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Blood Pressure The pressure in blood vessels decreases as the distance from the heart increases. The pressure starts high in the arteries leaving the heart, as they are most affected by contraction of the ventricles. The left ventricle with thicker muscle can contract with greater force than the thinner muscle of the right ventricle. Consequently, the pressure is higher in the aorta than in the pulmonary artery. Diagram below shows blood pressure in blood vessels from the left and right sides of the heart: The effects of the ventricle contracting and relaxing, can be clearly seen producing peaks and troughs in the pressure in the arteries. The pressure remains high during diastole, due to the springing back of the elastic wall of the arteries, in the elastic recoil mechanism. Moving away from the heart, the pressure falls due to increasing distance from the heart and increased friction. The pressure falls dramatically in the capillaries due both to the above and to loss of fluid through the capillaries. The main features of blood vessels are as follows: -In arteries, blood flows in spurts or pulses because the contraction of the ventricles is not continuous. -A high-pressure pulse is caused by contraction of the ventricles and blood is forced into the arteries. -Lower pressure between pulses is due to the ventricles relaxing. The fall in pressure is reduced by the elastic recoil mechanism. -The greatest drop in blood pressure takes place in the arterioles because of increased friction combined with increasing distance from the heart. -Elastic tissue allows expansion of the arteries without bursting, maintains high pressure and smooths out the flow. -Smooth muscle allows vasoconstriction and vasodilation which can be used to regulate the distribution of blood in temperature regulation and to divert more blood to the muscles during exercise (by reducing blood to the gut, for example). -Capillaries are thin and permeable, allowing exchange of materials between the cells and the blood. -Veins contain a lot of blood in their large lumens and there is very low pressure. -Blood is pushed along in veins by contraction of the large muscles they pass through, and semilunar valves keep the blood flowing in one direction. Blood pressure is a measure of the amount and the force of blood against the walls of blood vessels. The pressure in the arteries needs to be at a certain level to help to force some of the contents of the blood into the tissue fluid. A weakened heart may produce a low blood pressure and affect the composition of tissue fluid and the amount of substances delivered to the cells of the body. High blood pressure is often produced as a result of the build up of fat deposits in some of the arteries. Therefore monitoring of blood pressure can give an indication of the health of the circulatory system. The pumping of the heart, combined with the narrowness of the smaller blood vessels, produces a considerable pressure in the ARTERIES. This is what is meant by BLOOD PRESSURE. Blood only flows along the blood vessels because it is UNDER PRESSURE. Blood pressure is measured in millimetres of mercury (mmHg). Two readings are taken: (1) the pressure of the blood when the heart CONTRACTS (SYSTOLIC BLOOD PRESSURE) and (2) the pressure of the blood when the heart RELAXES (DIASTOLIC BLOOD PRESSURE). The normal systolic pressure of a young adult is about 120 mmHg; normal diastolic pressure is about 75 mmHg. Constant high blood pressure is HARMFUL. It makes the heart work harder. Eventually the overworked heart may fail altogether. High blood pressure can also damage the kidneys and eyes and increase the risk of an artery tearing open. A stroke can occur when one of the arteries that supplies blood to the brain ruptures. Hypotension: The technical term for low BP with dizziness and fainting episodes. Many healthy people have a lower BP than average for people of their age and have no symptoms. Others may have some underlying disorder that causes hypotension. The most common type is postural hypotension caused by suddenly sitting or standing up. Hypertension: Characterised by abnormally high BP at rest. The World Health Organisation (WHO) defines hypertension as being consistently above 165/95 mm Hg, but many people are classed as mildly hypertensive if their BP is over 140-160/90-95 mm Hg. Summary Changes in blood pressure -Blood pressure (BP) is the force exerted by the blood on the walls of the blood vessels. -It is generated by the walls of the left ventricle during contraction. -BP is measured in mmHg. -At first sight, BP recordings look like a fraction (e.g. 120/80 mmHg, the so-called average BP for a young adult). However, this is only a way of displaying an upper systolic reading when the ventricles are contracting and a lower diastolic reading when they are relaxed. -The highest BP is found in large arteries close to the heart, such as the aorta, the carotid arteries and the arteries in the arms. -There is a gradual drop in BP as the blood is forced through the medium and small arteries and the arterioles. -The veins have little BP and blood has to be assisted back to the right atrium by skeletal muscle pressure in the limbs and the presence of valves to prevent backflow. -Graph shows how BP varies in the circulation: