Biology Essays - Heart Disease

The Biological Basis and Risk Factors for Coronary Heart Disease.

Coronary heart disease is a blanket term which describes all types of heart disease caused by blockage of the arteries that supply blood to the heart. Lack of sufficient blood is associated with a lack of oxygen, also called ischemia. Hence coronary heart disease is also called ischemic heart disease. Angina, heart attack and heart failure and are all clinical forms of coronary heart disease.1

Overall, coronary heart disease is the leading cause of death in the Western world.2 In the UK, heart attacks are responsible for around 150,000 deaths every year and cost the NHS approximately £719 million per year. Almost half of all heart attack sufferers die within 20 days of the attack.3

The heart of the problem

The heart is a vital organ, pumping blood which carries essential oxygen and nutrients all around the body. As a powerfully active muscle, the heart needs a ready and plentiful supply of oxygen and nutrients itself; however these are not extracted from the blood that flows through it. Instead, the heart has its own specific blood supply – the coronary arteries. There are two major coronary arteries – the right and left. The volume of blood which flows to the heart is largely determined by the width of these vessels and is not so dependent on the power at which the blood is pumped through them. Unlike other organs which can use the oxygen within veins if needed, the heart is completely reliant on the coronary arteries’ supply. Hence their ability to expand and supply more blood when required is essential.4-6

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Coronary heart disease develops when various materials build up within the walls of the coronary arteries causing a narrowing of the vessels and potentially leading to complete blockage. The obstruction of blood flow to the heart which results from this build up within the coronary arteries can cause a range of cardiac problems. When the blood flow to the heart is only temporarily interrupted, usually during exercise and times of exertion when the heart muscle requires more oxygen, the condition is known as angina. A person with angina will experience short attacks of chest pain which usually clear up with rest. Ultimately, insufficient blood flow can result in actual physical damage to the heart tissue. In severe cases, where blood flow is completely blocked, the portion of the heart affected may die from lack of oxygen– this is termed myocardial infarction, commonly known as a heart attack. Symptoms of heart attack include prolonged and severe chest pain, weakness and shortness of breath. In rare cases, patients with coronary heart disease experience heart failure. This is a broad condition which describes any symptoms showing that a patient’s heart is not performing as it should. 4-6

Why do coronary arteries become blocked?

In healthy arteries, the walls are smooth and blood flows easily. The process leading to the development of coronary heart disease begins with the development of ‘fatty streaks’. These are yellow patches which appear within the wall of the coronary artery. The streaks are made up of a special type of cell called foam cells, which are rich in fats, plus smooth muscle cells. Over time, streaks can turn into plaques. Plaques, also known as atheroma, are fatty lumps which form within the lining of the arteries. There are various different kinds of plaques. Some are made up of a soft inner core of cholesterol covered by a thin firm outer layer. Others are completely solid and composed of smooth muscle cells and various other strong materials. Calcium may also build up on a plaque making the problem worse.4,7

What is the blockage like?

Plaques cause several problems to coronary blood flow. Firstly, a plaque itself can be big enough to narrow the artery and reduce the space available for blood to flow through it. Secondly, as blood is thrust against the side of the plaque with each heart beat, the outer shell of the plaque may crack open exposing the cholesterol core. When the cholesterol core is exposed, blood clotting is triggered within the artery. Clots reduce the blood flow even more. This whole process can happen repeatedly causing the artery to become narrower and narrower and eventually leading to complete blockage.4,7

Overall, the process of plaque formation and associated thickening, loss of elasticity and narrowing of the artery walls is known as atherosclerosis. Atherosclerosis is the major cause of coronary heart disease however, it is also present to some degree in almost every adult male in Western societies. What causes atherosclerosis to occur is complex and uncertain and there is much still to learn. Current thinking is that atherosclerosis results from some kind of ‘response to injury’ where initial non-specific injury to the lining of the artery wall triggers the whole process to start up.4,7,8

Risk factors

Everyone has some chance of experiencing heart disease, however, various factors have been identified which increase the risk of developing coronary heart disease. These include:4,6,7,8

• High cholesterol
• Smoking
• Lack of exercise
• Unhealthy diet
• Obesity
• Diabetes
• High blood pressure (hypertension)
• Mental stress
• High levels of certain blood clotting factors
• Family history
• Alcohol and coffee
• Male gender
• Ethnic group (e.g. Southern Asians in the UK are at higher risk)

Most of these risk factors increase the risk of coronary heart disease by increasing the likelihood that atherosclerosis will develop within the coronary arteries themselves. Also, these risk factors have been found to ‘interact’. Hence if you have two or more risk factors your overall chance of developing coronary heart disease is proportionally higher than if you just had one risk factor.8

Heredity as a risk factor

Coronary heart disease often occurs within several members of the same family, indicating that a genetic link may be involved. However, it is difficult to determine if the link between family members is due to genes or shared environmental factors, such as diet and exposure to smoke. Genetic factors have been shown to influence cholesterol levels and the amount of blood clotting factors a person has in their blood, which in turn has a knock-on effect on the risk of developing atherosclerosis. Overall, it seems that a combination of genetics and the environment would best explain the family link to heart disease. It has been estimated that 40% of the risk is controlled by genes, and 60% is due to environmental factors.4,6

High cholesterol as a risk factor

There are two types of cholesterol found in the blood – ‘good’ HDL cholesterol which can protect against the development of heart disease and ‘bad’ LDL cholesterol which promotes the development of atherosclerosis. There is lots of evidence to show that unhealthy amounts of these two types of fat in the blood, i.e. high LDL and low HDL levels, can increase the chance of developing coronary heart disease. This is because high levels of cholesterol in the blood encourage the formation of plaques within the artery wall and promote the overall process of atheroschlerosis.4,6

The cause of high LDL levels has not been clearly established but dietary factors are likely to be involved. There is evidence that a diet high in saturated, but lacking in polyunsaturated, fat may contribute to high cholesterol levels. There is relatively little actual cholesterol in the diet, but saturated fat is converted into cholesterol in the liver.4,6

Lack of exercise as a risk factor

Lack of exercise is a proven risk factor for the development of coronary heart disease. Overall, the risk is a third less in people who exercise compared to those who don’t.8 It seems that physical exercise has a protective effect on the development of coronary heart disease. The mechanism underlying this has not been fully elucidated but certain hypotheses have been put forward. Exercise may increase levels of good HDL cholesterol and also reduce the blood clotting which occurs within atherosclerotic arteries and causes further blockage. Exercise might also help enlarge the coronary arteries and hence increase blood supply to the heart. In order to gain full health benefits from exercise in the reduction of coronary heart disease risk, five bouts of moderate physical activity per week, each lasting a minimum of 30 minutes, are recommended.

Conclusion

Coronary heart disease is a serious condition caused by the build-up of fatty deposits within the walls of the vital arteries that supply blood to the heart. Coronary heart disease is the major cause of death in Western societies but to a large extent is preventable. Recognising the key risk factors which contribute to the development of atherosclerosis is an important step towards reducing the overall incidence of coronary heart disease.

References

1. Walton J, Beeson PB, Bodley Scott R, 1986. The Oxford Companion to Medicine: Volume I, A-M. Oxford: Oxford University Press.

2. Higgins M, 2001. Patients, families and populations at high risk for coronary heart disease. European Heart Journal, 22: 1682-1690.

3. Association of the British Pharmaceutical Industry, 2005. Hitting the Target: Medicines and Coronary Heart Disease and Stroke. Available at: http://www.abpi.org.uk/publications/publication_details/hitting_target/section5b.asp [Accessed 14 March 2005].

4. Julian DG & Campbell Cowan J, 1992. Cardiology: Sixth Edition. London: Bailliere Tindal.

5. Vander AJ, Sherman JH, Luciano DS, 1994. Human Physiology: The Mechanisms of Body Function. International Edition: McGraw-Hill.

6. Edwards CRW & Bouchier IAD (Eds), 1991. Davidson’s Principles and Practice of Medicine: Sixteenth Edition. London: Churchill Livingston.

7. Singh VN, 2005. Coronary Heart Disease. Emedicine. Available from: http://www.emedicinehealth.com/articles/10951-1.asp [Accessed 14 March 2005].

8. PRODIGY Guidance, 2004. Preventing Heart Disease and Stroke. Patient information leaflet. Available at: http://www.prodigy.nhs.uk/clinicalguidance/releasedguidance/webBrowser/pils/PL82.htm [Accessed 14 March 2005].