Arteries transport blood away from the heart at high pressure to supply the organs and body tissue with oxygen and nutrients required to maintain life. Their walls have a thick muscular structure, which adapts them for high pressure and any change in pressure due to increased activity.
The outer walls, tunica adventitia are connective tissue and the inner walls tunica media, are elastic tissue forming smooth muscle. A single layer of flattened endothelial cells form a smooth muscular tube within these walls. This structure enables diastole and systole with each pulse of blood pumped by the heart. Semilunar valves prevent the blood from returning to the heart and ensure a one way flow. Because of the elastic recoil of the arteries, blood pressure does not drop much therefore maintaining high pressure. This elasticity tends to decrease with age so diastole is less, producing higher blood pressure. ( Vascular System). (Boyle, M., Senior, K. 2002).
From the arteries, blood is pumped into smaller arterioles which in addition to distributing oxygenated blood, act as pressure reducing valves between the arteries and the capillaries. They are also important in determining blood pressure. They buffer the delicate capillaries from the high pressure of blood in the arteries. ( Vascular System).
Capillaries are the only blood vessels in the circulatory system that have the unique function of permitting the exchange of substances. They are composed of a single layer of flattened endothelial cells that together form a smooth continuous tube. At the arterial end of the capillaries, the blood pressure is high enough to cause tissue fluid containing oxygen and other nutrients to filter out through the thin walls to the surrounding cells. Branching of the capillaries is so extensive that no cell in the body is more than a few micrometers away from one and its life sustaining contents.
The venous end of the capillaries receive some of the tissue fluid back by osmosis, this time containing carbon dioxide and other waste materials.
( Vascular System). (Boyle, M., Senior, K. 2002).
From the capillary beds, the blood drains into the venous section and into the veins, which transport the blood at a lower pressure back to the heart. Although veins have the same composition as arteries, they have a much thinner layer of tunica adventitia and tunic media, as blood pressure is much lower. This structure has a wide lumen to reduce resistance in blood flow allowing for easier expansion and adapting them for their function as a reservoir for blood.
To prevent the downward pull of gravity resulting in an accumulation of blood in the lower part of the body, the veins contain one-way pocket valves. In addition to this, the contracting of the skeletal muscles press against the veins narrowing them so creating pressure which forces the blood towards the heart. During inspiration, reduced pressure in the thorax encourages blood to flow towards the heart. The pocket valves prevent back flow during expiration when thoracic pressure is high.
( Vascular System). (Boyle, M., Senior, K. 2002).
The coronary arteries are an extensive network of vessels that supply the myocardium or heart muscle itself. Coronary heart disease (CHD) or ischaemic heart disease is the most common and the most preventable of all heart disease and it occurs in this particular network. The main risk factors are high blood cholesterol, hypertension, smoking, obesity, diabetes mellitus, physical inactivity or the genetic predisposition familial hyperlipidaemia. Those most likely to develop CHD will always have one or more of these factors present and with the exception of the latter, the majority are preventable. (Boyle, M., Senior, K. 2002).
It is important to understand what happens during CHD and how certain lifestyles contribute to or cause the condition before any improvements can be made.
CHD begins when the walls of the coronary arteries become narrowed by a gradual build-up of fatty deposits. This condition is known as atherosclerosis and the fatty substance an atheroma. Gradually the muscle cells and fibres grow over the affected areas called fibrous plaque. This protrudes into the lumen and starts to obstruct the blood flow. During increased physical activity, a narrowed artery restricts the oxygen supply to the myocardium resulting in angina pectoris. This is normally felt as pain in the chest or left arm or shoulder and is usually the first indication of CHD. If the plaque bursts into a vessel it damages the endothelium and the cells underneath are exposed to the blood, triggering the blood clotting mechanism. A resulting blood clot or thrombus that completely blocks any artery is called thrombosis. When this occurs in a coronary artery it causes a heart attack and in the carotid artery, a stroke. When a blood clot becomes mobile, it is called an embolus resulting in a embolism when it blocks a vessel. A pulmonary embolism is usually fatal. An aneurysm is a burst artery that, if occurring close to the heart is also usually fatal. (Boyle, M., Senior, K. 2002).
A major contributor to the fatty deposits that cause CHD, are actually essential to the formation of cell membranes and energy stores. These are triglycerides and cholesterol lipids that combine with protein to form lipoproteins before the blood can transport them to required sites around the body. Most are produced by the liver but some are present in the foods we eat. Lipoproteins consist mainly of low density lipoproteins, LDL that carry cholesterol from the liver to the tissues and high density lipoproteins, HDL that return extra cholesterol not required to the liver. Ideally, there should be a higher ratio of HDL to LDL to ensure any excess is returned. When this ratio is unbalanced and the blood contains surplus LDL rich lipoproteins, they are engulfed by phagocytes within the artery walls just beneath the endothelium. Unable to break down the cholesterol contained within them, an accumulation gradually occurs inside the artery wall. This is the first sign of atherosclerosis. (Boyle, M., Senior, K. 2002).
Saturated fats in the diet lower HDL and increase LDL cholesterol therefore increasing the risk of an atheroma developing. Unsaturated fats lower the LDL cholesterol levels and are therefore encouraged in the diet. A high intake of saturated fats puts people at risk of developing hypertension, obesity and diabetes mellitus. Too much salt in the diet also increases the risk of hypertension by reducing the water potential of the blood, increasing the volume and in turn the pressure. (Eades, M.R., Eades, M.D. 2000). (British Heart Foundation, Eating for your Heart, 1999).
Regular physical activity raises HDL but does not affect LDL and can help to maintain a healthy weight and reduce obesity, hypertension and medication required for diabetes mellitus. It can also help to prevent hypertension and thrombosis by ensuring the artery walls are kept supple and less prone to the thickening that reduces the lumen, elevates pressure and increases the risk of damage to the endothelial cells. (British Heart Foundation, Physical Activity and your Heart, 1999). (Boyle, M., Senior, K. 2002).
Cigarette smoke contains a vasoconstrictor nicotine that narrows the blood vessels increasing blood pressure. It also increases lipoproteins in the blood and the amount of fibrinogen, which causes the blood to clot. (Boyle, M., Senior, K. 2002).
By reducing saturated fats, stopping smoking, increasing physical activity and generally eating a healthy balanced diet will reduce the risks of developing CHD. Unfortunately, the disease is still on the increase in developed countries and is a major killer. Luckily, there are many safe drugs available that can control symptoms or reduce them but, as an escalating problem, lifestyle changes are the safest, cheapest and healthiest ways of reducing or eliminating the risks. (Boyle, M., Senior, K. 2002).
(Word count: 1508)
References
Boyle, M., Senior, K. (2002). Human Biology. (2nd ed.). London: Harper Collins Publishers Ltd.
Eades, M.R., Eades, M.D. (2000). The Protein Power Lifeplan. New York: Warner Books Inc.
Roberts, M.B.V. (1976). Biology A Functional Approach. (2nd ed.). Sunbury-on-Thames: Thomas Nelson and Sons Ltd.
British Heart Foundation. (1999). Eating for your Heart.
British Heart Foundation. (1999). Physical Activity and your Heart.
http:// Vascular System.htm [Accessed: 2003, February 1].
[Accessed: 2003, February 1].
Catherine Graham\Unit B Biology Essay Page of