Short essay on physiology of cardiac regulation

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Introduction:

The heart is a muscular pulsatile pumping organ which acts as the centre for blood vascular system. The heart neat starts with the life and ends at death. The origin and mechanism of heart beat is still known to be a miraculous event.

Structure of the Heart:

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The wall of the heart consists of the following three layers.

1. Epicardium:

The outer serous membrane forms the epicardium and the visceral layer of the pericardium.

2. Myocardium:

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It is composed of specialized cardiac muscles. These peculiar striated muscles branch and anastomose (inter connected) showing intercalalted discs which marks the boundries of the cells. IT is thin in auricles and thick in ventricle.

3. Endocardium:

It is the innermost layer lined with flattened endothelium. It si thicker in auricle and thnner in ventricle.

Internal Structure of the Heart:

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Right Auricle:

The right auricle receives blood by superior and inferior vena cava. The eustachiab valve is present at the opening of post-cava; whicfh represents sinu-auricular calve. Near this, sinu-auricular node or S.A. Node or pace maker is present. The A.V. Node or Auriculo-ventricular node is situated at the lower right border of inter auricular septum. The right auricle opens into right ventricle through auriculo ventricular aperture guarded by tricuspid valve. The valves are attached to chordate-teninase.

Left Auricle:

To the left auricle a pair of pula monaryreins open. The left auriculo-ventricular aperture is guarded by bicuspid or mitral valves both the tricuspid and bicuspid value are kept in position to prevent backward flow.

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Right ventricle:

Right ventricle is separated from left ventricle buy thick interventricular septum containing. Bundle of his. It sreads over ventricle as Purkinje fibres. Thevalves are attached to chodaetendinae and papiliary muscles to the columnae carnae. The pulmonary artery starts from right ventricle, guarded by semilunar valve.

Left ventricle:

It is more muscular and the aortic arch starts whch is guarded by semilunar valves.

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Heart beat:

The heart beat comprises of rthymic contraction and expansion of cardiac muscles.

Types of Heart:

1. Neurogenic heart:

In this type of heart contrction is initiated by nervous stimulation.

Ex. Arthropods, annelids.

2. Myogenic Heart:

In myogenic heart the heart bear starts from a modified heart muscle or pace maker.

Ex. Molluces and vertebrates.

Physiology of Heart beat:

The rthymic contraction and expansion occuts fro the ginning to the end of life. IN a normal man of 70 years life span there are 2700000000 heart beats. The heart beat comprises of two phases i.e.,

1. Systole or contraction

2. Diastole or relaxation

The complete heart beat takes nearly 0.8 seconds in human at rest.

Origin of heart beat:

The contraction of auricles is initiated by a special modified heart muscles.

Pacemaker or S.A. Node:

The sinu-auricular ode or pace maker. IT is present near the opening of the post-caal. It is made up of thin elongated fibres tapperng at both ends, longitudinally striated and it is interconnected. It is innervated by autonomic nervous system. So it is calle as Heart of Hearts. The fibres fo heart muscles, like all cells are charged positively on the outside and negatively inside. Inpace maker, this chargebreaks down spontaneously per 72 times per minute. This is turn initiates breakdown of charges in the surrounding cells. Thus generating a tiny wave of current, which spreads over the entire heart. Because of this built in signal generating mechanism, the vertebrate’s heart exhibits intrinsic beat for which it can maintain when isolated from the body.

Auriculo-ventricular Node – A.V. Node

When this wave of contraction of pacemaker reaches the A.V. Node, it is stimulated to emit an impulse of contraction which spreads to the ventricular muscles through Bundle of his and Purkinje fibre. The A.V. node is present at theposterior right border of interauricular septum.

In abnormal condition when S.A. nide fails the A.V. node cn generate the wave of contraction and the rate of heart-beat is about 45/min.

Bundle of His:

It is a set of specialoized muscle strands which originates from the A.V. Node and passed downwards in the interventricular septum discovered by His in 1893.

Purkinje Fibres:

1. Each fibre has a large diameter (50-70).

2. The Bundle of His divides into two branches which breaks up withi the myocardium of ventricle into a network of fine branching anastomsing filaments.

3. IT contains large amount of glycogen with more sarcoplasm.

Blood Supply:

1. The coronary arteries supply blood to the heart.

2. The coronary sinus receives blood in the right atrium.

Nervous Regulation of heart beat:

The heart is under the control of two pairs of nerves, arising from cardiac centre of meulla oblongata. These are accelator or sympatheitic and inhibitor or parasympathetic.

1. Accelatr Nerves:

It arises from second, third thoracic segments of spinal cord and secrete sympathin. These are responsible for pounding of heart and also accelerated by vigorous exercise.

2. Inhibitor Nerves:

It runs from the vagus cranial nerve and secretes acetylcholine to reduce the heart beat. It is destroyed by acetyl choline esterase.

Other Factors affectin Heart beat:

1. Environmental changes communicated through vision, audition and other senses.

2. Emotions.

3. Internal activities of the body.

4. Temperature.

5. pH.

6. Hormones like adrenaline, thyroxine etc.

Cardiac Cycle:

It consists of a regular contraction and relaxation sequences of events called cardiac cycle. It takes about 0.8 secs to complete.

(a) Atrial systole:

The S.A. Node generates an impulse of contraction which spreads as a Wace of contraction over the atrial myocardium. IT takes about 0.1 secs. The blood, collected by auricle is pushed into the ventricle.

(b) Ventricular systole:

When the wave of contraction reaches the A.V. Node, it is stimulated to emit an impulse of contraction, which spreads into the ventricular muscle through Bundle of His and Purkinje Fibres. It takes 0.3 secs. The blood is pushed from respective ventricles into pulmonary aorta and aortic arch. A complete heart beat consists of one systole and one diastole and a pause.

(c) Complete diastole:

The auricular and ventricular muscles relax after a systole for a period of 0.4 secs, it is called as complete diastole.

Flow of Blood:

Right auricle receives blood from different parts of the body through pre and post caval veins. The left auricle at the seme time gets blood from the lungs through pulmonary veins. The blood from auricle passed into respective ventricles. Blood from right ventricle during cystole, pass through pulmonary aorta into lungs and form left ventricle into aortic arch.

Electro cardiogram:

When muscles contract there is a change in elelctric potential across the membrane of muscle fibre. This recording is called as ECG.

Blood Pressure:

It is defined as the force which the blood exet on the walls of artery. IT is the systolic pressure by diastolic pressure. It is measured by sphygmomanometer.

Normal man B.P.=120/80 mm Hg.

Cardiac Output:

Cardiac output = Heart rate * stroke volume i.e. cardiac output in litres /min

= heart beats/ minutes * litres/beat

Ina normal man, cardiac output = 72 * 0.8 = 5.5litres. Cardiac output is the volume of blood pumped by each ventricle per minute.

Conclusion:

The heart in vertebrate is nature’s marvel, depicting self-regulating mechanism which works not stop till death.

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