How the Heart is Nourished?

The two coronary arteries arise from the aorta and pass throughout the heart muscles and ultimately from capillaries which give rich supply of blood to the wall of the heart. This capillary network pours its blood into the coronary veins which lead into the coronary sinus and the latter empties vessels into the right atrium. Naturally any damage to the coronary vessels could have serious consequences. The heart muscles cannot function without a continuing supply of oxygen and food materials.

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.

Beating is an inherent capacity of the heart. The heart of resting human beings 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.

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.

Origin of Heart Beat:

The mammalian heart is myogenic (myo = muscle, genie = originating from). It means the heart beat originated 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:

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 network 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:

The rate of heart beat is regulated by two mechanisms.

(i) Nervous Regulation: 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 vena cava, aorta and carotid sinuses to the cardiovascular centre of 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: Adrenalin (epinephrine) and noradrenalin (nor epinephrine) hormones are secreted by the medulla of the adrenal glands. Noradrenalin accelerates the heart beat under normal conditions while adrenalin 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.

Heart Rate :

Pulse per minute is called the heart rate. As we know human heart beats about 72 times per minute in an adult person at rest, this is heart rate of that person. The 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 normal heart rate of about 25 per minute whereas mouse has a normal heart rate of several hundred per minute.

Cardiac (Heart) Output:

The amount of blood pumped by heart per minute is called cardiac output. Heart of a normal person beats 72 timer per minute and pumps out about 70 millilitres of blood per beat. Thus the cardiac output is 72 x 70 or 5040 millilitres per minute i.e., about 5 litres per minute which is equivalent to the total body blood volume (about 5.5 litres).

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 in to 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, (it) The pulse rate is more rapid in the female than in the male. (Hi) 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, feat, etc. (v) Any exercise increases the rate of the pulse.

Cardiac Cycle :

The cardiac cycle consists of one heart beat or one cycle of contraction and relaxation of the cardiac muscle. During a heart beat 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. 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 semi lunar valves open,

(d) Beginning of ventricular diastole. The ventricles ralax and the semi lunar 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.

Heart Sounds :

The beating heart produce 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 sound have been described in words as “lubb dub” 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.

Autorhythmicity of Heart Beat :

The automatic rhythm city of the heart is the ability to contract spontaneously and at a regular rate. As we know, human heart is myogenic, 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 heat beat while the vagus 10th cranial nerve (carries parasympathetic nerve fibres) decreases the rate of impulse formation from the SA node and its conduction. AV node, bundle of His and Purkinge fibres together provide autorhythmicity to the heart.

Blood Pressure (BP) :

Blood pressure does not mean a disease at all. It is essential for life and every person has blood pressure to a certain degree. The pressure exerted by the flow of the Wood on the elastic -walls of the arteries is called blood pressure.

The blood pressure is greater during, the systole than during the diastole. The blood pressure is measured b\ the use sphygmomanometer (invented in Italy in b^ Dr. Sapione Hiva-Rocci). In normal young person, the systolic pressure is 120 mm Hg and diastolic pressure is 80 mm Hg. It is normally expressed as 120/80. The blood pressure varies with age. It is influenced by the rate of heartbeat. “Normal heart-beats of a man are too 72 per minute.

A persistent rise in blood pressure is called high blood pressure or hypertension. Fall in BP is termed as low blood pressure or hypotension. A survey by the Indian Council of Medical Research recently showed that one out of every six Indians is hypertensive. In metropolitan cities, lime Bombay, Calcutta and Delhi, this aliment is more prevalent.

Electrocardiogram (ECG):

ECG is graphic record of the electric current produced the excitation of the cax&ac. mêlées. The instrument -used to record the electrocardiograph. Waller (1881″) first recorded the sea to cardiograph hut Einthoven (,1903) studies ECG in details, therefore, he got “Nobel Prize in 1924 for discovery of ECG. He is also considered “father of 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 indicates the depolarisation of the atria (a trial contraction). It is caused by the activation of SA node.

2. The QRS wave (complex) beings after a fraction of a second of the P wave. It represents ventricular depolarisation (ventricular contraction). It is caused by the impulses of the contraction from AV node through the bundle of His and the Purkinje fibres and the contraction of the ventricular muscles.

3. The T wave indicates ventricula’r repolarisation (Ventricular relaxation). The potential generated by the recovery of the ventricle from the depolarisation state is called the repolarisation wave.

During atherosclerotic heart disease (i.e., formation of plaques and calcification) and rheumatric 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).

T wave is flat when the heart muscle receives insufficient oxygen as in atherosclerotic heart disease. It may by 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.

Pacemaker, SA node is called the natural pacemaker. When SA node does not send impulses to the atria, the ventricles fail to receive atrial impulses. The conducting system of the heart is disrupted. In such patients normal heart beat can be restored and maintained with an artificial pacemaker.

Temporary Pacemaker is used in emergency such as arrythmia control (e.g. bradycardia – slow heart beat). Eletrodes of temporary pacemaker is introduced from juguler, sub clavian and femoral veins as well as from veins of the upper extremity.

Permanent pacemaker is used in atrioventricular (AV) block, SA node dysfunction, etc. In the patients of Stokes adams syndrome (ventricular escape or ventricular asystole) atrial impulse suddenly fails to be transmitted to the ventricles. In such patients, the permanent pacemaker is implanted.

An artificial pacemaker is an artificial electronic device which regularly sends small amount of electrical charges that stimulate the heart. The artificial pacemaker consists of (a) a pulse – generator containing cell (solid state lithium cell) to produce electrical impulse, (b) the lead in the form of a wire which transmits the impulse and (c) an electrode, which is connected to the portion of the heart where impulse is to be transmitted.

During implantation, the lead is inserted into the cephalic vein, following an incision inferior to the clavicle. The lead is then guided into the right axillary vein, right sub clavian vein, superiorvena cava, right atrium and finally into the right ventricle to make contact with the ventricular myocardium. The pulse generator is placed in a subcutaneous pocket. The patients with artificial pacemaker should not go for MRI because MRI can cause inhibition of the pacemaker. Such patients should not be exposed to airport security metal detectors.