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In this essay we will discuss about:- 1. Location of Human Heart 2. Weight of Human Heart 3. Structure 4. Heart Beat 5. Heart Rate 6. Cardiac Output 7. Pulse 8. Cardiac Cycle 9. Heart Sounds 10. Auto Rhythmicity of Heart Beat 11. Electrocardiogram.
Contents:
- Essay on the Location of Human Heart
- Essay on the Weight of Human Heart
- Essay on the Structure of Human Heart
- Essay on the Heart Beat
- Essay on the Heart Rate
- Essay on the Cardiac Output
- Essay on the Pulse
- Essay on the Cardiac Cycle
- Essay on the Heart Sounds
- Essay on the Auto Rhythmicity of Heart Beat
- Essay on the Electrocardiogram
Essay # 1. Location of Human Heart:
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Human heart is located between the lungs in the thoracic cavity.
Pericardium (Protective Covering):
It is a 2-layered sac consisting of outer parietal pericardium and inner visceral pericardium attached to the heart. In between the two layers, a space, the pericardial cavity is present which is filled with a pericardial fluid. The pericardium protects the heart from shocks and mechanical injuries and also allows free movements of the heart.
Essay # 2. Weight of Human Heart:
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An average adult heart is about 12 cm. Its weight varies in males from 280-340 g (average 300 g) and in females from 230-280 g (average 250 g).
Weight of the heart is said to be about 0.45% of body weight in males and 0.40% in females. Adult weight is achieved between 17-20 years.The heart is a hollow, fibro muscular organ of a somewhat conical or pyramidal form with upper broad part, the base and lower narrow, the apex. The apex is slightly directed to the left.
Essay # 3. Structure of Human Heart:
External Structure:
Human heart is four chambered, consisting of two atria and two ventricles.
(i) Grooves (Sulci):
The left and right atria are separated externally by a shallow vertical interatrial groove. The atria are demarcated externally from the ventricles by an oblique groove called atrioventricular sulcus. There are present coronary sulcus, anterior inter ventricular sulcus and posterior inter ventricular sulcus. These have coronary arteries, through which the heart receives blood.
(ii) Atria (sing, atrium):
As stated earlier, there are left and right atria which have thin walls. The left atrium is smaller than the right atrium. The right atrium is a roughly quadrangular chamber. Each atrium has an appendage called an auricle (L auris- ear), so named because its shape resembles a dog’s ear. The auricle increases the atrium’s surface area. The superior vena cava, inferior vena cava and coronary sinus open into the right atrium.
(a) The superior vena cava carries blood from the body’s upper region,
(b) The inferior vena cava is larger than the superior and carries blood from the lower body’s region,
(c) The coronary sinus carries the majority of blood from the heart itself. The coronary veins open into the coronary sinus. The right atrium receives deoxygenated blood. The left atrium is less in volume than that of right atrium but it has thicker walls. The left atrium is roughly cuboidal. It projects as the left auricle towards the left side.
(d) The left atrium receives oxygenated blood from the lungs through two pairs of pulmonary veins.
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(iii) Ventricles:
There are present left and right ventricles with thick walls. The wall of the right ventricle is thinner than that of the left ventricle. The left ventricle is longer and narrower than the right ventricle. The walls of the left ventricle are about three times thicker than the right ventricle.
(iv) Pulmonary Trunk and Aorta:
The pulmonary trunk arises from the right ventricle. It divides into left and right pulmonary arteries that carry deoxygenated blood to the lungs. The aorta arises from the left ventricle. It is divisible into the ascending aorta, arch of aorta and descending aorta.
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The right and left coronary arteries arise from the ascending aorta. The arch of the aorta (also called aortic arch) gives rise to the brachiocephalic artery (Innominate artery), left common carotid artery and left subclavian artery. The descending aorta runs through the thorax and abdomen and hence it is divisible into thoracic and abdominal parts.
The pulmonary trunk is connected with the aorta by the ligamentum arteriosum that represents the remnant of an embryonic connection between the pulmonary trunk and aorta. In embryo the ligamentum arteriosum is called ductus arteriosus. Coronary arteries arise from the ascending aorta and supply blood to the heart.
Internal Structure:
The internal structure of the heart can be better studied by dissecting it from the ventral side.
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(i) Atria:
The two thin walled atria are separated from each other by the interatrial septum. The right atrium receives the openings of superior vena cava, inferior vena cava and coronary sinus. The opening of inferior vena cava is guarded by Eustachian valve.
The opening of the coronary sinus has coronary or Thebasian valve. In the right atrium adjoining the interatrial septum, an oval depression, the fossa ovalis is present. It marks the position of an opening, the foramen ovale, between the two atria in the foetus, but in the adult it persists only as a depression. The left artrium receives four openings of pulmonary veins.
(ii) Bicuspid and Tricuspid Valves:
The artrioventricular opening between the left atrium and the left ventricle is guarded by the bicuspid valve, also called mitral valve (having two flaps). The right atrio-ventricular opening is guarded by the tricuspid valve, as it has three flaps.
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(iii) Ventricles:
Attached to the flaps of the bicuspid and tricuspid valves are special fibrous cords, the chordae tendineae, which are joined to the other ends with the special muscles of the ventricular wall, the papillary muscles. The chordae tendineae prevent the bicuspid and tricuspid valves from collapsing back into the atria during powerful ventricular contractions.
The chordae tendineae can be seen extending from the valves to the columnae carneae, which are the muscular ridges or projections on the walls of the ventricles. The columnae carneae divide the cavity of the ventricles into smaller spaces, known as fissures. The walls of the ventricles are thicker than the atria. The thickest portion of the human heart is the wall of the left ventricle.
(iv) Semilunar valves:
As stated in the external structure, the pulmonary trunk and aorta arise from the right and left ventricles respectively. At the base of the pulmonary trunk and aorta are located three half-moon shaped pockets known as pulmonary semilunar valves and aortic semilunar valves respectively. These valves allow the free and forward flow of blood, but prevent any backward flow.
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Thus the valves of the heart are:
(a) Bicuspid valve,
(b) Tricuspid valve,
(c) Aortic semilunar valves and
(d) Pulmonary semilunar valves.
Histologically, the heart has an outermost smooth coelomic epithelium— the visceral pericardium, the middle thick muscular layer— the myocardium, composed of cardiac muscle fibres and the innermost layer, the endothelium consisting of simple squamous epithelial cells.
The heart collects blood through both the atria and then distributes it through the ventricles. The action of heart includes contractions and relaxations of the atria and ventricles. A contraction of the heart is called a systole and its relaxation a diastole. The atria and ventricles contract alternately.
The contraction of heart (systole) and the relaxation of heart (diastole) constitute the heart beat. The contraction of atria is initiated and activated by the sinoatrial node (SA Node— pace maker; Fig. 18.10) which spreads waves of contraction across the walls of the atria via muscle fibres at regular intervals.
When the wave of contraction originating from the sinoatrial node reaches the atrioventricular node (AV Node— pace setter), the latter is stimulated and excitatory impulses are rapidly transmitted from it to all parts of the ventricles via bundle of His and Purkinje’s fibres.
These impulses stimulate the ventricles to contract simultaneously. The ventricles force blood through long system of arteries and hence must exert great pressure on the blood.
Essay # 4. Heart Beat:
What is heart beat? Heart beat is the rhythmic contraction and relaxation of the heart. Each heart beat includes one systole (contraction phase) and one diastole (relaxation phase) of the heart to distribute and receive blood to and from the body. The heart of a healthy person beats 72 times per minute (average).
Beating is an inherent capacity of the heart. The heart of a resting human being pumps about 5 litres of blood per minute. This means that a quantity of blood equal to the total amount contained in the body passes through the heart each minute. During exercise both the number of beats per minute and the amount of blood pumped per beat are greatly increased.
Types:
The heart beat is of two types: neurogenic and myogenic. The neurogenic heart beat is initiated by a nerve impulse coming from a nerve ganglion (mass of nerve cells) situated near the heart. It is present in the heart of some annelids and most arthropods. The myogenic heart beat is initiated by a patch of modified heart muscle itself. It is found in hearts of molluscs and vertebrates including human beings.
Origin of heart beat (Fig. 18.10):
The mammalian heart is myogenic (myomuscle, genic originating from). It means the heart beat originates from a muscle, (however, it is regulated by the nerves). The heart beat originates from the sinoatrial node (SA Node)— pace maker, which 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. Sometimes, the SA-node may become damaged or defective. So the heart does not function properly. This can be remedied by the surgical grafting of an artificial pace maker in the chest of the patient. The artificial pace maker stimulates the heart at regular intervals to maintain its beat.
Conduction of heart beat (Fig. 18.10):
Another mass of neuromuscular tissue, the atrio-ventricular node (AV node) is situated in the wall of the right atrium. The AV node picks up the wave of contraction propagated by SA node. A mass of specialized fibres, the bundle of His, originates from the AV node. The bundle of His divides into two branches, one going to each ventricle.
Within the mycocardium of the ventricles the branches of bundle of His divide into a net work of fine fibres called the Purkinje fibres. The bundle of His and the Purkinje fibres convey impulse of contraction from the AV node to the myocardium of the ventricles. Regulation of heart beat (= Regulation of Cardiac Activity).
The rate of heart beat is regulated by two mechanisms:
(i) Neural Regulation (Fig. 18.11):
The cardiac centre lies in the medulla oblongata of the brain. The cardiac centre is formed of cardio-inhibitor and cardio-accelerator parts. The former decreases the rate of heart beat and the latter accelerates it. The cardio-inhibitor is connected with the heart through vagus nerve (it carries parasympathetic nerve fibres) and cardio accelerator through sympathetic nerve fibres.
Sensory fibres extend from the receptors present in the superior vena cava, aorta and carotid sinuses to the cardiovascular centre in the medulla oblongata. The impulses received from the aorta and carotid sinuses decrease the heart rate, whereas the impulses from the vena cava increase the heart rate.
(ii) Hormonal Regulation:
Adrenaline (epinephrine) and noradrenaline (norepinephrine) hormones are secreted by the medulla of the adrenal glands. Noradrenaline accelerates the heart beat under normal conditions while adrenaline does this function at the time of emergency. These hormones directly influence the SA node.
Thyroxine hormone secreted by thyroid gland increases oxidative metabolism of the body cells. This requires more oxygen and thus indirectly increases heart beat.
Essay # 5. Heart Rate:
Human heart beats about 72 times per minute in an adult person at rest. This is called heart rate of that person. The heart rate increases during exercise, fever, fear and anger. Because smaller animals have the higher metabolic rate, their heart rate is higher than larger animals. An elephant has normal heart rate of about 25 per minute whereas mouse has a normal heart rate of several hundred per minute.
Essay # 6. Cardiac Output (Heart Output):
The amount of blood pumped by heart per minute is called sraidiae output or heart output. Heart of a normal person beats 72 times per minute and pumps out about 70 mL of blood per beat. Thus the cardiac output is 72 x 70 or 5040 mL per minute i.e., about 5 litres per minute which is equivalent to the total body blood volume (about 5.5 litres).
Essay # 7. Pulse:
Pulse is the rhythmic contraction and relaxation in the aorta and its main arteries. Thus pulse is a wave of increase which passes through arteries as the left ventricle pumps its blood into the aorta. Pulse is a regular jerk of an artery. Therefore, it is also called arterial pulse.
The pulse rate is exactly the same as the heart rate because an artery pulses every time the heart beats. Pulse is usually taken on the radial artery in the wrist but it can be taken on any artery that flows near enough to the surface of the body to be felt.
The factors which affect the pulse rate are as follows:
(i) The pulse rate in children is more rapid than in adults,
(ii) The pulse rate is more rapid in the female than in the male,
(iii) When the person is standing up the pulse rate is more rapid than when he/she is lying down,
(iv) When any strong emotion is experienced the pulse rate is increased, for example, anger, excitement, fear, etc. (v) Any exercise increases the rate of the pulse.
Essay # 8. Cardiac Cycle (Fig. 18.12 & 18.13):
The cardiac cycle consists of one heart beat or one cycle of contraction and relaxation of the cardiac muscle. During a heartbeat there is contraction and relaxation of atria and ventricles. The contraction phase is called the systole while the relaxation phase is called the diastole. When either the atria and ventricles are in diastolic or relaxed phase, this is referred to as a joint diastole.
During this phase, the blood flows from the superior and inferior venae cavae into the atria and from the atria to the respective ventricles through auriculo ventricular valves. But there is no flow of blood from the ventricles to the aorta and pulmonary trunk as the semilunar valves remain closed.
The successive stages of the cardiac cycle are briefly described below.
(a) Atrial Systole:
The atria contract due to a wave of contraction, stimulated by the SA node. The blood is forced into the ventricles as the bicuspid and tricuspid valves are open.
(b) Beginning of Ventricular Systole:
The ventricles begin to contract due to a wave of contraction, stimulated by the AV node. The bicuspid and tricuspid valves close immediately producing part of the first heart sound.
(c) Complete Ventricular Systole:
When the ventricles complete their contraction, the blood flows into the pulmonary trunk and aorta as the semilunar valves open.
(d) Beginning of Ventricular diastole:
The ventricles relax and the semilunar valves are closed. This causes the second heart sound.
(e) Complete Ventricular Diastole:
The tricuspid and bicuspid valves open when the pressure in the ventricles falls and blood flows from the atria into the ventricles. Contraction of the heart does not cause this blood flow. It is due to the fact that the pressure within the relaxed ventricles is less than that in the atria and veins. The duration of a cardiac cycle is 0.8 sec.
Essay # 9. Heart Sounds:
The beating heart produces characteristic sounds which can be heard by placing the ear against the chest or by using stethoscope (an instrument which magnifies sounds and conducts them to ear). In a normal person, two sounds are produced per heart beat.
(i) First sound:
This is caused partly by the closure of the bicuspid and tricuspid valves and partly by the contraction of the muscles in the ventricles. The first sound, ‘lubb’ is low pitched, not very loud and of long duration,
(ii) Second sound:
This is caused by the closure of the semilunar valves and marks the end of ventricular systole.
The second sound ‘dup’ is highly pitched, louder, sharper and shorter in duration. The two sounds have been described in words as “lubb dup” and their quality indicates the state of the valves. Damage to the bicuspid or tricuspid valve effects the quality of the first heart sound.
When the semilunar valves are injured, a soft hissing noise “lubb shhh” is heard in place of the second sound. This is called a heart murmur. It may be caused by the syphilis, rheumatic fever or any other disease which injures the semilunar valves and affects their working. Thus, the blood can leak back from the pulmonary trunk and aorta into the ventricles.
Essay # 10. Auto Rhythmicity of Heart Beat:
The automatic rhythmicity of the heart is its ability to contract spontaneously and at a regular rate. As we know, human heart is myogenic (myomuscle, genic originating from), the cardiac impulse normally originates from the SA node (a node of specialized cardiac muscle fibres).
Although the origin of cardiac impulse is myogenic, the rate of its formation and conduction may be changed by the action of nerves.
For example, the sympathetic nerve fibres increase the activity of the SA node to accelerate the heart beat while the vagus 10th cranial nerve (carries parasympathetic nerve fibres) decreases the rate of impulse formation from the SA node and its conduction. Thus vagus cranial nerve decreases the heart beat. AV node, bundle of His and Purkinje fibres together provide auto rhythmicity to the heart.
Essay # 11. Electrocardiogram (ECG):
ECG is graphic record of the electric current produced by the excitation of the cardiac muscles. The instrument used to record the changes is an electrocardiograph. Waller (1887) first recorded the electrocardiogram but Einthoven (1903) studied ECG in details, therefore, he got Nobel Prize in 1924 for the discovery of ECG. He is also considered “father of the electrocardiography” (the device used).
A normal electro gram (ECG) is composed of a P wave, a QRS wave (complex) and a T wave. The letters are arbitrarily selected and do not stand for any particular words.
1. The P Wave is a small upward wave that represents electrical excitation or the atrial depolarization which leads to contraction of both the atria (atrial contraction). It is caused by the activation of SA node. The impulses of contraction start from the SA node and spread throughout the artia.
2. The QRS Wave (complex) begins after a fraction of second of the P wave. It begins as a small downward deflection (Q) and continues as large upright (R) and triangular wave, ending as downward wave(S) at its base. It represents ventricular de-polarisation (ventricular contraction).
It is caused by the impulses of the contraction from AV node through the bundle of His and Purkinje fibres and the contraction of the ventricular muscles. Thus this wave is due to the spread of electrical impulse through the ventricles.
3. The T Wave is dome-shaped which represents ventricular repolarisation (ventricular relaxation). The potential generated by the recovery of the ventricle from the de-polarisation state is called the repolarisation wave. The end of the T-wave marks the end of systole.
Normal P—R interval is < 0.12 to 0.2 sec. Normal QRS complex duration is < 0.10 sec Normal Q—T interval is < 0.42 sec.
Enlargement of the P Wave indicates enlargement of the artia. P—R interval (also called P—Q interval) is the time required for an impulse to travel through the atria and AV node to the remaining conductive tissues. During atherosclerotic heart disease (i.e., formation of plaques and calcification) and rheumatic fever, the P—R interval is lengthened. This is due to the inflammation of atria and AV node.
The enlarged Q and R waves indicate a myocardial infarction (heart attack). The S—T interval represents the time between the end of the spread of impulse through ventricles and its repolarisation. Thus, the S—T segment begins at the end of the S wave and terminates at the beginning of the T wave.
The S—T segment is elevated in acute myocardial infarction and depressed when the heart muscle receives insufficient oxygen. T wave is flat when the heart muscles receive insufficient oxygen as in atherosclerotic heart disease. It may be elevated when the body’s potassium level is increased.
When ECG of a person is to be recorded, four leads (metal electrodes) are attached in the arms and legs. It is done after cleaning and putting a special jelly, which improves electrical conduction. With the help of a rubber suction cup, an additional electrode is placed on the chest.
Now the electrocardiograph is switched on which detects and amplifies the electrical current of the heart and transmits to the recording pen. The latter draws a wavy line that is called the deflection waves (Electrocardiogram). The importance of ECG is that it gives accurate information about the heart. Therefore, ECG is of great diagnostic value in cardiac diseases.
