The Heart

 The human heart is s conical, hollow, muscular organ which works continuously throughout the life of a person. It is about the size of a clenched fist and weighs 300 grams.

     The heart lies in the chest just behind the breast stone and between the two lungs. It is enveloped in a two layered tough membranous bag. The pericardium. The space between the two layers is filed with liquid. This reduces the friction , caused by the pumping movements of the heart, between the heart wall and the surrounding tissues.

     When at rest, the average human heart contracts and relaxes about 70 times per minute and pumps about 14,000 litres of blood over 100, thereby increase swing the supply kg oxygen and food to the body cells. To Carry out such hard work, the wall of the heart is made up of a special muscle, the cardiac muscle, found only in the heart. It contract and relaxes rhythmically, and can work continuously without becoming tired like the skeletal muscles. To work hard and continuously, the heart muscles need a good supply of blood to provide oxygen and nutrients and remove its wastes. This is supplied by the coronary arteries and veins which spread all over the heart wall.

 Structure of the Heart

 The heart has four chambers: two upper thin walled atria or auricles and two lower thick walled ventricles. The right side of the heart is completely separated from the left side by a wall called the septum.

     Two large veins, the superior (anterior) and inferior (posterior) venue cavae, bring dark red, deoxygenerated blood from the various parts of the body (except the lungs), open into the right auricle. The pulmonary veins bringing bright red, oxygenerated blood from the lungs open into the left auricle. The openings of the veins into the auricles are controlled by rings of muscles found at the vein. Their contraction closes the openings while their relaxation opens them.

      The right auricle opens into the right ventricle. This valve has three flaps which are attached by cord like tendons (chordae tendinae) to the walls of the right ventricle. These cords allow the flaps to open outwards into the ventricle. Hence, the valves only allow a unidirectional blood flow from the auricles into the ventricle. The opening of the left auricle into the left ventricle is guarded by the bicuspid or mitral valve. This valve is structurally and functionally similar to the tricuspid valve except that it has two flaps instead of three. The right ventricle opens out into the pulmonary artery, which branches into two, one leading to the right lung and the other to the left lung. These carry deoxygenerated blood to the lungs. The left ventricle opens into the large aorta which branches distributes oxygenerated blood to all parts of the body except the lungs. Semi lunar valves in these arteries prevent back flow of blood from the arteries into the ventricles.

       The wall of the left ventricle is at least three times thicker than that of the right ventricle. This is because the contraction of the left ventricle must send the blood round the much longer systemic circulation. While that of the right ventricle only needs to send blood round the shorter pulmonary circulation. Thus , blood entering the aorta is at a very high pressure (about 105 mmHg), while that entering the pulmonary artery is at a much lower pressure (about 16 mmHg).

 One heartbeat

 The two sides of the heart work together. The relaxed or resting period of the heart chambers, especially the ventricles, is known as diastole. The period when the chambers, especially the ventricles, Contract is known as systole. The two auricles relax at the same time to receive blood from the respective veins. The blood entering the auricles is at low pressure. The auricles contract causing the bicuspid and tricuspid valves to open, and the openings of the veins to close. The blood then flows into the relaxed ventricles. Almost at once, the ventricles contract. Back flow of blood into the auricles is prevented by the sudden closing of these valves as a loud 'lub' sound in a heart beat. Blood from the ventricles enters the arteries. After fully contracting, the ventricles start to relax. The blood in the arteries tend to flow back into the ventricles. This is prevented by the sudden closing IG the semi Luna valves ad a soft 'dub' sound in a heart beat. Thus, one complete heartbeat consists of one contraction (systole) and one relaxation (diastole) of the ventricles, and lasts for about 0.8 seconds.

 Pulse

 Each time the ventricles contract, they force blood into the arteries (already filled with blood) at high pressure. When this happens, the elastic walls of the arteries dilate suddenly. Then when the ventricles relax and the semi lunar valves close, the walls of the arteries recoil, and at the same time the muscles in these walls contract. These two forces cause blood to flow away from the heart in a series of waves. Each wave is called a pulse. We can feel it by placing a finger over an artery which lies near the surface of the skin, such as the arteries at the wrists and neck.