Heart and its functions – The Cardiac Cycle

In this article, you will learn about the heart and its functions. The following topics are about heart and its functions:

  • The Heart as a Pump
  • Blood Supply to the Heart—The Coronary Circulation
  • The Cardiac Cycle
  • Cardiac Function
  • Valves of the Heart
  • Electrical Excitation of the Heart
  • Abnormal Rhythms of the Heart
  • Electrocardiogram

Heart and its Functions

Heart and its functions

1. The Heart as a Pump – Heart and its functions

1. The heart is made up of 2 sides, the left, and the right. Each side of the heart is divided into an upper chamber which is called the atrium function of which is to collect blood from the veins which drain into it and pass it on to the lower chamber called the ventricle. Most of the blood collected by the auricle (atrium) just tickles to the ventricle and only the last 40% is forcibly pumped by the auricle into the ventricle. The ventricle then contracts forcibly and pumps out the blood into the arteries.

The right ventricle pumps out the blood into pulmonary circulation (pulmonary artery). The de-oxygenated during its passage through the pulmonary circulation gives out carbon dioxide and collects oxygen. The oxygenated blood returns to the left auricle via the pulmonary veins and then reaches the left ventricle. The left ventricle then contracts forcibly and pumps the blood into the general circulation or systemic circulation via the aorta.

2. From the aorta, the blood proceeds to the smaller branches and ultimately reaches the capillaries where oxygen is given up to the tissues and carbon dioxide is collected. The deoxygenated blood returns to the heart (right auricle) via the veins.

Heart functions

2. Blood Supply to the Heart—The Coronary Circulation

1. The 2 main arteries supplying blood to the heart are the left coronary artery which supplies blood to the left side of the heart and the right coronary artery which supplies blood to the right side of the heart.
2. The left coronary artery (also called left main artery) divides into 2 branches. The more important branch goes right down to the tip of the heart along the anterior or front surface. It is called left anterior descending artery (LAD). The smaller branch goes round to the back of the heart and is called left circumflex artery.
3. The right main artery divides into two similar branches, one being called the right descending artery (RDA) and the second, right circumflex artery.
4. The right and the left circumflex arteries meet and form a ring round the heart.
5. It is important to know the position of the coronary arteries. When an angiogram (X-ray with contrast dye) is done, the blockage of the arteries and the degree of blockage are visualized and these are crucial factors in deciding whether a coronary bypass operation should be done.

Functions of heart

3. The Cardiac Cycle – Heart and its functions

1. To function as a pump, the heart has to alternately contract and relax.
2. The contraction of the heart is called systole. During systole, the heart pumps out blood into the arteries.
3. After contraction is over the heart relaxes, relaxation of the heart is called diastole. During diastole, the heart becomes flaccid and allows incoming blood to enter the heart.

4. Cardiac Function

1. A normal heart pumps out all the blood that it receives.
2. All the muscle fibers of the heart contract at each stroke.
3. The strength of contraction depends on the volume of blood inside the heart. The greater the volume of blood inside the heart, the more the stretch of the cardiac muscle and the more forceful the contraction.
4. The amount of blood pumped out at each heartbeat is called the cardiac output per stroke or the “Stroke Output”.
5. The heart normally beats 60—90 times/min. and this is called cardiac rate or heart rate.
6. At each systole, the pressure in the heart increases, the maximum pressure being in the left ventricle which may go up to 300 mm Hg. During diastole, the pressure in the heart becomes zero and may even be slightly negative.
7. The pressure in the left ventricle at the end of diastole, just before the heart starts to contract is called the preload.
8. The pressure in the left ventricle when the ventricle starts contracting and pushing out the blood in the aorta is called afterload.
9. The heart is supplied by both sympathetic and parasympathetic nerves.
10. Stimulation of sympathetic nerves increases the heart rate. Stimulation of para-sympathetic nerves slows the heart rate.
11. Sympathetic new act by the release of catecholamines which combine with beta receptors in the heart and stimulate the heart to beat more frequently and more forcefully.
12. At rest, the volume of blood in the left ventricle at the end of diastole (just before it contracts) is about 120 ml. The volume of blood ejected on contraction is about 70 ml. A residue of 50 ml remains at the end of the contraction, The ejection fraction is 70/120 ml, i.e. about 60%.
13. During exercise, the stroke output increases and the heart rate also increases. The cardiac output per minute increases many times.

5. Valves of the Heart – Heart and its functions

1. To prevent the backflow of blood there are valves at many sites of the heart and blood vessels.
2. There are valves between the auricles and the ventricles. These valves allow the flow of blood from the auricles to the ventricles but do not permit backflow.
3. One set of valves prevent backflow of blood from the pulmonary artery into the right ventricle. This is called semilunar valve.
4. One set of valves prevent the blood from flowing back from the aorta to the left ventricle. ms is called aortic valve.
5. The presence of these valves ensures that blood always flows forward and never in the reverse direction.
6. If these valves are damaged, the efficiency of the heart suffers and the heart may fail.
7. Mitral valves have to withstand tremendous pressure, especially because the left ventricle during its contraction generates pressures of up to 300 mm Hg and at the other extreme, during relaxation the pressure is0 mm Hg. To withstand this severe pressure, the valves are strengthened by rope-like muscles and tendons called papillary muscle and chordaetendinae.
8. Valves can malfunction in 2 ways. Narrowing of valves, called stenosis, obstructs the free forward flow of blood and this results in the heart chambers to overexert and ultimately fail.

When the valves are damaged and allow backflow of blood, the heart chamber has to pump a very large volume of blood to make up for the blood which has flowed back. This is called regurgitation.Both stenosis and regurgitation result in extra work for the heart, which ultimately may fail and cause death.

6. Electrical Excitation of the Heart

1. The entire heart does not beat simultaneously. The wave of contraction starts in the auricles and proceeds down to the ventricles. This ensures maximum efficiency as a pump.
2. Heart muscle fibers can contract and relax alternately on their own, but the rate is slow—around 40/min. for the ventricular muscle. To have a faster rate (around 72/min.), the heart muscle is driven by impulses coming down from a very special place in the heart which beats at a rhythm of approximately 72/min. This special part of the heart is called the pacemaker.

The pacemaker is situated at the extreme upper part of the right auricle. It is also called the sino-auricular (S.A) node. Electrical impulses originating in the sino-atrial rode spread down to the junction called atrioventricular (A.V) node and then spread down along the conducting bundle called the bundle of His to reach the apex of the heart (ventricle). During its spread, it is held up for a moment at the A.V. node.This enables the blood to be emptied by the auricle into the ventricle. By the time the impulse reaches the ventricle, the auricles have already emptied blood into the ventricles and the vermcles are full. The ventricles then contract and pump the blood into the arteries. Because the S.A. node has the fastest rhythm (72/min.), it beats earlier and
the whole heart has to follow its rate of 72/min.

7. Abnormal Rhythms of the Heart

Many abnormal rhythms are possible in the heart. When the heart rate is fast it is called tachycardia (over 90/min) When the rate is slow it is called bradycardia. In some persons, the heart rate accelerates during inspiration and
decelerates during expiration. This is sinus arrhythmia. The grossly abnormal heart rhythms are as follows:
1. Heart block. When there is blockage of the transmission of an electrical impulse along the pathway for conduction, abnormal rhythms appear, e.g. if the pacemaker is non-functional, the whole heart picks up the ventricular rate of 40/min. The auricles and the ventricles may beat independently producing atrioventricular dissociation. Heart block may be complete or partial, e.g. right heart block (RBBB) or the left heart block (LBBB).
2. Auricular flutter. In this disorder, the auricular rhythm is around 200/min. Ventricles become inefficient at this high rate and the heart fails.
3. Auricular fibrillation. The auricular rate is 400-600 times a minute. All the impulses cannot go through the A. V. node and the ventricles, therefore, beat at the much lower rate but irregularly, depending on the number of impulses, which get through the A. V. node. Here also the heart may fail and more importantly, clots may form in the auricles.
4. Ventricular tachycardia. This is a potentially fatal condition. The ventricles beat at a high rate and the heart fails.
5. Ventricular fibrillation. Ventricles beat at a very very rapid rate, and the patient dies.
6. Ventricular extra-systole. From time to time, an extrasystolic beat is produced by the ventricle. This is a very common condition and is not always due to disease. It may be innocuous.

Treatment of cardiac arrhythmias depends on the diagnosis, the correct drug treatment and the use of a machine called the defibrillator. The defibrillator gives an electric shock which stops all the electric currents in the heart and when the heart resumes heating, it is usually with a normal rhythm, the pacemaker setting the heart rate.

8. Electrocardiogram

An electrocardiogram is the recording of the electrical currents in the heart. To record an electrocardiogram, wires or electrodes are connected to the left arm, right arm, and the left leg. These are called limb leads. In addition, movable leads in 6 positions are placed in the precordial area of the chest (over the heart). By this method, the standard normal electrocardiogram gives 12 different tracings. The reading of the electrocardiogram is very specialized and gives an immense amount of information.

The normal electrocardiogram has a P wave due to auricular electrical changes, and a QRS wave which gives the ventricular electrical activity and then a T wave which gives the process of disappearing electrical activity. With the help of the electrocardiogram, the following diagnosis can be made:
— Tachycardia (fast heart rate)
— Bradycardia (slow heart rate)
— Sinus arrhythmia
— Heart block
— Ventricular extrasystole
— Auricular fibrillation
— Auricular flutter
— Ventricular tachycardia
— Ventricular fibrillation.
These are all disturbances in rhythm. More importantly, cardiogram will give information on:
1. Myocardial infarction (heart attack).
2. Myocardial ischemia (inadequate blood supply to the heart).
3. Hypertrophy or enlargement of the individual chambers of the heart, i.e. left auricle, right auricle, left ventricle and the right ventricle.
4. Presence of heart failure.
5. Toxic effect of drugs, e.g. digitalis.
6. Presence of other diseases such as pericardial effusion, thyrotoxicosis (excessive thyroid function) and gross changes in serum potassium.

Of all the investigations done on the heart, electrocardiogram or ECG is the most important, informative and most useful and easy to record. The tracing is then labeled and read by the physician or the cardiologist.

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