The heart muscle contracts to pump the blood from the Right Ventricle through the Pulmonary Semi Lunar valve and into the Pulmonary artery, which splits into two branches leading to each of the lungs. This is the only artery that carries deoxygenated blood, because it is needed to carry blood to the lungs where it loses it’s carbon dioxide and absorbs oxygen through a network of fine capillaries.
The blood returning from the lungs is oxygenated, and enters the heart again through the two Left Pulmonary veins, into the Left Atrium. The blood flows from here through the Mitral (Bicuspid) valve, which consists of two membrane flaps into the Left Ventricle.
The Left Ventricle has a thicker muscular wall than the Right Ventricle because it has to be capable of pushing blood through the arteries at a high enough pressure to be able force it through all of the small arteries and capillaries to reach all parts of the body.
The strong heart muscle pushes the blood through the Aortic Valve into the Aorta, which branches out into smaller arteries which distribute blood to the brain, trunk and limbs.
The heart muscle needs its own blood supply in order to operate; this comes via the three Coronary Arteries, which lead from the Aorta, and branch and spread over the surface of the heart, eventually re-entering the heart cavity via the Coronary Sinus into the Right Atrium. The electrical stimulation that the muscle needs in order to contract is generated in the Sinus Node, or Pacemaker, which is an area of specialised cells located in the wall of the Right Atrium. This sets the heart beat rhythm, and is under the control of the autonomic nervous system which determines the heart rate, depending on the amount of carbon dioxide in the blood, which varies due to action, exercise or emotional state. The atria is caused to contract first, and the electrical impulse then reaches the ventricles shortly after through the Atrioventricular node, across a band of neuromuscular fibres called the Bundle of His, and spreads across the ventricles through the Purkinje fibres. These cause the ventricle to contract shortly after the atria, in a sequential rhythm of contraction and relaxation. The contraction stage is known as systole, and the relaxation stage is called diastole. The area between the atria and the ventricles is insulated by another fibrous band which prevents the signal from prematurely entering the ventricle, and throwing this rhythm into chaos.
How does coronary heart disease effect the normal functions of the heart?
Coronary heart disease is the name given to the disorder arising from the narrowing of the coronary arteries, which prevents the heart muscle from receiving a sufficient amount of blood; this is called Ischaemia. The reduced blood supply eventually causes a Myocardial Infarction, or heart attack, which means that the muscle cells die, and the heart muscle weakens. A loss of 40% of the heart muscle and there is not enough power left to maintain circulation.
The dead muscle is replaced with fibrous tissue, which is weak, and may rupture or cause leakage into the pericardium or the septum between the two heart cavities. A weak area in the wall of the heart may become ballooned, and the heart’s efficiency is reduced, this is called an aneurysm. Aneurysms may be removed by surgery.
Coronary heart disease is the biggest killer in the Western world, and is caused by high levels of cholesterol and other fatty substances in the blood accumulating inside the arteries. As the coronary arteries become increasingly blocked, a sufferer begins to suffer from angina pectoris, a tightening of the chest which spreads out through the arms and shoulders, and shortness of breath whenever strain is put on the heart e.g. during exercise, or brought on by emotional stress. This discomfort disappears when the sufferer rests and the heart rate returns to normal.
The coronary arteries are elastic so that the blood flow through them can be controlled due to demand, and have an inner cell lining, surrounded by a muscular wall, wrapped in a fibrous outer layer. The build up of fat occurs under the delicate lining layer, and causes it to swell out. Rather than remaining soft, it calcifies into a hard plaque, which eventually ruptures the inner layer of the coronary artery and causes a thrombosis to be formed inside the damaged artery, from adhering platelets forming a clot. This process is known as Atheroma. The clot may break off and cause a blockage, called an occlusion further along the artery. This sudden decrease in oxygen supply to the heart is almost always fatal. Persons who survive a heart attack must under go careful rehabilitation, and are at risk from recurrence.
The immediate cause of death in a heart attack is called ventricular fibrillation or cardiac arrest. This is where the heart loses its rhythm and beats weakly and rapidly. The normal heart beat can be re established by a massive electrical shock across the chest.
People suffering from severe angina due to atherosclerotic disease may be given nitrate which is sprayed of dissolved under the tongue, and help the heart to work more efficiently. Some people take aspirin to thin the blood and reduce clotting. If these measures cause no improvement, the next step is an operation called a coronary bypass, where a section of vein from the leg is removed and grafted across the blocked coronary artery to form a bridge through which the blood can travel. Another treatment is called catheterisation, or percutaneous transluminal coronary angioplasty. This operation involves inserting a wire with a tiny balloon into an artery in the leg, and feeding it through the aorta into the blocked coronary artery. When the balloon reaches the area blocked by atherosclorosis, the balloon is inflated so as to compress the plaque, and re-establish normal blood flow.
Bibliography:
Microsoft Encarta 95
Coronary Heart Disease – The Facts, Desmond Julian and Claire Marley 91
The Hutchinson Educational Encyclopaedia 1999
