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Chapter: Biology of Disease: Disorders of the Cardiovascular System

Clinical Problems Associated with Blood Pressure

Low Blood Pressure (Hypotension), ,Shock, High Blood Pressure (Hypertension).


The blood pressure of any individual varies with their age, sex, physical activity and emotional state. For example, the normal upper limits at age 20 are 150/90 mmHg, which can increase to 160/95 and 170/95 at 40 and 60 years of age. If these pressures are found consistently then they are abnormally high. Blood pressure also varies with ethnic origin. American blacks, for example, are much more likely to have high blood pressure than whites.

The three major factors that determine the blood pressure are the amount of blood pumped by the heart, the volume of blood in the blood vessels and the elasticity of the blood vessels. Thus the more blood pumped by the heart, the higher the pressure. If the heart beats more slowly or its contractions are weakened, such as may happen after myocardial infarction, then less blood will be pumped. Equally, a rapid heartbeat can result in inefficient pumping. The greater the volume of blood in circulation, the more likely the blood pressure is to be high. Conversely, a loss of blood by bleeding or as a result of dehydration will have the opposite effect. The smaller the capacity of the blood vessels, the higher will be the resulting blood pressure. Consequently, a dilation of the blood vessels will lower the blood pressure.

Sensors in the neck and chest continually monitor the blood pressure and trigger physiological changes if the blood pressure changes . The actions taken might be to modify and strengthen the heartbeat, to regulate the kidneys to alter the amount of water excreted which, in turn, changes the volume of blood circulating or to constrict or dilate the blood vessels. However, these compensatory mechanisms have limits, for example if too much blood is lost as a result of bleeding there is little that can be done. Action must be taken to stop the bleeding and to transfuse blood or fluid to make up the volume. Furthermore, these compensatory mechanisms may themselves fail in certain conditions.


Clearly the heart must pump hard enough so that the blood pressure is maintained. Pressures below 100/60 mmHg at any age are abnormally low. If an individual’s blood pressure is too low it can lead to dizziness and fainting. Fainting or syncope is the result of a temporary inadequacy of oxygen and nutrients and is usually associated with a temporary decrease in blood flow. This can happen in people with an abnormal heart rhythm when they suddenly begin to exercise or the heart rate is too slow. However, there are many other possible causes of fainting including anemia, hypoglycemia, hypocapnia (a lower than usual concentration of CO2 in the blood) or hyperventilation. The latter may be caused by anxiety. Usually lying flat is all that is needed for the individual to regain consciousness but checks should be performed to eliminate more serious conditions and this may mean, for example, an ECG examination.


If the blood pressure falls too low to sustain life, the body is said to have gone into shock. This is more severe and prolonged than in fainting, since if body cells are deprived of oxygen and nutrients for any length of time then they quickly become irreversibly damaged and die. Shock may result from low blood volume, inadequate pumping by the heart or excessive vasodilation as can occur in extreme allergic reactions . Low blood volume may be the result of bleeding following serious trauma. Inadequate pumping by the heart may be the result of a heart attack, pulmonary embolism, the failure of a heart valve or an irregular heartbeat or drug toxicity. Head injuries, liver failure, poisoning, severe bacterial infections or drug reactions may all lead to excessive blood vessel dilation.

Unless treated promptly, shock is usually fatal. When shock results from a sudden loss of blood for example in an accident or a hemorrhage the first person on the scene should aim to stop the bleeding, keep the victim warm and raise the legs slightly to improve the return of blood to the heart. Emergency personnel may provide mechanically assisted breathing, if it has stopped, and fluid or blood to increase the blood volume. Other treatments will depend on the cause of the shock.


Hypertension refers simply to the condition where a person has a bloodpressure that is higher than that which is regarded as normal, regardless of its cause. High blood pressure is defined as a systolic pressure at rest of 140 mmHg or greater and a diastolic pressure of 90 mmHg or greater, or both of these. In fact both are usually elevated in hypertension. It does not usually cause symptoms, at least for many years, and often tends to go undetected unless the person’s blood pressure happens to be measured for some other reason. Nevertheless, it is sometimes referred to as the ‘silent killer’ because there tend to be no symptoms until some vital organ is damaged. Mortality and morbidity rise continuously with increasing blood pressure. However, the risk is not linear and rises more steeply at higher pressures.


Initially an abnormal heart sound indicating hypertension may be detected using a stethoscope. A diagnosis of hypertension can be made on the basis of an elevated blood pressure reading of 140/90 mmHg or more when measured several times. A single reading on a given day is unreliable. Obviously the higher the values, the more serious the condition must be considered. It is possible to judge the seriousness of the condition by examining the arterioles at the back of the eye to determine the degree of damage to the retina, as hypertension is known to cause retinopathy. In addition, ECG and echocardiography can detect an enlargement of the heart brought on by the increased workload. Kidney damage may be detected by urine analyses.

The cause of hypertension can be identified in less than 10% of patients. These are usually kidney disease, a hormonal disorder or the use of oral contraceptives. Thus in most cases the primary cause cannot be identified and this form is referred to as essential hypertension. Many factors are probably responsible. For example, in older people the larger arteries lose their flexibility and become stiffer. Consequently when the heart pumps blood they cannot expand and the pressure increases. If the kidneys malfunction such that the urinary output is decreased, more fluid will be added to the system. Obesity, stress, a sedentary lifestyle, excessive amounts of alcohol and too much salt in the diet can also contribute.

Untreated hypertension increases the chances of a person developing heart diseases, such as cardiac failure or myocardial infarction, kidney failure, or a stroke (see above). Stopping smoking, reducing weight, salt intake and cholesterol levels reduces the risk. In general, patients do not have to restrict their activities as long as their blood pressure is controlled. Various drugs are available as part of a treatment program. These include diuretics to help the kidneys eliminate water and salt, adrenergic blockers to block the effects of the sympathetic nervous system and ACE inhibitors which lower the blood pressure by stimulating arterial dilation.

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