Describe how the structure of arteries, veins and capillaries enables them to fulfil their functions - Discuss the possible links between lifestyle and coronary heart disease.

UNIT B ASSIGNMENT- ESSAY.

Describe how the structure of arteries, veins and capillaries enables them to fulfil their functions.

Discuss the possible links between lifestyle and coronary heart disease.

The human heart is a muscular pump which has four chambers, two atria which receive blood from the veins and two ventricles which pump blood away from the heart. The heart is responsible for pumping the blood to every cell in the body. It is also responsible for pumping blood to the lungs, where the blood gives up carbon dioxide and takes on oxygen. This is known as double circulation as the blood passes through the heart twice on its journey around the body, the blood is returned to the heart after passing through the lungs, before being pumped over the body tissues to supply nutrients and oxygen. This helps to sustain a high blood pressure which allows rapid circulation. (See diagram 1.1)

Blood is pumped around the body by the heart through vessels. There are three types of vessels, arteries, veins and capillaries. These aren’t just tubes through which blood flows and aren’t anatomically the same. Their structure is related to the function they perform, the diameter of arteries and veins slowly reduces the further away from the heart they are. The smaller arteries are called arterioles and the smaller veins are called venules. At the centre of each is the Lumen which is the hole through which blood travels, this is surrounded by the Endothelium which has a smooth layer of cells that prevent the blood from sticking to either side which could cause clotting. (See diagram 1.2)

Arteries carry oxygenated blood away from the heart which is under high pressure; as a result they have a thick muscular wall and many more elastic fibres to cope with the pressure. These arteries aren’t permeable and are capable of constriction due to the fact they have a large quantity of elastic fibres around the endothelium. The blood moves through the arteries rapidly and in pulses, they expand to accept the blood being forced into them from the heart, then when the heart relaxes and the pressure is released they contract back to their original size squeezing the blood onto the veins. It is this action that keeps the pressure on the blood when the heart relaxes keeping it flowing forward. The only valves separate the atria and ventricles on either side of the heart, these are called atrioventricular valves and work in the same way as the semi-lunar valves which we will discuss later. (See diagram 1.3) The largest artery is the aorta leaving the heart which has cardiac muscle fibres in its walls for the first few inches of its length to withstand the high pressure of the blood leaving the heart.

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Veins are responsible for transporting the deoxygenated blood back to the heart except in the pulmonary vein, which transports the blood from the lungs to the left side of the heart before it begins its journey around the body. Blood in the veins is under low pressure, it doesn’t move in pulses as it does through the arteries but instead flows slowly. As a result the lumen is large in relation to the diameter to allow a larger volume of blood to pass through. Veins have the smooth muscle wall the same as the arteries but don’t have many elastic fibres as the blood doesn’t pulse through and so have no need to constrict. As they don’t constrict they have valves throughout to stop the blood pooling or flowing back towards the legs under the influence of gravity. Veins are usually surrounded by skeletal muscle which contracts as a result of movement and pushes the blood along the vein in one direction. Semi-lunar valves stop the blood flowing backwards, they work by allowing the blood to flow past but as it flows backwards it opens the flaps of tissue that make the valve and it fills with blood which closes the lumen and stops blood going back past the valve.(see diagram 1.3)

Capillaries are the smallest of the vessels and it is in these that exchanges between the blood and body cells take place. This is possible because the capillary walls are very thin and permeable allowing small soluble molecules to pass through. (See diagram 1.2) When the blood flowing through them is seen through a microscope the cells are seen to pass through one at a time. Capillary beds, which are like a spider’s web in the liver, allow the blood to give up waste products which are then disposed of in urine. They also allow the transfer of carbon dioxide and oxygen from the blood and enable white blood cells to leave the blood and fight against foreign bodies.

Coronary heart disease is a leading cause of death and an illness that affects millions of people. It occurs when fatty deposits called plaque build up on the inner walls of the coronary arteries, this progressive process is called atherosclerosis. The blood flow to the heart is reduced because of the plaque and the heart muscle isn’t able to increase its performance when needed, any extra strain on the heart causes a chest pain called angina or even sudden death. The development of coronary heart disease can start at an early age however there are factors that can affect the rate at which this happens such as age, family history, lifestyle, diet and gender.

If a member of your immediate family has suffered from heart disease then your risk of developing it is twice as high as normal and the risk is even higher if your relative developed the disease before the age of 50. Coronary heart disease is not only genetic it results from the interaction between genes and the environment. If you are in a high risk category already then smoking or being overweight will make the probability of the disease occurring even greater.

Smoking a cigarette increases the heart beat 15-30 beats per minute, blood pressure also increases which damages the artery walls. Long term smoking also greatly increases the formation of plaque-type substances in the coronary arteries.

Diet also plays a large part in the chances of developing heart disease. A diet which is high in low-density lipoproteins is bad, as they pick up cholesterol molecules which accumulate on the artery walls forming plaques and arteriosclerosis. High-density lipoproteins are good as they help clear cholesterol out of arteries taking them to the liver to be disposed of. Unsaturated fatty acids are also known to reduce LDL’s and increase HDL’s in the blood stream, also reducing the risk of blood clots forming. These unsaturated fatty acids are also known as omega-3 fatty acids and can be found in foods like salmon, trout, mackerel, herring and sardines. A balanced diet that doesn’t contain high amounts of saturated fat would reduce the chances of heart disease.

Exercise is very important in reducing the chances of heart disease. The heart being a muscle gets stronger with exercise; repeated exercise will increase the amount of heart muscle and the size of the heart chambers. In return the heart will pump a greater volume of blood per beat and so beat fewer times.

Although global trends show the same increase in heart disease the levels around the world differ. In developing countries where the economic situation isn’t good, the food that is available is rarely deep fried or contains high levels of fat. The people get more exercise as their jobs are more physical, as a result they burn off excess fat. In a country such as the U.K where food is in plentiful supply and often contains high levels of fat i.e. chocolate, crisps, chips etc, most people are employed in jobs that involve much less physical activity and so don’t burn off fat as easily. As a result of these different lifestyles heart disease is much higher in developed parts of the world.

WORD COUNT 1,329

SOURCES Collins advanced science biology Mike Boyle, Kathryn Senior

Bio Fact sheet number 37 January 1999

The heart and the circulatory system

Roger E Phillips

Understanding biology for advanced level Glenn and Susan Toole

University of Bath Science Biology Martin Rowland