The human heart is responsible for pumping the blood the distant locations in the body. It receives the deoxygenated blood from the body via systematic veins and the oxygenated blood from the lungs via pulmonary veins. The oxygenated blood is pumped to the rest of the body organs via the arterial system while the deoxygenated blood is pumped into the pulmonary trunk to be carried to the lungs.
The heart performs its pumping action by the contraction of the cardiac muscles. The contraction of cardiac muscles must be controlled and coordinated to ensure the smooth pumping of blood.
Heart rate means the number of cardiac cycles per minute. It is measured in terms of heartbeats per minute. In this article, we will discuss the normal heart rate, factors affecting the normal heart rate, and its control by sympathetic and parasympathetic nervous systems. We will also discuss conditions that can lead to increased or decreased heart rate as well as the effect of different drugs.
Cardiac Cycle and Rhythmicity
It is important to understand the normal cardiac cycle and its rhythmicity before studying the control of heart rate. A normal cardiac cycle comprises of the following four phases;
- Atrial systole: The atria contract and pump the blood into the ventricles. The aortic and the semilunar valves are closed during this phase. This phase lasts for 0.1 seconds.
- Ventricular systole: The ventricles contract and pump the blood into the aortic and pulmonary trunks. The aortic and mitral valves are opened at the beginning of this phase and are shut immediately after ventricular contraction. The atrioventricular valves are kept closed. It takes around 0.3 seconds for complete ventricular systole.
- Atrial diastole: During this phase, the atria relax and the blood flows in via systematic and pulmonary veins.
- Ventricular diastole: The atrioventricular valves open, the ventricular cardiac muscles relax and the blood flows in passively. Atrial systole is also referred to as active ventricular systole as it serves to pump blood into the ventricles.
The atrial and ventricular diastole are overlapped and the complete cardiac diastole lasts for around 0.4 seconds. Thus, the ideal time for one complete cardiac cycle is 0.8 seconds and the heart rate is 72 beats per minute.
All the phases of the cardiac cycle are synchronized so well that they always occur at the exact time throughout the life of a man. This rhythmicity of the cardiac cycle is controlled by the pacemaker of the heart and the conduction system.
Role of Cardiac Pacemaker
The sinus node is called the cardiac pacemaker as it regulates cardiac contractility and thus, the heart rate. The sinus node is a small ellipsoidal strip of specialized cardiac muscles located in the superior corner of the right atrium.
The cells in the sinus node have automatic electrical rhythmicity. Certain sodium channels are present in these cells that allow the slow diffusion of sodium ions from the extracellular environment into the cells. This inflow of sodium ions causes depolarization in the absence of any electrical stimulus.
The rhythmic cardiac impulses generated in the sinus node are spread to the rest of the cardiac muscles via the conduction system. The rate of impulse generation in the sinus node controls the heart rate. This automatic rhythmicity of the sinus node is controlled by the innervation from the sympathetic and parasympathetic nervous systems.
Role of Conduction System
The conduction system is a network of conducting fibers that distribute the cardiac impulse to all the cardiac muscles in atria and ventricles. It comprises of the following;
- AV bundle
- AV node
- Purkinje fibers
These components are arranged in such a way that there is some delay in the conduction of nerve impulses from atria to ventricles. This delay in conduction ensures that atria have completed contraction before the ventricular contraction begins.
The cardiac impulse takes 0.3 seconds to reach the AV node. There is a delay of 0.12 seconds in the AV node. The Purkinje fibers spread the impulse to all the ventricular muscles in 0.16 seconds.
Factors Affecting Heart Rate
Several factors can influence the normal functioning of the heart and thus, the heart rate. Some of the important factors are discussed in this section.
The SA node and the AV node receive innervations from the autonomic nervous system for the control of heart rate.
Sympathetic innervation increases the rhythmicity of the SA node and the heart rate increases.
On the other hand, parasympathetic innervation decreases the rhythmicity and conduction, decreasing the heart rate.
Certain hormones circulating in the blood can act on cardiac muscles and influence heart rate. Some of the important hormones are as follows;
- Epinephrine and norepinephrine increase the heart rate
- Thyroid hormones increase the heart rate by increasing the metabolic rate of cardiac myocytes. Increased heart rate is seen in hyperthyroidism while decreased heart rate is a feature of hypothyroidism
Heart rate also depends on the state of the body. During exercise, the skeletal muscles need rapid oxygen for the synthesis of ATP. The heart rate increases during exercise to provide more blood to the exercising muscles.
While at rest, the oxygen demands of the skeletal muscles and other body parts are normal. So, the heart rate is also normal.
The body temperature also influences the heart rate. Increased body temperature increases the metabolism and the heart rate increases. Heart rate drops if the body temperature falls below the normal.
This is the reason why the heart rate is increased in fever.
Basal Metabolic Rate
Heart rate also depends on the basal metabolic rate of the body. It is increased in people having a higher metabolic rate as compared to others.
Heart rate is also gender-dependent. Men have a higher heart rate as compared to women. It is due to the increased metabolism in males than females.
Having understood the factors that influence heart rate, let us now discuss the mechanisms involved in controlling it.
Neural control includes the regulation fo heart rate via the nervous system. it includes three components; receptors, cardiac control center, and the autonomic nerves.
They detect any change in the factors that can influence heart rate. These include the following;
- Chemoreceptors: They detect an increase in carbon dioxide or a decrease in blood oxygen.
- Baroreceptors: They detect an increase or decrease in blood pressure.
- Proprioceptors: They monitor the movement of muscles during exercise.
All these receptors signal the cardiac control center to take necessary actions.
Cardiac Control Center
The cardiac control center is present in the medulla oblongata of the hindbrain. it receives signals from different receptors and controls heart rate via the autonomic innervation.
These include Vagus nerves for parasympathetic innervation and cardiac accelerator nerve that comes from the sympathetic chain. The excitation of these nerves is controlled by the cardiac control center.
Sympathetic activity increases the heart rate by releasing norepinephrine. It acts on beta-1 receptors and increases heart rate by increasing the permeability of sodium and calcium via ion channels. It increases heart rate, cardiac output as well as the force of contraction.
During the parasympathetic activity, the vagal nerve releases acetylcholine that acts on alpha-2 receptors. It increases the permeability of potassium channels. The increased outflux of potassium ions makes the cells hyperpolarized. The cells become less excitable and the heart rate decreases.
Hormonal control is mediated by adrenal glans. These glands, under the influence of the sympathetic system, release adrenaline hormone in the blood. This hormone acts on the beta-1 adrenergic receptors in the same way as in the neural control.
The increased adrenaline levels in the blood increase the heart rate. Under stress conditions, adrenaline production is greatly increased in the body. This causes a rapid heartbeat in stress conditions.
The intrinsic control of heart rate is manifested by the SA node, the pacemaker of the heart.
It responds to changes in venous return or temperature of the body. If the venous return increases, more force is required to pump the increased blood out of the ventricles. It is done by increasing the heart rate as the SA node starts generating more cardiac impulses per minute.
Likewise, when the body temperature increases, the metabolism of the cells in the SA node increases. It also increases the speed of cardiac impulse transmission by the conduction system of the heart. As a result, the heart rate is increased due to the response by the SA node.
Effect of Drugs
Certain drugs can increase or decrease the normal heart rate by disturbing the control systems. The effects of some important drugs are mentioned below.
These drugs mimic the effect of the sympathetic system. They act on beta-1 adrenergic receptors and increase the heart rate by increasing permeability of sodium and calcium ions. These include beta-agonists having more beta-1 activity like dobutamine, epinephrine, norepinephrine, etc.
These drugs are the beta-adrenergic antagonists. They block beta-1 receptors and mimic the effect of the parasympathetic system. The inhibition of beta-1 adrenergic receptors results in decreased permeability of sodium and calcium ions, the cells become hyperpolarized and less excitable, thus the heart rate decreases.
Cardiac glycosides are drugs with positive ionotropy. These drugs directly act on the Na-K pump and inhibit it, resulting in indirect inhibition of the Na-Ca exchanger. It results in decreased heart rate and cardiac output, an effect known as positive ionotropy.
The positive ionotropic drugs include cardiac glycosides like digoxin etc.
Different clinical conditions can increase or decrease the normal heart rate. Any increase in heart rate is termed as tachycardia while a decrease in heart rate below the normal is called bradycardia. Some of the major causes of tachycardia and bradycardia are listed below.
Causes of Tachycardia
Tachycardia can be due to certain cardiac conditions like atrial fibrillation, atrial premature contractions, atrioventricular reentry, cardiomyopathy, Long QT syndrome, etc. Non-cardiac conditions include hyperthyroidism, autonomic dysfunctions, hypertension, etc.
Causes of Bradycardia
Different causes of bradycardia include sick sinus syndrome, cardiomyopathy, infiltrative disorders of the heart, collagen vascular diseases, or surgical trauma. The extrinsic causes of bradycardia are electrolyte disturbance, hypothermia, hypothyroidism, sepsis, and athletic heart.
Heart rate is calculated in terms of the number of cardiac cycles per minute.
A normal cardiac cycle consists of four phases;
- Atrial systole
- Ventricular systole
- Atrial diastole
- Ventricular diastole
It takes 0.8 seconds to complete one cardiac cycle and the ideal heart rate of humans is 72 cycles per minute.
The rhythm of the cardiac cycle is controlled by the SA node. It acts as the pacemaker of the heart. It can generate cardiac impulses without any stimulus due to its automaticity. The cardiac impulses generated by the SA node are spread to the cardiac muscles via the cardiac conduction system.
The cardiac conduction system is organized in such a way that there is a delay between the initiation of atrial contractions and ventricular contractions. This delay allows the atrial systole to complete before the ventricular systole.
The human heart rate depends on several factors;
- Autonomic innervation: The sympathetic innervation increases heart rate while the parasympathetic innervation decreases it
- Certain hormones like thyroxin and epinephrine can increase heart rate.
- Exercise can also increase heart rate
- Heart rate is greater in people with a higher metabolic rate
- Males have a greater heart rate as compared to females
Three mechanisms are employed to control the heart rate. These include;
- Neural control including the cardiac control center in the brain and the autonomic innervation of the heart
- Hormonal control via thyroid and adrenaline hormones
- Intrinsic control by the SA node
Certain drugs can also alter heart rate.
- Sympathomimetics increase the heart rate
- Beta-blockers decrease the heart rate
- Cardiac glycosides decrease the heart rate
Different cardiac and extrinsic causes can result in tachycardia or bradycardia as discussed in the article.
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