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blood pressure

blood pressure In a resting individual the left ventricle of the heart pumps typically 5 litres of blood each minute into the aorta and arteries of the body. Downstream, the small arterioles restrict the outflow of blood from the arteries and are therefore known as the main ‘resistance vessels’. The combined effect of the energy generated by the heart and the outflow restriction results in a distending pressure in the arterial system which is referred to as the blood pressure.

The first report of a direct measurement of arterial blood pressure was by Revd Stephen Hales in 1733. He inserted a tube into an abnormally exposed artery of a horse and observed that a column of blood rose in a glass tube to a vertical height of 8 ft 3 in. This represents the force generated by the heart and transmitted to all the major arteries in the body. We do not now express blood pressure as height in feet and inches of blood. However, we sometimes use centimeters of water, so that the horse's blood pressure would be 250 cm of water or blood. Such a column has obvious practical problems for measuring arterial pressure. But for venous pressures which are much lower, a column of saline connected to a major vein is often used clinically to assess the degree of filling of the circulation. Arterial pressure is usually expressed as millimetres of mercury (mm Hg) because mercury is 13.6 times as dense as water and a mercury column of that height is more practicable. Thus the horse's pressure blood would be 185 mm Hg. An alternative unit for expressing blood pressures, which has not been widely adopted in clinical practice, is the SI unit, the pascal or kilopascal (kPa). One kPa is approximately 7.5 mm Hg.

Blood pressure is not normally expressed as a single figure but rather as two, for example 120/80. This means that the pressure in the arteries varies with each heart beat to a peak, called systolic pressure, of 120 mm Hg, and then declines to a minimum value, called diastolic pressure, of 80 mm Hg just before the next beat. These phasic values of blood pressure can be recorded accurately using modern transducers (electronic measuring devices) connected to catheters (fine tubes) inserted into arteries. However, except for research and measurements during complex investigations in patients, blood pressure is not usually determined by direct puncture of an artery. The most common method is to use the device known as a sphygmomanometer. This is an inflatable cuff which fits round the upper arm and is connected to a mercury manometer. A stethoscope is applied to listen to the artery below the cuff. The cuff is first inflated with a pressure well above systolic and then slowly deflated. The systolic pressure is taken as the pressure in the cuff when the artery just opens and a sound is first heard. The diastolic pressure is that when the sound either becomes muffled or disappears completely.

Blood pressure, like all biological variables, varies widely in different people and, in the same individual, at different times of the day. Typically a normal value for systolic blood pressure would be 120 mm Hg at age 20, increasing perhaps to 140 mm Hg at 60. Diastolic pressure also increases with age but rather less. Estimates of blood pressure in apparently healthy people show values that can be 20 or even 40 mm Hg higher or lower than the average values. This, and the fact that blood pressure varies considerably during the day, particularly in response to stresses such as visiting a doctor, mean that it is very difficult to decide on the basis of a single measurement whether a patient suffers from hypertension (high blood pressure). Definitions of hypertension are constantly changing but, generally, if systolic pressure is consistently greater than 160 mm Hg or diastolic more than 95, a person is considered to be hypertensive.

At rest, each time the heart contracts, it ejects typically 70 ml of blood into the arterial system. This causes a steep increase in arterial pressure, the magnitude of which is dependent both on the volume ejected and on the distensibility of the arteries. Older people have less distensible arteries, which explains why their systolic blood pressure is usually higher than in younger subjects. Because the shape of the arterial pressure pulse is roughly triangular, the mean level of pressure is nearer to the diastolic value.

The importance of blood pressure is that it effectively provides a store of energy, generated by the heart, available to cause blood to flow through the working tissues. It is actually the flow of blood, providing oxygen and nutrients and removing waste products including carbon dioxide, which is really the important factor, but without pressure there would be no flow. Humans, being upright bipedal animals, have a particular problem in supplying blood to all parts of the body. Due just to gravity, pressure in arteries supplying the head is about 100 mm Hg less than that in arteries in the feet. The fact that the brain must have an adequate arterial pressure places a limitation on the range of effective pressures in the upright person.

Control of blood pressure

Mean blood pressure depends on the flow of blood from the heart (cardiac output) and the resistance to flow in the small arteries and microscopic resistance vessels (arterioles).BP = CO × PVRwhere BP is blood pressure, CO is cardiac output, and PVR is the peripheral vascular resistance or the net resistance to blood flow in all the small arteries and microscopic arterioles.

Peripheral vascular resistance is dependent on the radius (r) of the small blood vessels. In fact it turns out to be proportional to 1/r4. The equation for blood pressure can now be changed:BP ∝ CO/r4

The importance of the degree of constriction of resistance vessels can be seen from this equation because if cardiac output is unchanged a reduction in the average radius of the resistance vessels of only 10% would increase blood pressure by more than 50%. The physiological control of blood pressure is thus effected mainly by regulating the radius of the resistance vessels and, to a smaller extent, the cardiac output. Baroreceptors provide an effective means for detecting changes in blood pressure and bringing about appropriate responses, via the autonomic nervous system. If blood pressure started to fall the baroreceptor stimulation would decrease and the reflex response would cause the small resistance vessels to constrict and the heart to beat faster and harder, by action of the sympathetic nerves. This negative feedback mechanism largely restores the blood pressure. Conversely, if blood pressure increases, stimulation of baroreceptors gives rise to nerve impulses which run to the brain and stimulate activity in the parasympathetic pathway in the vagus nerves, which slows the heart; also inhibition of activity in sympathetic nerves decreases both the rate and force of contraction of the heart and dilates of both the resistance and the capacitance vessels (veins) (Fig. 1).

Some factors which affect blood pressure

Baroreceptors are important for minimizing changes in blood pressure: animal studies have shown that blood pressure is much more variable if the influence of baroreceptors is removed. However, they do not prevent all fluctuations from occurring. Continuous 24-hour recordings have been made in healthy volunteers and have shown variations of 30–80 mm Hg in systolic pressure and of 10–80 mm Hg in diastolic pressure. Blood pressure is particularly low during sleep, and high during physical activity or emotional stress.

Physical exercise

causes very major effects on the circulation. Due to the enormously increased blood flow through the exercising muscle, the amount of blood pumped by the heart may increase four-fold, or in elite athletes as much as six-fold. The increased volume of blood ejected at each heart beat causes systolic blood pressure to increase, perhaps to 180 mm Hg. However, because blood flows very rapidly out of the arteries, particularly to the working muscle where the resistance vessels are widely dilated, diastolic pressure remains relatively unchanged or may even decrease. Isometric exercise has quite a different effect. Here there is a much smaller effect on the total amount of blood pumped by the heart, but reflexes, particularly those arising from the contracting muscle itself, cause blood vessels elsewhere to constrict, and consequently both systolic and diastolic blood pressure rise sharply. This response may also be augmented by a straining effect (see below).

Emotional stress

can cause quite large increases in blood pressure. Prominent amongst the physiological responses to stress is an increase in activity in the sympathetic nerves. Sympathetic overactivity increases heart rate and force, and constricts resistance blood vessels (Fig. 1). All these effects increase both systolic and diastolic blood pressure and are augmented by increased secretion into the blood of adrenaline and noradrenaline.

Postural changes

exert stresses on the cardiovascular system requiring effective reflex responses to constrict arteries and veins and stimulate the heart, to control blood pressure, maintain brain blood flow, and prevent loss of consciousness. The upright position means that blood vessels below the level of the heart are subjected to increased distending pressures due to the effects of gravity. Veins are particularly susceptible to gravitational stress due to their distensibility, and blood ‘pools’ in dependent veins when we stand. Because of this, less blood flows back to the heart and, were it not for effective reflexes, involving baroreceptors, blood pressure would fall catastrophically, particularly in the brain, resulting in insufficient brain blood flow and consequent loss of consciousness. Blood pressure frequently falls transiently when we stand. This is particularly noticeable if we stand suddenly when warm, for example on getting out of a hot bath, because the resistance blood vessels initially will be dilated. In some people blood pressure control may be inadequate to counter the stress of postural changes and the result is that they faint.

Straining (the Valsalva manoeuvre)

induces large and complex variations in blood pressure. The sort of stresses that induce these changes include blowing against a resistance, lifting heavy objects, and straining at stool. The effects on the circulation are illustrated in Fig. 2. The primary change is caused by an increase in pressure within the chest (intrathoracic pressure) and within the abdomen. Normally, intrathoracic pressure is lower than atmospheric, due to the tendency of lungs to collapse and their prevention from so doing by the chest wall. This negative intrathoracic pressure aids the flow of blood to the heart from the peripheral veins. Straining causes the pressure in both the chest and the abdomen to become positive. Initially the compression of the heart and large arteries causes an increase in blood pressure. Then, the high pressure in the chest impedes the inflow of blood from peripheral veins (veins in the neck can be seen to distend), so the cardiac output decreases and blood pressure falls. Baroreceptors detect this fall and initiate constriction of blood vessels and an increase in heart rate, so that mean blood pressure is restored. At the end of the strain there is a transient fall in pressure before blood rushes back to the heart, causing an overshoot and often a transient slowing of the heart. In people with some autonomic nerve disorders these responses may be deficient: blood pressure falls continuously, and the overshoot is absent.

Roger Hainsworth


See also autonomic nervous system; baroreceptors; blood circulation; heart.

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Blood Pressure

BLOOD PRESSURE

Blood pressure is a physiological variablelike body temperature, respiratory rate, or heart rate. Blood pressure is not constant throughout the day; each time the heart squeezes and relaxes, there is a new blood pressure. It increases before awakening and declines with sleep. The level of blood pressure is regulated by the kidneys, brain, heart, endocrine glands, and blood vessels. In the United States, the actual level of blood pressure gradually increases from birth to adulthood. Due to difference in diet and activity levels in nonindustrialized countries, however, blood pressure does not increase beyond the age of eighteen.

Whereas temperature is measured with a thermometer, blood pressure is measured with a sphygmomanometer, preferably a mercury sphygmo-manometer, though aneroid and electronic devices are sometimes used.

Blood pressure should be measured after a five-minute period of rest, with the back supported and the legs uncrossed. Constrictive clothing should be removed from around the upper arm, which must be resting on a table at heart level. The blood pressure cuff is evenly and snugly applied around the upper arm above the elbow, and a stethoscope is placed over the crease of the elbow. The cuff is inflated to 15 millimeters of mercury (mmHg) above the point where radial artery pulse (the artery above the thumb at the wrist) disappears. The pressure in the cuff is then slowly released at 2 mmHg per second. The first of two consecutive sounds as cuff pressure decreases is called the systolic blood pressurethe pressure to open the artery occluded with the cuff. The diastolic blood pressure is recorded at the absence of sounds with continued deflation of the blood pressure cuff. Blood pressure is generally recorded to the nearest 2 mmHg. For example, a blood pressure of 142/86 mmHg indicates a systolic blood pressure of 142 mmHg and a diastolic blood pressure of 86 mmHg. Pain and emotional disturbance, as well as caffeine, tobacco, and alcohol, can elevate systolic blood pressure.

HYPERTENSION

An abnormal blood pressure requires confirmation on two subsequent days. An optimal blood pressure is less than 120/80 mmHg. High blood pressure, or hypertension, is defined as either a systolic blood pressure greater than 140 mmHg or a diastolic blood pressure greater than 90 mmHg. Systolic blood pressure is a more powerful predictor of cardiovascular events than diastolic blood pressure. With increasing age, the diastolic blood pressure may actually decrease while systolic blood pressure increases; this indicates increased stiffening of the arteries throughout the body.

Hypertension is not a nervous disorder or an anxiety state, but rather a disease of the blood vessels that increases blood vessel constriction of the small arteries. It particularly damages the blood vessels inside the brain, heart, kidneys, eyes, and the largest artery, the aorta. Damaged arteries may rupture, thicken, or harden and narrowresulting in strokes, heart attacks, kidney failure, visual impairment, or tearing or rupture of the aorta. Also, the left heart chamber thickens as a consequence of increased blood pressure. When the heart can no longer thicken or enlarge to overcome the increased pressure in the blood vessels, the squeezing function of the heart decreases, resulting in congestive heart failure.

CAUSES OF HYPERTENSION

Fifty million Americans (about one-fifth of the U.S. population) have hypertension (see Figure 1). Over 90 percent of the causes of hypertension remain unknown. Four groups are predisposed to developing hypertension: the obese, the elderly, diabetics, and African Americans. Certain drugs are known to elevate blood pressure, including most arthritis medications (except acetaminophen and aspirin), many cold remedies, nose sprays, weight-reducing pills, and alcohol. Increased heart rate, anemia, excessive thyroid hormone, or stiff

Figure 1

(nondistendible) arteries can increase systolic blood pressure. Blocked arteries to the kidney, kidney failure, and decreased production of thyroid hormone are common causes of hypertension. Other rare causes include tumors of the adrenal gland.

TREATMENT OF HYPERTENSION

Nondrug treatment of hypertension should include weight loss, salt restriction, smoking cessation, and alcohol restriction. A reduced saturatedand total-fat diet that is rich in fruits, vegetables, and low-fat dairy products lowers blood pressure in some individuals, avoiding the need for drug treatment. The treatment goal for uncomplicated hypertensives is below 140/90 mmHg. To achieve that goal consistently, most individuals will need to be treated with more than one drug. Treatment has been proven to decrease heart attacks, strokes, and heart failure, and is usually required throughout life.

L. Michael Prisant

(see also: Atherosclerosis; Cardiovascular Diseases; Coronary Artery Disease; Foods and Diets; Lifestyle; Nutrition )

Bibliography

"The Sixth Report of the Joint National Committee on Prevention, Detection, Evaluation, and Treatment of High Blood Pressure." (1997). Arch Intern Med 157:24132446.

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blood pressure

blood pressure, force exerted by the blood upon the walls of the arteries. The pressure in the arteries originates in the pumping action of the heart, and pressure waves can be felt at the wrist and at other points where arteries lie near the surface of the body (see pulse). Since the heart can pump blood into the large arteries more quickly than it can be absorbed and released by the tiny arterioles and capillaries, considerable inner pressure always exists in the arteries. The contraction of the heart (systole) causes the blood pressure to rise to its highest point, and relaxation of the heart (diastole) brings the pressure down to its lowest point.

Blood pressure is strongest in the aorta, where the blood leaves the heart. It diminishes progressively in the smaller blood vessels and reaches its lowest point in the veins (see circulatory system). Blood pressure manifests itself dramatically when an artery is severed or pierced and the blood (under pressure) ejects in spurts.

Since blood pressure varies in different arteries, the pressure in the brachial artery of the forearm serves as a standard. A sphygmomanometer measures blood pressure in millimeters of mercury; blood pressure gauges that do not use mercury also produce readings that are expressed in terms of millimeters of mercury. Normal blood pressure readings for healthy young people should be below 120 mm for systolic pressure and 80 mm for diastolic pressure, commonly written as 120/80 and read as "one-twenty over eighty." With age, and the constriction of the small arteries and then the larger ones, blood pressure increases, so that at 50 years, a person may typically have a systolic pressure between 140 and 150, and a diastolic pressure of about 90.

Factors other than heart action and the condition of the arteries also influence blood pressure. Temporary high blood pressure usually occurs during or following physical activity, nervous strain, and periods of rage or fear. Therapy for persistent high blood pressure, sometimes called hypertension, consists of sufficient rest, a diet low in salt and alcohol, reduction in weight where there is obesity, and increased exercise. Drug therapy may include diuretics, beta-blockers, calcium-channel blockers, or ACE inhibitors. Low blood pressure (hypotension) has not been studied as extensively as high blood pressure. If not caused by disease or injury, it is generally considered to be a benign or even advantageous condition; however, studies have linked hypotension with feelings of tiredness or faintness and minor psychiatric conditions in some people.

See N. H. Naqvi and M. D. Blaufox, Blood Pressure Measurement: An Illustrated History (1998).

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blood pressure

blood pressure The pressure exerted by the flow of blood through the major arteries of the body. This pressure is greatest during the contraction of the ventricles of the heart (systolic pressure; see systole), which forces blood into the arterial system. Pressure falls to its lowest level when the heart is filling with blood (diastolic pressure; see diastole). Blood pressure is measured in millimetres of mercury using an instrument called a sphygmomanometer. Normal blood pressure for a young average adult human is in the region of 120/80 mmHg (the higher number is the systolic blood pressure; the lower number the diastolic blood pressure), but individual variations are common. Abnormally high blood pressure (hypertension) may be associated with disease or it may occur without an apparent cause.

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blood pressure

blood pressure (BP) n. the pressure of blood against the walls of the main arteries. Pressure is highest during systole, when the ventricles are contracting (systolic pressure), and lowest during diastole, when the ventricles are relaxing and refilling (diastolic pressure). Blood pressure is measured – in millimetres of mercury – by means of a sphygmomanometer at the brachial artery of the arm. A young adult would be expected to have a systolic pressure of around 120 mm and a diastolic pressure of 80 mm. These are recorded as 120/80. See also hypertension, hypotension.

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blood pressure

blood pressure Force exerted by circulating blood on the walls of blood vessels due to the pumping action of the heart. This is measured, using a gauge known as a sphygmomanometer. It is greatest when the heart contracts and lowest when it relaxes. High blood pressure is associated with an increased risk of heart attacks and strokes; abnormally low blood pressure is mostly seen in people in shock or following excessive loss of fluid or blood.

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blood pressure

blood pressure When the heart contracts (systolic pressure) the normal blood pressure of females is 120 mm mercury at the age of 12 rising to 175 at 70. In males it is 120 rising to 160. When the heart relaxes (diastolic pressure) the normal range for females is 70 rising to 95; males 70 to 85. Diastolic blood pressure above 105 is moderate, and above 115 severe, hypertension.

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blood pressure

blood pres·sure • n. the pressure of the blood in the circulatory system, often measured for diagnosis since it is closely related to the force and rate of the heartbeat and the diameter and elasticity of the arterial walls.

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pressure, blood

pressure, blood See hypertension.

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