Essay on the Mammalian Heart (1572 Words)!
Human heart is situated in the thorax between the lungs with its apex (formed by left ventricles) resting on the diaphragm. Usually it measures about 12 cm in length and 9 cm in breadth. It is enclosed in double membrane fibro serous sac called pericardium.
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Outer membrane is parietal pericardial membrane (fibrous) and inner one is visceral pericardial membrane (serous) that encloses a narrow pericardial cavity containing pericardial fluid for frictionless movement, protection from shock and mechanical injury.
Heart wall consists of three layers – epicardium (outermost), myocardium (middle one) and endocardium (innermost). Among these three layers, myocardium consists of cardiac muscles, resembling the striated muscles structurally and smooth muscles functionally because of involuntary muscle. Cardiac muscle is similar to sketetal muscle in many ways.
Both types appear striated as a result of the arrangement of the actin and myosin filaments in the sarcomeres of the muscle fibres. The sarcolemma (plasma membrane) of both types has invaginations called T-tubules that spread depolarization throughout the cell.
The signal is transmitted to the terminal cisternae of the sarcoplasmic reticulum, which are adjacent to the T – tubules.
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The release of calcium from the cisternae allows contraction to take place. The mechanism of muscle contraction (sliding filament) is the same in the cardiac and skeletal muscle.
Hence the heart is constructed from cardiac muscle which has several properties that are relevant to its function in the heart like
i. Involuntary
ii. Consists of interwoven intercalating fibres – which are the connections between two adjacent cardiac cells. Intercalated discs help multiple cardiac muscle cells to contract rapidly as a unit. This is important for the heart to function properly.
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iii. Incapable of building up an oxygen debt.
Internal structure:
Heart is four chambered with two anterior auricles and two posterior ventricles. The atria lie above and behind the ventricles.
On the surface of heart they are separated from the ventricles by an atrio-ventricular groove. Auricle or atrium is divided by an interatrial septum into left and right auricles. On this septum a depression called fossa ovalis is present which a remnant of embryonic foramen ovale is.
The ventricles are separated from each other by an inter-ventricular septum. A prominent muscular trabeculum called moderator band extends from the inter-ventricular septum to the anterior papillary muscle in the right ventricle.
Ventricles are far thicker walled than atria and left ventricles are at least three times thicker than the right one.
This is because the ventricles pump out blood with force away from heart, the right one to pulmonary artery and the left one to aorta. The left one is thicker because aorta has to take the blood to organs far away from heart.
The right auricle receives two large veins – inferior and superior vena cava. The opening of inferior vena cava is guarded by a membranous falciform fold called eustachian valve. It is present between the fossa ovalis and post caval. The right auricle also receives a small coronary sinus whose opening is guarded by a small fold called coronary or the basian valve.
Myocytes in the atrial differ from myocytes into the ventricles.
Circulation of Blood:
The right atrium receives deoxygenated blood from the whole body through superior and inferior venae cavae and the coronary sinus. It contracts and sends the blood through the right Atrioventricular orifice to the right ventricle.
The right ventricle contracts and propels the blood into the pulmonary trunk, the pulmonary arteries and finally the lungs where the blood is oxygenated.
The oxygenated blood returns to the heart through the 4 pulmonary veins and enters the left atrium. The left atrium contracts and sends its blood through the left Atrioventricular orifice into the left ventricle, which in turn contracts and drives the blood into the ascending aorta and its ramifications.
Valves of Heart:
The valves of heart maintain unidirectional flow of blood and prevent its regurgitation in the opposite direction. Opening and closing of valve depends upon pressure on opposite sides. Each valve has a set of cusps or flaps. The cusps are the fold of endocardium strengthened by an intervening layer of fibrous tissue. These valves act like one-way doors, allowing blood to flow either forward into the next chamber, or out of the heart via one of two main blood vessels that carry blood away from the heart.
The valves close to prevent back flow. Without these valves the heart would have to works much harder to push blood into adjacent chambers. When these valves break down, as often happens in older or inactive people, the blood does flow back and pool in the weakened vein legs. The result is varicose veins which often appear as large purplish tubes in the lower legs.
Artificial Heart Valve:
An artificial heart valve is a man-made device that is used to replace one of a patient’s own damaged or diseased heart valves that cannot be repaired.
The artificial heart valve is inserted into the patient’s heart as part of an open-heart surgery called heart valve replacement. On average, an artificial heart valve can last 30 years or more. There are two types of artificial heart valve – mechanical & bioprosthetic valve.
Mechanical valves are made of artificial materials such as plastics and metal. In this use of anticoagulants is necessary to reduce the risk of blood clot formation.
Mechanical valves are more resistant to the constant demands on them (the valve must work each time the heart beats, so it must open and close at least 100,000 times each day.
Bio-prosthetic valves are the other type of artificial heart valves. These are taken from animal sources, usually the pig. Artificial valves for malfunctioning leg veins may be able to help people with venous disease, which can cause varicose veins.
These valves are more acceptable to the human immune system as they are made from compatible biological matter.
Types of valves:
1. Atrioventricular Valves – Bicuspid & Tricuspid
Bicuspid valve, also called mitral valve present on the left side i.e. between the left atrium and left ventricles. It consists of 2 cusps/flaps.
Tricuspid valve:
It consists of three flaps/ cusps and present between the right atrium and right ventricles.
Both cuspid valves (i.e. mitral and tricuspid) are connected below to the walls
Heart valve disease fall into two categories – Stenos is (narrowing of valve) & incompetence (or regurgitation). Steno tic heart valve (results in high pitched voice when heard through stethoscope) prevents the valve from opening fully, due to stiffened valve tissue.
Hence there is more work required to push blood through the valve. Whereas, the incompetent valve causes inefficient blood circulation by permitting backflow of blood in the heart because the valves are supposed to prevent any backflow same is the case in veins of legs causing varicose vein.
The contractions of papillary muscles bring the tightening of chordae tendinae which in turn prevent the valves from turning inside out or from being forced upwards during contraction of ventricles.
II. Semilunar Valve:
They are so called because their cusps (3) are semi lunar in shape i.e. look like half moons in cross section.
It is present where arteries leave heart. These are of two types – pulmonary valve and aortic valve which are present at the base of pulmonary artery and the aorta respectively.
The pulmonic and aortic valves are virtually identical although the aortic valve consists of a thicker fibrous structure than the pulmonic valve.
Pulmonary valve controls flow from the right ventricle into the pulmonary artery which carries blood to the lungs for oxygenation whereas the aortic valve on the left side opens the way for oxygenated blood to pass from the left ventricle into the aorta (the body’s largest artery).
Automatic Rhythm city of Heart:
Automatic rhythmicity of the heart is the ability to contract spontaneously and at a regular interval of time. Mammalian heart is myogenic. It means heart beat results from a wave of electrical potential called cardiac impulse arising from sinoatrial node (SA node) and spreading over the cardiac chambers through a special conducting system. SA node is a specialised cardiac muscle fibre (or nodal fibres) which lies in the wall of right atrium near the opening of superior vena cava and which contracts on its own about 72 times per minute.
In contrast, the muscle in the rest of the atrium contracts on its own only 40 or 50 times per minute. From SA node cardiac impulse travels to Atrioventricular node (AV node lies between the right atrium and ventricle) then passes along AV bundle (also called bundle of His because these are present in the intraventricular septum connected to the AV bundle) and its branches to reach the Purkinje fibres in the ventricles.
AV bundles provide the only route for the transmission of wave of excitation from the atria to the ventricles. Purkinje fibres conduct impulses five times more rapidly than surrounding cells.
It forms a pathway for conduction of impulse that ensures that the heart muscle cells contract in the most efficient pattern.
SA node is known as the pacemaker of heart because the cells in the SA node contract the most number of times per minute and because each wave of excitation begins here and acts as the stimulus for the next wave of excitation.
In a diseased heart the AV node can acts as pacemaker though it beats at a comparatively less frequency (around 40-50 per min.).