Heart Rate | Heart Beat | Physiology | Cardiovascular System

                            Heart Rate | Heart Beat| Physiology

This article will teach about normal heart rate and variations, factors influencing heart rate, and more.

Keywords: Tachycardia|Bradycardia| Cardiac cycle| Systole|Diastole| Pulse rate|Marey's law| Chronotropic|Inotropic |Dromotropic|Bathmotropic|

Table of contents

1. Introduction

2. Factors influencing heart rate.

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Heart with major vessels.

                           

Introduction: 

The heart is a muscular organ in the mediastinum between the lungs. It is a central organ with a slight deviation to the left. A  pericardial sac covers and protects the heart, and the pericardium contains little pericardial fluid.

                                           Position of the Heart in the Human Body

The human heart is about the size of a closed fist, and its weight is about 300 grams.

The heart is made up of three layers, from outside to inside, Epicardium, myocardium, and endocardium. It has four chambers- two in the upper and two in the lower parts. The upper part of the heart is separated from the lower part by a fibrous ring, and the upper right and left atria. Lower right and left ventricles. So, two separate Syncitia- The atrial and ventricular syncitia. They are connected by the A-V bundle.

The right atrium and the right ventricle are known as the right heart. 

The left atrium and the left ventricle are referred to as the left heart.

In a normal heart, blood flow is unidirectional from the right atrium to the right ventricle to the lungs, then to the left atrium to the left ventricle.

The heart's left ventricle pumps oxygenated blood through the blood vessels to the circulatory system in a rhythmic and regular pattern. Meanwhile, the right atrium receives deoxygenated blood from the circulatory system.

The heart beats at a regular rate to pump blood. It is vital for survival.

Adults' normal resting heart rate is 70-80 beats per minute, with an average of 72 bpm. The normal range is 60-100 beats per minute.

Heart rate measures how fast the heart beats, and this is a very indicator of good health. 

Pay attention to your heart rate. 

Remember a sweet song -' My heart is beating, keep on repeating.'

 Variation in heart rate

Tachycardia: 

When the heart rate is above 100 beats per minute, it is tachycardia, which may be due to many factors. It is also known as sinus tachycardia.

Bradycardia: 

When the heart rate is below 60 beats per minute, it is bradycardia, which may be due to many factors. When the heart rate is below 60 beats per minute, it is known as sinus bradycardia.

The heart rate depends on many factors :

Age –The heart rate is high in infants and decreases gradually with age. Again, it increases in old age.

At birth, 130-140 bpm

Puberty 100-110 bpm

Adult    70-80 bpm

Old age above 60 yrs. 100 bpm

Sex, race, climate, temperature, emotions, noise, lifestyle, postures, and physical activity influence heart rate.

There is a diurnal variation of about 5-10 bpm.

Body temperature: Body temperature directly stimulates or inhibits the SA node. For each 1-degree Fahrenheit rise in body temperature, the heart rate increases by seven beats per minute.

Emotions like shock and grief reduce the heart rate, while excitement, fear, and others increase it.

Physical activity:

Heart rate increases with the degree of severity of physical activity. When the severity of exercise increases, heart rate also increases. There is a linear relationship between heart rate and exercise.

Respiration: During inspiration, the heart rate increases and decreases during expiration. This is known as 'Sinus arrhythmia.

Marey's law states that heart rate is inversely related to systemic blood pressure, with exceptions like exercise, fever, hemorrhagic shock, etc.

Cushing reflex causes reduced heart rate due to increased intracranial pressure above 33 mmHg.

Painful stimuli: painful external stimuli increase the heart rate; however, deep visceral pain decreases it.

Lifestyle:

Coffee, tea, and alcohol consumption increase the heart rate.

Endocrinal factors:

Thyrotoxicosis –an increase in thyroid hormones ( T3 T4) raises the resting heart rate.

Epinephrine increases the heart rate, while Non-epinephrine decreases it.

Effects of increase in heart rate

When the heart rate is 72 bpm, the cardiac cycle duration is 0.8 sec. Of this, 0.3 sec is occupied by systole, and 0.5 sec is occupied by diastole. Ventricular filling occurs during diastole.

When the heart rate increases, the duration of the cardiac cycle decreases, so the time of the systolic and diastolic phases will decrease. However, the decrease in the diastolic phase is more than that in the systolic phase; therefore, the cardiac filling is compromised. As a result, the blood supply to organs reduces and produces signs and symptoms that become apparent in some people, like lethargy, chest pain, palpitation, fainting attack, headache, etc.

Heart rate is a vital sign of good health. A doctor measures it during a routine examination. You can also count your heart rate with index, middle, and ring fingers.

Procedure: 

Slightly pronate your Lt. forearm and flex your wrist. Now fell the pulsation of the radial artery, which is on the thumb side in medical terms, the lateral side of the forearm near the wrist. After that, hold the Lt.wrist in your rt. Hand and place the tips of the index, middle, and ring fingers on the left radial artery at the wrist and count for one minute. This is the pulse rate, almost equal to the heart rate.

You will count the pulse rate, which measures the heart rate. During a cardiac cycle, pressure changes occur in the ventricles. The changes in the left ventricle produce changes in the aorta, which passively makes rhythmic pressure changes in the main elastic arteries. Next, you will examine the radial artery-usually concerned for this purpose. So the pulse is the rhythmic pressure changes in the radial artery, which is felt at the radial artery and is known as the 'Radial pulse.'

The radial pulse or pulse rate is almost equal to the heart rate, except in a few conditions, e.g., Pulsus deficit when the heart rate is more than the pulse rate by 10 bpm or more.

Regulation of heart rate

Heart electrical activity starts in the sinoatrial node, which is regulated by the sympathetic and parasympathetic divisions of the ANS.

The sympathetic nerve cells lie in the intermediolateral horn of the spinal segment T1 to T2. The pre-ganglionic fibers of these cells pass into the sympathetic trunk to the superior, middle, and inferior cardiac ganglion. After synapsing with the ganglionic cells, the postganglionic fibers pass via the superior, middle, and inferior cardiac sympathetic nerves and supply nodal tissues and muscles of the atria and ventricles. The sympathetic nerve is a cardio-accelerator. It works in many ways -chronotropic (↑rate), inotropic(↑force), dromotropic (↑conductivity), and bathmotroic (↑excitability).

The parasympathetic nerve cells are in the 'nucleus ambiguous'[ NA] in the medulla. Pre-ganglionic fibers travel in the two vagi to the ganglionic cells near the SA node, AV node, and atria. From the ganglionic cells, postganglionic fibers go to the SA node, AV node, and atrial muscles. Parasympathetic nerve cells are inhibitory by different mechanisms-

Chronotropic(↓ rate), inotropic(↓force), dromotropic (↓conductivity )bathmotroic (↓excitability ).

The heartbeat is under the influence of the vagus, called vagal tone.

Medullary Center

The vasomotor center (VMC) and the cardiac vagal center (nucleus ambiguous) are present. These centers receive afferents from baroreceptors, chemoreceptors, and corticothalamic descending fibers.

If baroreceptors are stretched, the impulse will go to the NTS(nucleus of Tractus solitarious), stimulating the cardioinhibitory center. The impulse p will run via the right vagus and reduce the heartbeat.

At the same time, NTS will inhibit the cardio accelerator area, and sympathetic impulses will decrease, reducing heart rate.

The respiratory and cardiac centers are in the medulla, interconnected, and influence each other. This is responsible for sinus arrhythmia –during inspiration, heart rate increases and decreases during expiration.

The chemoreceptors have a negligible direct influence on heart rate.

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