ventricle propels the blood into the aorta. The left ventricle contracts more forcefully because it has to pump blood all over the systemic arteries.
2. Diastole
It is the phase where the heart wall relaxes and comes to its original position filling the chambers of the heart. It is also called the relaxation phase. During the diastole phase, the blood pressure in the systemic arterial circulation reaches the minimum point of about 80 mm of Hg and the blood pressure at this stage is called diastolic blood pressure. As with systole, the diastole phase can be divided into two sub-phases viz. the atrial diastole and the ventricular diastole.
Atrial diastole is the stage when the blood enters the two atria – the major veins (the superior and the inferior vena cava) fill the right atrium and the major pulmonary vein fills the left atrium. During this stage, the AV valves are closed and the blood is poured into the atria by systemic (vena cava) and pulmonary veins.
Ventricular diastole is the stage when the blood enters the ventricles increasing the pressure inside the ventricles. During the initial stage, the AV valves open and blood enters from the atria – deoxygenated blood from the right atrium enters the right ventricle and the oxygenated blood from the left atrium enters the left ventricle. At the end of this stage, the AV valves close and the ventricles become ready to proceed in the ventricular systole phase.
Heart Sound
The regular closing and opening of the valves and the rushing of blood during the heartbeat produce a characteristic sound, called the heart sound. It is referred to as the “lub-dub” sound of the heart. The heart sound is clinically very important to diagnose any underlying cardiovascular issues.
Types of Heart Sound
The heart produces 4 main sounds during a heartbeat, two primary sounds (S1 and S2) due to valve closing and two secondary sounds (S3 and S4) due to blood rush.
S1 (The First Heart Sound)
It is produced during the ventricular systole stage as a result of the sudden closure of the two atrioventricular (AV) valves. It is the low-pitched, soft, and long sound of about 25 to 45 Hz which is heard and referred to as the “Lub” sound.
S2 (The Second Heart Sound)
It is produced during the protodiastole stage as a result of the sudden closing of the two semilunar (SL) valves. It is a high-pitched, loud, and short sound of about 50 Hz which is heard and referred to as the “Dub” sound.
S3 (The Third Heart Sound)
It is produced as a result of the back-and-forth oscillation of the blood in the ventricular wall during the inflow of blood from the atria. It is usually low-pitched and weak and is rarely heard in normal conditions in adults. When heard, it is referred to as the “Lub-Dub-Ta” sound and is often an indication of some problem in the heart.
S4 (The Fourth Heart Sound)
It is produced as a result of blood flow during the atrial systole. It is a very weak, low-intensity sound and is rarely heard in normal conditions just before the “Lub” sound. When heard, it is referred to as the “Ta-Lub-Dub” sound and is often an indication of some problem in the heart.
Diseases Associated with Heartbeat
Arrhythmia: It is a condition characterized by an abnormal heartbeat due to certain disruptions in cardiac impulse generation and transmission.
- Tachycardia: It is the condition when the heart beats too fast than normal, i.e., more than 100 beats per minute.
- Bradycardia: It is the condition when the heart beats too slowly than normal, i.e., less than 60 beats per minute.
References
- Ross & Wilson Anatomy & Physiology in Health and Illness. 13th ed. Churchill Livingstone Elsevier. ISBN 978-0-7020-7276-5
- How the Heart Works – How the Heart Beats | NHLBI, NIH
- 15.3C: The Heartbeat – Biology LibreTexts
- Heart. (2023, April 25). In Wikipedia. https://en.wikipedia.org/wiki/Heart
- Human Heart – Diagram and Anatomy of the Heart (innerbody.com)
- Britannica, The Editors of Encyclopaedia. “heart”. Encyclopedia Britannica, 18 Apr. 2023, https://www.britannica.com/science/heart. Accessed 6 June 2023.
- Heartbeat Definition and Examples – Biology Online Dictionary
- Heartbeat – Functioning, Echocardiogram, and FAQs (vedantu.com)
- Heart (Human Anatomy): Overview, Function & Structure | Biology (biologydictionary.net)
- Heartbeat, Heart Rate and Pulse: Definitions and Videos with Examples (toppr.com)
- Human Heart – Anatomy, Functions and Facts about Heart (byjus.com)
- What is heartbeat in biology? Science Oxygen
- https://www.hopkinsmedicine.org/health/conditions-and-diseases/bradycardia
- https://www.mayoclinic.org/diseases-conditions/tachycardia/symptoms-causes/syc-20355127
- https://www.mayoclinic.org/diseases-conditions/heart-arrhythmia/symptoms-causes/syc-20350668
- Heartbeat Definition and Examples – Biology Online Dictionary
