blood pressure

Dictionary: blood pressure

n. (Abbr. BP)
The pressure exerted by the blood against the walls of the blood vessels, especially the arteries. It varies with the strength of the heartbeat, the elasticity of the arterial walls, the volume and viscosity of the blood, and a person's health, age, and physical condition.

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Britannica Concise Encyclopedia: blood pressure

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Force originating when the heart's pumping pushes the blood against the walls of the blood vessels. Their stretching and contraction help maintain blood flow. Usually measured over an arm or leg artery in humans, blood pressure is expressed as two numbers; normal adult blood pressure is about 120/80 mm of mercury. The higher number (systolic) is measured when the heart's ventricles contract and the lower (diastolic) when they relax. See also hypertension, hypotension.

For more information on blood pressure, visit Britannica.com.

World of the Body: blood pressure

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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 × PVR

where 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).

Fig. 1 The baroreceptor reflex. An increase in blood pressure increases the rate of nerve impulses from baroreceptorto brain. Vagal centres are stimulated: increased activity in the vagus nerve to the pacemaker slows the heart. Sympathetic centres are inhibited causing less activity in sympathetic nerves. As these nerves are excitatory, the effect of inhibition is to slow heart rate, weaken force of contraction, and dilate blood vessels. All these changes lower the blood pressure. The opposite effects would be seen in response to a decrease in blood pressure

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.

Fig. 2 The Valsalva manoeuvre. The subject blows against a fixed resistance to generate a pressure in the mouth(MP) of 40 mm Hg. This causes 4 phases of blood pressure change. Phase 1: blood pressure (BP) rises due to the pressure transmitted to the arteries. Phase 2: BP falls and pulse pressure (difference between systolic and diastolic pressures) particularly falls due to the reduced filling, and therefore pumping, of the heart. Pressure subsequently recovers due to the reflex changes. Note also the reflex increase in heart rate. Phase 3: BP falls as the pressure is taken off the arteries in the chest and abdomen. Phase 4: there is an overshoot as the 'dammed back' blood rushes into the heart and is pumped into a constricted circulation

— Roger Hainsworth

Food and Nutrition: blood pressure

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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.

Food and Fitness: blood pressure

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The pressure exerted by the heart and arteries to push blood around the body. The magnitude of blood pressure is determined by the amount of blood being pumped out of the heart per beat (the stroke volume) and the resistance encountered as it passes through the blood vessels (peripheral resistance). Blood pressure is usually expressed as two measurements: systolic blood pressure, indicating the pressure when the heart is actually pumping; and diastolic blood pressure, the pressure when the heart is filling up with blood. Systolic pressure is always the higher and is expressed first. The pressures are measured in millimetres of mercury. Thus a blood pressure of 130/80, or 130 over 80, refers to a systolic blood pressure which will support a column of mercury 130 mm high, and a diastolic pressure which will support a column 80 mm high. Systolic pressure in children is about 100 and in young adults the value is about 120. It tends to rise with age as arteries thicken. A systolic pressure of 180 is not uncommon and it may be as high as 280. The value varies according to a person's position. It tends to drop when you stand up after lying down; this is called postural hypotensive drop. A typical value for diastolic pressure is 80 mm of mercury. Although it is difficult to define precisely what is ‘normal’ blood pressure, there is general agreement that a desirable blood pressure is less than 140/90. See also hypertension and hypotension.

Dental Dictionary: blood pressure

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The pressure exerted on arterial walls by the blood when the heart is in systole (systolic pressure), and the pressure maintained by the elasticity of the arteries when the heart is in diastole (diastolic pressure). A consistent arterial pressure greater than 140/90 is considered abnormally high and suggestive of hypertensive vascular disease.

Encyclopedia of Public Health: Blood Pressure

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Blood pressure is a physiological variable—like 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 pressure—the 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 narrow—resulting 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. 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

(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.

Bibliography "The Sixth Report of the Joint National Committee on Prevention, Detection, Evaluation, and Treatment of High Blood Pressure." (1997). Arch Intern Med 157:2413–2446.

— L. MICHAEL PRISANT

Sports Science and Medicine: blood pressure

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The force exerted by blood against a unit area of blond vessel. A blood pressure gradient from the arteries leaving the left ventricle to the veins entering the right atrium enables blond to circulate the body. The magnitude of the blond pressure is determined by the amount blood pumped out of the heart per beat (stroke volume) and the resistance the blond encounters as it passes through the blond vessels (peripheral resistance). Usually two measurements of blood pressure are made: systolic and diastolic pressures, traditionally expressed as two figures in millimetres of mercury (mmHg), e.g. 130/80 mmHg (or 130 over 80). The first figure is always the highest. It refers to the systolic blood pressure, obtained when the blond is ejected into the arteries from the heart. In children, systolic pressure is about 100 mmHg; in young adults, 120 mmHg; thereafter it tends to rise with age as arterial walls thicken. The second figure is the diastolic pressure. It is obtained when the blond drains from the arteries. Blood pressure varies according to the subject's body position (see postural hypotension drop). It is difficult to define precisely what is ‘normal’ blood pressure, but there is general agreement that a desirable blond pressure is less than 140/90 mmHg. See also hypertension.

Columbia Encyclopedia: blood pressure

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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.

Bibliography

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

Health Dictionary: blood pressure

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The pressure of the blood against the walls of the blood vessels, especially the arteries. It is expressed in two figures, said to be one “over” the other: the systolic pressure, which is the pressure when the left ventricle of the heart contracts to push the blood through the body; and the diastolic pressure, which is the pressure when the ventricle relaxes and fills with blood. Blood pressure is affected by the strength of the heartbeat, the volume of blood in the body, the elasticity of the blood vessels, and the age and general health of the person. (See circulatory system.)

Veterinary Dictionary: blood pressure

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The pressure of the blood in the blood vessels. The term usually refers to the pressure of the blood within the arteries, or arterial blood pressure. This pressure is determined by several interrelated factors, including the pumping action of the heart, the resistance to the flow of blood in the arterioles, the elasticity of the walls of the main arteries, the blood volume and extracellular fluid volume, and the blood's viscosity, or thickness.
Relatively simple Doppler instruments can provide accurate blood pressure measurements in dogs and cats. The systolic pressure in dogs is 132±22 mmHg; in cats it is 108±23 mmHg. Thoroughbreds have been shown to be 112/77 mmHg. Indwelling catheters can be used in dogs to monitor central venous pressure.

arterial b. p. — the common measure of blood pressure. The measurement in animal patients must be by a method that does not require entrance to an artery, i.e. noninvasive. Standard methods use an inflatable cuff around a limb, around the tail in the horse, and measurement of the air pressure required to obliterate the pulse wave—the systolic blood pressure, and permit the re-entry of the pulse wave—the diastolic blood pressure.
b. p. homeostasis — the maintenance of a steady state of blood pressure. The mechanisms involved include the baroreceptor mechanism, the chemoreceptor mechanism, the ischemic response of the central nervous system (the Cushing response), the renin–angiotensin vasoconstrictor and the renin–angiotensin–aldosterone system, the capillary fluid-shift mechanism, the regulation of body fluid level by the kidney and the stress–relaxation mechanism of the arterial wall.
b. p. impedance — the resistance to pulsatile flow, as in arteries.
pulmonary wedge b. p. — see wedge pressure.
b. p. regulation — the complex regulatory system which controls arterial blood pressure is dependent on sensory inputs related to cardiac output, peripheral resistance to blood flow at the arterioles, the viscosity of the blood, the volume of blood in the arterial system, the elasticity of the arterial walls. Changes in blood pressure are brought about by the control exerted on the same physiological mechanisms.
venous b. p. — see central venous pressure.

Wikipedia: Blood pressure

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See Hypertension for more information about high blood pressure.

A sphygmomanometer, a device used for measuring arterial pressure.

Blood pressure (BP) is the pressure exerted by circulating blood on the walls of blood vessels, and is one of the principal vital signs. During each heartbeat, BP varies between a maximum (systolic) and a minimum (diastolic) pressure. The mean BP decreases as the circulating blood moves away from the heart through arteries, has its greatest decrease in the small arteries and arterioles, and continues to decrease as the blood moves through the capillaries and back to the heart through veins.^[1]

The term blood pressure usually refers to the pressure measured at a person's upper arm. It is measured on the inside of an elbow at the brachial artery, which is the upper arm's major blood vessel that carries blood away from the heart. A person's BP is usually expressed in terms of the systolic pressure and diastolic pressure, for example 115/75.

BP is sometimes measured at other places, for instance at an ankle. The ratio of the BP measured at the main artery of an ankle, to the BP measured at the brachial artery of an upper arm, gives the Ankle Brachial Pressure Index (ABPI).

Measurement

A medical student checking blood pressure using a sphygmomanometer and stethoscope.

Arterial pressure is most commonly measured via a sphygmomanometer, which historically used the height of a column of mercury to reflect the circulating pressure.^[2] Today BP values are still reported in millimetres of mercury (mmHg), though aneroid and electronic devices do not use mercury.

For each heartbeat, BP varies between systolic and diastolic pressures. Systolic pressure is peak pressure in the arteries, which occurs near the end of the cardiac cycle when the ventricles are contracting. Diastolic pressure is minimum pressure in the arteries, which occurs near the beginning of the cardiac cycle when the ventricles are filled with blood. An example of normal measured values for a resting, healthy adult human is 115 mmHg systolic and 75 mmHg diastolic (written as 115/75 mmHg, and spoken [in the US] as "one-fifteen over seventy-five"). Pulse pressure is the difference between systolic and diastolic pressures.

Systolic and diastolic arterial BPs are not static but undergo natural variations from one heartbeat to another and throughout the day (in a circadian rhythm). They also change in response to stress, nutritional factors, drugs, disease, exercise, and momentarily from standing up. Sometimes the variations are large. Hypertension refers to arterial pressure being abnormally high, as opposed to hypotension, when it is abnormally low. Along with body temperature, respiratory rate, and pulse rate, BP measurements are the most commonly measured physiological parameters.

Arterial pressures are usually measured non-invasively, without penetrating skin or artery. Measuring pressure invasively, by penetrating the arterial wall to take the measurement, is much less common, and usually restricted to a hospital setting.

Noninvasive measurement

The noninvasive auscultatory and oscillometric measurements are simpler and quicker than invasive measurements, require less expertise in fitting, have virtually no complications, and are less unpleasant and painful for the patient. However, noninvasive methods may yield somewhat lower accuracy and small systematic differences in numerical results. Non-invasive measurement methods are more commonly used for routine examinations and monitoring.

Palpation method

A minimum systolic value can be roughly estimated without any equipment by palpation, most often used in emergency situations. Historically, students have been taught that palpation of a radial pulse indicates a minimum BP of 80 mmHg, a femoral pulse indicates at least 70 mmHg, and a carotid pulse indicates a minimum of 60 mmHg. However, at least one study indicated that this method often overestimates patients' systolic BP.^[3] A more accurate value of systolic BP can be obtained with a sphygmomanometer and palpating for when a radial pulse returns.^[4] The diastolic blood pressure can not be estimated by this method.^[5] Sometimes palpation is used to get an estimate before using the auscultatory method.

Auscultatory method

Auscultatory method aneroid sphygmomanometer with stethoscope

Mercury manometer

The auscultatory method (from the Latin for listening) uses a stethoscope and a sphygmomanometer. This comprises an inflatable (Riva-Rocci) cuff placed around the upper arm at roughly the same vertical height as the heart, attached to a mercury or aneroid manometer. The mercury manometer measures the height of a column of mercury, giving an absolute result without need for calibration, and consequently not subject to the errors and drift of calibration which affect other methods. The use of mercury manometers is often required in clinical trials and for the clinical measurement of hypertension in high risk patients, such as pregnant women.

A cuff of appropriate size is fitted smoothly and snugly, then inflated manually by repeatedly squeezing a rubber bulb until the artery is completely occluded. Listening with the stethoscope to the brachial artery at the elbow, the examiner slowly releases the pressure in the cuff. When blood just starts to flow in the artery, the turbulent flow creates a "whooshing" or pounding (first Korotkoff sound). The pressure at which this sound is first heard is the systolic BP. The cuff pressure is further released until no sound can be heard (fifth Korotkoff sound), at the diastolic arterial pressure.

The auscultatory method has been predominant since the beginning of BP measurements but is in some cases is being replaced by other noninvasive techniques.^[6]

Oscillometric method

The Oscillometric method was first demonstrated in 1876 and involves the observation of oscillations in the sphygmomanometer cuff pressure^[7] which are caused by the oscillations of blood flow, i.e. the pulse.^[8] The electronic version of this method is sometimes used in long-term measurements and general practice. It uses a sphygmomanometer cuff like the auscultatory method, but with an electronic pressure sensor (transducer) to observe cuff pressure oscillations, electronics to automatically interpret them, and automatic inflation and deflation of the cuff. The pressure sensor should be calibrated periodically to maintain accuracy.

Oscillometric measurement requires less skill than the auscultatory technique, and may be suitable for use by untrained staff and for automated patient home monitoring.

The cuff is inflated to a pressure initially in excess of the systolic arterial pressure, and then reduces to below diastolic pressure over a period of about 30 seconds. When blood flow is nil (cuff pressure exceeding systolic pressure) or unimpeded (cuff pressure below diastolic pressure), cuff pressure will be essentially constant. It is essential that the cuff size is correct: undersized cuffs may yield too high a pressure, whereas oversized cuffs yield too low a pressure. When blood flow is present, but restricted, the cuff pressure, which is monitored by the pressure sensor, will vary periodically in synchrony with the cyclic expansion and contraction of the brachial artery, i.e., it will oscillate. The values of systolic and diastolic pressure are computed, not actually measured from the raw data, using an algorithm; the computed results are displayed.

Oscillometric monitors may produce inaccurate readings in patients with heart and circulation problems, that include arterial sclerosis, arrhythmia, preeclampsia, pulsus alternans, and pulsus paradoxus.

In practice the different methods do not give identical results; an algorithm and experimentally obtained coefficients are used to adjust the oscillometric results to give readings which match the auscultatory results as well as possible. Some equipment uses computer-aided analysis of the instantaneous arterial pressure waveform to determine the systolic, mean, and diastolic points. Since many oscillometric devices have not been validated, caution must be given as most are not suitable in clinical and acute care settings.

The term NIBP, for Non-Invasive Blood Pressure, is often used to describe oscillometric monitoring equipment.

White-coat hypertension

For some patients, BP measurements taken in a doctor's office may not correctly characterize their typical BP.^[9] In up to 25% of patients, the office measurement is higher than their typical BP. This type of error is called white-coat hypertension (WCH) and can result from anxiety related to an examination by a health care professional.^[10] The misdiagnosis of hypertension for these patients can result in needless and possibly harmful medication. WCH can be reduced (but not eliminated) with automated BP measurements over 15 to 20 minutes in a quiet part of the office or clinic.^[11]

Debate continues regarding the significance of this effect.^{[citation needed]} Some reactive patients will also react to many other stimuli throughout their daily lives, and require treatment. In some cases a lower BP reading occurs at the doctor's office.^[12]

Home monitoring

Ambulatory blood pressure devices that take readings every half hour throughout the day and night have been used for identifying and mitigating measurement problems like white-coat hypertension. Except for periods during sleep, home monitoring could be used for these purposes instead of ambulatory blood pressure monitoring.^[13] Home monitoring may also be used to improve hypertension management and to monitor the effects of lifestyle changes and medication related to BP.^[14] Compared to ambulatory blood pressure measurements, home monitoring has been found to be an effective and lower cost alternative.^[13]^[15]^[16]

Aside from the white coat effect, BP readings outside of a clinical setting are usually slightly lower in the majority of people. The studies that looked into the risks from hypertension and the benefits of lowering BP in affected patients were based on readings in a clinical environment.

When measuring BP, an accurate reading requires that one not drink coffee, smoke cigarettes, or engage in strenuous exercise for 30 minutes before taking the reading. A full bladder may have a small effect on BP readings, so if the urge to urinate exists, one should do so before the reading. For 5 minutes before the reading, one should sit upright in a chair with one's feet flat on the floor and with limbs uncrossed. The BP cuff should always be against bare skin, as readings taken over a shirt sleeve are less accurate. During the reading, the arm that is used should be relaxed and kept at heart level, for example by resting it on a table.^[17]

Since BP varies throughout the day, measurements intended to monitor changes over longer time frames should be taken at the same time of day to ensure that the readings are comparable. Suitable times are:

immediately after awakening (before washing/dressing and taking breakfast/drink), while the body is still resting,
immediately after finishing work.

Automatic self-contained BP monitors are available at reasonable prices, some of which are capable of Korotkoff's measurement in addition to oscillometric methods, enabling irregular heartbeat patients to accurately measure their blood pressure at home.

Invasive measurement

Arterial blood pressure (BP) is most accurately measured invasively through an arterial line. Invasive arterial pressure measurement with intravascular cannulae involves direct measurement of arterial pressure by placing a cannula needle in an artery (usually radial, femoral, dorsalis pedis or brachial). This procedure can be done by any licensed doctor or a Respiratory Therapist.

The cannula must be connected to a sterile, fluid-filled system, which is connected to an electronic pressure transducer. The advantage of this system is that pressure is constantly monitored beat-by-beat, and a waveform (a graph of pressure against time) can be displayed. This invasive technique is regularly employed in human and veterinary intensive care medicine, anesthesiology, and for research purposes.

Cannulation for invasive vascular pressure monitoring is infrequently associated with complications such as thrombosis, infection, and bleeding. Patients with invasive arterial monitoring require very close supervision, as there is a danger of severe bleeding if the line becomes disconnected. It is generally reserved for patients where rapid variations in arterial pressure are anticipated.

Invasive vascular pressure monitors are pressure monitoring systems designed to acquire pressure information for display and processing. There are a variety of invasive vascular pressure monitors for trauma, critical care, and operating room applications. These include single pressure, dual pressure, and multi-parameter (i.e. pressure / temperature). The monitors can be used for measurement and follow-up of arterial, central venous, pulmonary arterial, left atrial, right atrial, femoral arterial, umbilical venous, umbilical arterial, and intracranial pressures.

Classification

The following classification of blood pressure applies to adults aged 18 and older. It is based on the average of seated BP readings that were properly measured during 2 or more office visits.^[14]^[18]

Classification of blood pressure for adults
Category	systolic, mmHg	diastolic, mmHg
Hypotension	< 90	< 60
Normal	90 – 119	60 – 79
Prehypertension	120 – 139	80 – 89
Stage 1 Hypertension	140 – 159	90 – 99
Stage 2 Hypertension	≥ 160	≥ 100

Normal values

While average values for arterial pressure could be computed for any given population, there is often a large variation from person to person; arterial pressure also varies in individuals from moment to moment. Additionally, the average of any given population may have a questionable correlation with its general health, thus the relevance of such average values is equally questionable. However, in a study of 100 subjects with no known history of hypertension, an average blood pressure of 112/64 mmHg was found,^[19] which is in the normal range.

Various factors influence a person's average BP and variations. Factors such as age and gender^[20] influence average values. In children, the normal ranges are lower than for adults and depend on height.^[21] As adults age, systolic pressure tends to rise and diastolic tends to fall.^[22] In the elderly, BP tends to be above the normal adult range,^[23] largely because of reduced flexibility of the arteries. Also, an individual's BP varies with exercise, emotional reactions, sleep, digestion and time of day.

Differences between left and right arm BP measurements tend to be random and average to nearly zero if enough measurements are taken. However, in a small percentage of cases there is a consistently present difference greater than 10 mmHg which may need further investigation, e.g. for obstructive arterial disease.^[24]^[25]

The risk of cardiovascular disease increases progressively above 115/75 mmHg.^[26] In the past, hypertension was only diagnosed if secondary signs of high arterial pressure were present, along with a prolonged high systolic pressure reading over several visits. In the UK, patients’ readings are considered normal up to 140/90 mmHg.^[27]

Clinical trials demonstrate that people who maintain arterial pressures at the low end of these pressure ranges have much better long term cardiovascular health. The principal medical debate concerns the aggressiveness and relative value of methods used to lower pressures into this range for those who do not maintain such pressure on their own. Elevations, more commonly seen in older people, though often considered normal, are associated with increased morbidity and mortality.

Physiology

The physics of the circulatory system are very complex. That said, there are many physical factors that influence arterial pressure. Each of these may in turn be influenced by physiological factors, such as diet, exercise, disease, drugs or alcohol, stress, obesity, and so-forth.

Some physical factors are:

Rate of pumping. In the circulatory system, this rate is called heart rate, the rate at which blood (the fluid) is pumped by the heart. The volume of blood flow from the heart is called the cardiac output which is the heart rate (the rate of contraction) multiplied by the stroke volume (the amount of blood pumped out from the heart with each contraction). The higher the heart rate, the higher the arterial pressure, assuming no reduction in stroke volume.
Volume of fluid or blood volume, the amount of blood that is present in the body. The more blood present in the body, the higher the rate of blood return to the heart and the resulting cardiac output. There is some relationship between dietary salt intake and increased blood volume, potentially resulting in higher arterial pressure, though this varies with the individual and is highly dependent on autonomic nervous system response and the renin-angiotensin system.
Resistance. In the circulatory system, this is the resistance of the blood vessels. The higher the resistance, the higher the arterial pressure upstream from the resistance to blood flow. Resistance is related to vessel radius (the larger the radius, the lower the resistance), vessel length (the longer the vessel, the higher the resistance), as well as the smoothness of the blood vessel walls. Smoothness is reduced by the build up of fatty deposits on the arterial walls. Substances called vasoconstrictors can reduce the size of blood vessels, thereby increasing BP. Vasodilators (such as nitroglycerin) increase the size of blood vessels, thereby decreasing arterial pressure. Resistance, and its relation to volumetric flow rate (Q) and pressure difference between the two ends of a vessel are described by Poiseuille's Law.
Viscosity, or thickness of the fluid. If the blood gets thicker, the result is an increase in arterial pressure. Certain medical conditions can change the viscosity of the blood. For instance, low red blood cell concentration, anemia, reduces viscosity, whereas increased red blood cell concentration increases viscosity. Viscosity also increases with blood sugar concentration—visualize pumping syrup. It had been thought that aspirin and related "blood thinner" drugs decreased the viscosity of blood, but studies found^[28] that they act by reducing the tendency of the blood to clot instead.

In practice, each individual's autonomic nervous system responds to and regulates all these interacting factors so that, although the above issues are important, the actual arterial pressure response of a given individual varies widely because of both split-second and slow-moving responses of the nervous system and end organs. These responses are very effective in changing the variables and resulting BP from moment to moment.

Mean arterial pressure

The mean arterial pressure (MAP) is the average over a cardiac cycle and is determined by the cardiac output (CO), systemic vascular resistance (SVR), and central venous pressure (CVP),^[29]

$\! MAP = (CO \cdot SVR) + CVP.$

MAP can be approximately determined from measurements of the systolic pressure $\! P_{sys}$ and the diastolic pressure $\! P_{dias}$ while there is a normal resting heart rate,^[29]

$\! MAP \approxeq P_{dias} + \frac{1}{3} (P_{sys} - P_{dias}).$

Pulse pressure

The up and down fluctuation of the arterial pressure results from the pulsatile nature of the cardiac output, i.e. the heartbeat. The pulse pressure is determined by the interaction of the stroke volume of the heart, compliance (ability to expand) of the aorta, and the resistance to flow in the arterial tree. By expanding under pressure, the aorta absorbs some of the force of the blood surge from the heart during a heartbeat. In this way the pulse pressure is reduced from what it would be if the aorta wasn't compliant.^[30]

The pulse pressure can be simply calculated from the difference of the measured systolic and diastolic pressures,^[30]

$\! P_{pulse} = P_{sys} - P_{dias}.$

Vascular resistance

The larger arteries, including all large enough to see without magnification, are low resistance conduits (assuming no advanced atherosclerotic changes) with high flow rates that generate only small drops in pressure.

Vascular pressure wave

Modern physiology developed the concept of the vascular pressure wave (VPW). This wave is created by the heart during the systole and originates in the ascending aorta. Much faster than the stream of blood itself, it is then transported through the vessel walls to the peripheral arteries. There the pressure wave can be palpated as the peripheral pulse. As the wave is reflected at the peripheral veins it runs back in a centripetal fashion. Where the crests of the reflected and the original wave meet, the pressure inside the vessel is higher than the true pressure in the aorta. This concept explains why the arterial pressure inside the peripheral arteries of the legs and arms is higher than the arterial pressure in the aorta,^[31]^[32]^[33] and in turn for the higher pressures seen at the ankle compared to the arm with normal ankle brachial pressure index values.

Regulation

The endogenous regulation of arterial pressure is not completely understood. Currently, three mechanisms of regulating arterial pressure have been well-characterized:

Baroreceptor reflex: Baroreceptors detect changes in arterial pressure and send signals ultimately to the medulla of the brain stem. The medulla, by way of the autonomic nervous system, adjusts the mean arterial pressure by altering both the force and speed of the heart's contractions, as well as the total peripheral resistance. The most important arterial baroreceptors are located in the left and right carotid sinuses and in the aortic arch.^[34]

Renin-angiotensin system (RAS): This system is generally known for its long-term adjustment of arterial pressure. This system allows the kidney to compensate for loss in blood volume or drops in arterial pressure by activating an endogenous vasoconstrictor known as angiotensin II.

Aldosterone release: This steroid hormone is released from the adrenal cortex in response to angiotensin II or high serum potassium levels. Aldosterone stimulates sodium retention and potassium excretion by the kidneys. Since sodium is the main ion that determines the amount of fluid in the blood vessels by osmosis, aldosterone will increase fluid retention, and indirectly, arterial pressure.

These different mechanisms are not necessarily independent of each other, as indicated by the link between the RAS and aldosterone release. Currently, the RAS system is targeted pharmacologically by ACE inhibitors and angiotensin II receptor antagonists. The aldosterone system is directly targeted by spironolactone, an aldosterone antagonist. The fluid retention may be targeted by diuretics; the antihypertensive effect of diuretics is due to its effect on blood volume. Generally, the baroreceptor reflex is not targeted in hypertension because if blocked, individuals may suffer from orthostatic hypotension and fainting.

Pathophysiology

High arterial pressure

Main article: Hypertension

Overview of main complications of persistent high blood pressure.

Arterial hypertension can be an indicator of other problems and may have long-term adverse effects. Sometimes it can be an acute problem, for example hypertensive emergency.

All levels of arterial pressure put mechanical stress on the arterial walls. Higher pressures increase heart workload and progression of unhealthy tissue growth (atheroma) that develops within the walls of arteries. The higher the pressure, the more stress that is present and the more atheroma tend to progress and the heart muscle tends to thicken, enlarge and become weaker over time.

Persistent hypertension is one of the risk factors for strokes, heart attacks, heart failure and arterial aneurysms, and is the leading cause of chronic renal failure. Even moderate elevation of arterial pressure leads to shortened life expectancy. At severely high pressures, mean arterial pressures 50% or more above average, a person can expect to live no more than a few years unless appropriately treated.^[35]

In the past, most attention was paid to diastolic pressure; but nowadays it is recognised that both high systolic pressure and high pulse pressure (the numerical difference between systolic and diastolic pressures) are also risk factors. In some cases, it appears that a decrease in excessive diastolic pressure can actually increase risk, due probably to the increased difference between systolic and diastolic pressures (see the article on pulse pressure).

Low arterial pressure

Main article: Hypotension

Blood pressure that is too low is known as hypotension. The similarity in pronunciation with hypertension can cause confusion. Hypotension is a medical concern only if it causes signs or symptoms, such as dizziness, fainting, or in extreme cases, shock.^[18]

When arterial pressure and blood flow decrease beyond a certain point, the perfusion of the brain becomes critically decreased (i.e., the blood supply is not sufficient), causing lightheadedness, dizziness, weakness or fainting.

Sometimes the arterial pressure drops significantly when a patient stands up from sitting. This is known as orthostatic hypotension (postural hypotension); gravity reduces the rate of blood return from the body veins below the heart back to the heart, thus reducing stroke volume and cardiac output.

When people are healthy, the veins below their heart quickly constrict and the heart rate increases to minimize and compensate for the gravity effect. This is carried out involuntarily by the autonomic nervous system. The system usually requires a few seconds to fully adjust and if the compensations are too slow or inadequate, the individual will suffer reduced blood flow to the brain, dizziness and potential blackout. Increases in G-loading, such as routinely experienced by aerobatic or combat pilots 'pulling Gs', greatly increases this effect. Repositioning the body perpendicular to gravity largely eliminates the problem.

Other causes of low arterial pressure include:

Sepsis
Hemorrhage - blood loss
Toxins including toxic doses of BP medicine
Hormonal abnormalities, such as Addison's disease

Shock is a complex condition which leads to critically decreased perfusion. The usual mechanisms are loss of blood volume, pooling of blood within the veins reducing adequate return to the heart and/or low effective heart pumping. Low arterial pressure, especially low pulse pressure, is a sign of shock and contributes to and reflects decreased perfusion.

If there is a significant difference in the pressure from one arm to the other, that may indicate a narrowing (for example, due to aortic coarctation, aortic dissection, thrombosis or embolism) of an artery.

Other sites

Blood pressure generally refers to the arterial pressure in the systemic circulation. However, measurement of pressures in the venous system and the pulmonary vessels plays an important role in intensive care medicine but requires an invasive central venous catheter.

Venous pressure

Venous pressure is the vascular pressure in a vein or in the atria of the heart. It is much less than arterial pressure, with common values of 5 mmHg in the right atrium and 8 mmHg in the left atrium.

Pulmonary pressure

Normally, the pressure in the pulmonary artery is about 15 mmHg at rest.^[36]

Increased BP in the capillaries of the lung cause pulmonary hypertension, with interstitial edema if the pressure increases to above 20 mmHg, and to frank pulmonary edema at pressures above 25 mmHg.^[37]

Fetal blood pressure

Further information: Fetal circulation#Blood pressure

In pregnancy, it is the fetal heart and not the mother's heart that builds up the fetal BP to drive its blood through the fetal circulation.

The BP in the fetal aorta is approximately 30 mmHg at 20 weeks of gestation, and increases to ca 45 mmHg at 40 weeks of gestation.^[38]
The average BP for full-term infants:
Systolic 65–95 mm Hg
Diastolic 30–60 mm Hg ^[39]

References

Pickering, TG; Hall, JE; Appel, LJ et al. (2005). "Recommendations for blood pressure measurement in humans and experimental animals: Part 1: blood pressure measurement in humans: a statement for professionals from the Subcommittee of Professional and Public Education of the American Heart Association Council on High Blood Pressure Research". Hypertension 45 (5): 142–61. doi:10.1161/01.HYP.0000150859.47929.8e. PMID 15611362. http://hyper.ahajournals.org/cgi/content/full/45/1/142. Retrieved 2009-10-01.

Footnotes

^ Klabunde, Richard (2005). Cardiovascular Physiology Concepts. Lippincott Williams & Wilkins. pp. 93–4. ISBN 978-0781750301.
^ Booth, J (1977). "A short history of blood pressure measurement". Proceedings of the Royal Society of Medicine 70 (11): 793–9. PMID 341169. http://www.pubmedcentral.nih.gov/articlerender.fcgi?artid=1543468. Retrieved 2009-10-06.
^ Deakin CD, Low JL (September 2000). "Accuracy of the advanced trauma life support guidelines for predicting systolic blood pressure using carotid, femoral, and radial pulses: observational study". BMJ 321 (7262): 673–4. doi:10.1136/bmj.321.7262.673. PMID 10987771. PMC 27481. http://bmj.com/cgi/pmidlookup?view=long&pmid=10987771.
^ Interpretation - Blood Pressure - Vitals, "University of Florida". Retrieved on 2008-03-18.
^ G8 Secondary Survey, "Manitoba". Retrieved on 2008-03-18.
^ (Pickering et al. 2005, p. 146) See Blood Pressure Measurement Methods.
^ (Pickering et al. 2005, p. 147) See The Oscillometric Technique.
^ Laurent, P (2003-09-28). "Blood Pressure & Hypertension". http://www.blood-pressure-hypertension.com/how-to-measure/measure-blood-pressure-8.shtml. Retrieved 2009-10-05.
^ Elliot, Victoria Stagg (2007-06-11). "Blood pressure readings often unreliable". American Medical News (American Medical Association). http://www.ama-assn.org/amednews/2007/06/11/hlsa0611.htm. Retrieved 2008-08-16.
^ Jhalani, Juhee; Tanya Goyal, Lynn Clemow, et al (2005). "Anxiety and outcome expectations predict the white-coat effect". Blood Pressure Monitoring 10 (6): 317–9. PMID 16496447. http://journals.lww.com/bpmonitoring/pages/articleviewer.aspx?year=2005&issue=12000&article=00006&type=abstract. Retrieved 2009-10-03.
^ (Pickering et al. 2005, p. 145) See White Coat Hypertension or Isolated Office Hypertension.
^ (Pickering et al. 2005, p. 146) See Masked Hypertension or Isolated Ambulatory Hypertension.
^ ^a ^b Mancia G, De Backer G, Dominiczak A, et al (June 2007). "2007 Guidelines for the management of arterial hypertension: The Task Force for the Management of Arterial Hypertension of the European Society of Hypertension (ESH) and of the European Society of Cardiology (ESC)". Eur Heart J 28 (12): 1462–536. doi:10.1093/eurheartj/ehm236. PMID 17562668.
^ ^a ^b Chobanian AV, Bakris GL, Black HR, et al (December 2003). "Seventh report of the Joint National Committee on Prevention, Detection, Evaluation, and Treatment of High Blood Pressure". Hypertension 42 (6): 1206–52. doi:10.1161/01.HYP.0000107251.49515.c2. PMID 14656957. http://www.nhlbi.nih.gov/guidelines/hypertension/.
^ Niiranen, TJ; Kantola IM, Vesalainen R, et al (2006). "A comparison of home measurement and ambulatory monitoring of blood pressure in the adjustment of antihypertensive treatment". Am J Hypertens 19 (5): 468–74. doi:10.1016/j.amjhyper.2005.10.017. PMID 16647616.
^ Shimbo, Daichi; Thomas G. Pickering, Tanya M. Spruill, et al (2007). "Relative utility of home, ambulatory, and office blood pressures in the prediction of end-organ damage" (^{[dead link]}). Am J Hypertens 20 (5): 476–82. doi:10.1016/j.amjhyper.2006.12.011. PMID 17485006. http://www.nature.com/ajh/journal/v20/n5/abs/ajh200783a.html.
^ National Heart, Lung and Blood Institute. Tips for having your blood pressure taken. http://www.nhlbi.nih.gov/hbp/detect/tips.htm.
^ ^a ^b "Diseases and Conditions Index - Hypotension". National Heart Lung and Blood Institute. September 2008. http://www.nhlbi.nih.gov/health/dci/Diseases/hyp/hyp_whatis.html. Retrieved 2008-09-16.
^ Pesola GR, Pesola HR, Nelson MJ, Westfal RE (January 2001). "The normal difference in bilateral indirect BP recordings in normotensive individuals". Am J Emerg Med 19 (1): 43–5. doi:10.1053/ajem.2001.20021. PMID 11146017. http://www.sciencedirect.com/science?_ob=ArticleURL&_udi=B6W9K-45SRDHC-C&_user=10&_coverDate=01%2F31%2F2001&_rdoc=1&_fmt=&_orig=search&_sort=d&view=c&_acct=C000050221&_version=1&_urlVersion=0&_userid=10&md5=74f2b32e088d88986cd307f6c7219331.
^ Reckelhoff, Jane F. (2001 May). "Gender Differences in the Regulation of Blood Pressure". Hypertension 37 (5): 1199–208. PMID 11358929. PMID 11358929. http://hyper.ahajournals.org/cgi/content/abstract/hypertensionaha;37/5/1199.
^ National Heart, Lung and Blood Institute. Blood Pressure Tables for Children and Adolescents. http://www.nhlbi.nih.gov/guidelines/hypertension/child_tbl.htm. (Note that the median BP is given by the 50th percentile and hypertension is defined by the 95th percentile for a given age, height, and gender.)
^ (Pickering et al. 2005, p. 145) See Isolated Systolic Hypertension.
^ "...more than half of all Americans aged 65 or older have hypertension." (Pickering et al. 2005, p. 144)
^ Eguchi K, Yacoub M, Jhalani J, Gerin W, Schwartz JE, Pickering TG (February 2007). "Consistency of blood pressure differences between the left and right arms". Arch Intern Med 167 (4): 388–93. doi:10.1001/archinte.167.4.388. PMID 17325301. http://archinte.ama-assn.org/cgi/content/full/167/4/388.
^ Agarwal R, Bunaye Z, Bekele DM (March 2008). "Prognostic significance of between-arm blood pressure differences". Hypertension 51 (3): 657–62. doi:10.1161/HYPERTENSIONAHA.107.104943. PMID 18212263.
^ Appel LJ, Brands MW, Daniels SR, Karanja N, Elmer PJ, Sacks FM (February 2006). "Dietary approaches to prevent and treat hypertension: a scientific statement from the American Heart Association". Hypertension 47 (2): 296–308. doi:10.1161/01.HYP.0000202568.01167.B6. PMID 16434724.
^ "Hypertension: management of hypertension in adults in primary care", NICE Clinical Guideline 34, London, England: National Institute for Health and Clinical Excellence (NICE), June 2006, http://www.nice.org.uk/nicemedia/pdf/CG034NICEguideline.pdf, retrieved 2008-09-15
^ Rosenson RS, Wolff D, Green D, Boss AH, Kensey KR (February 2004). "Aspirin. Aspirin does not alter native blood viscosity". J. Thromb. Haemost. 2 (2): 340–1. PMID 14996003.
^ ^a ^b Klabunde, RE (2007). "Cardiovascular Physiology Concepts - Mean Arterial Pressure". http://www.cvphysiology.com/Blood%20Pressure/BP006.htm. Retrieved 2008-09-29. Archived version 2009-10-03
^ ^a ^b Klabunde, RE (2007). "Cardiovascular Physiology Concepts - Pulse Pressure". http://www.cvphysiology.com/Blood%20Pressure/BP003.htm. Retrieved 2008-10-02. Archived version 2009-10-03
^ Messerli FH, Williams B, Ritz E (2007). "Essential hypertension". Lancet 370 (9587): 591–603. doi:10.1016/S0140-6736(07)61299-9. PMID 17707755.
^ O'Rourke M (01 Jul 1995). "Mechanical principles in arterial disease". Hypertension 26 (1): 2–9. PMID 7607724. http://hyper.ahajournals.org/cgi/content/full/26/1/2.
^ Mitchell GF (2006). "Triangulating the peaks of arterial pressure". Hypertension 48 (4): 543–5. doi:10.1161/01.HYP.0000238325.41764.41. PMID 16940226. http://hyper.ahajournals.org/cgi/content/full/48/4/543.
^ Klabunde, RE (2007). "Cardiovascular Physiology Concepts - Arterial Baroreceptors". http://www.cvphysiology.com/Blood%20Pressure/BP012.htm. Retrieved 2008-09-09. Archived version 2009-10-03
^ Textbook of Medical Physiology, 7th Ed., Guyton & Hall, Elsevier-Saunders, ISBN 0-7216-0240-1, page 220.
^ What Is Pulmonary Hypertension? From Diseases and Conditions Index (DCI). National Heart, Lung, and Blood Institute. Last updated September 2008. Retrieved on 6 April, 2009.
^ Chapter 41, page 210 in: Cardiology secrets By Olivia Vynn Adair Edition: 2, illustrated Published by Elsevier Health Sciences, 2001 ISBN 1560534206, 9781560534204
^ Struijk PC, Mathews VJ, Loupas T, et al (October 2008). "Blood pressure estimation in the human fetal descending aorta". Ultrasound Obstet Gynecol 32 (5): 673–81. doi:10.1002/uog.6137. PMID 18816497.
^ Sharon, S. M. & Emily, S. M.(2006). Fundations of Maternal-Newborn Nursing. (4th ed p.476). Philadelphia:Elsevier.

External links

Cardiovascular system, physiology: cardiovascular physiology

Heart

Volumes	Stroke volume = End-diastolic volume – End-systolic volume Cardiac output = Heart rate · Stroke volume Afterload · Preload Frank-Starling law of the heart · Cardiac function curve · Venous return curve Aortic valve area calculation · Ejection fraction · Cardiac index

Interaction diagrams	Cardiac cycle · Wiggers diagram · Pressure volume diagram

Tropism	Chronotropic (Heart rate) · Dromotropic (Conduction velocity) · Inotropic (Contractility) · Batmotropic (Excitability) · Lusitropic (Relaxation)

Conduction system / Cardiac electrophysiology	Cardiac action potential (Atrial action potential, Ventricular action potential) · Effective refractory period · Pacemaker potential · EKG (P wave, PR interval, QRS complex, QT interval, ST segment, T wave, U wave) · Hexaxial reference system

Chamber pressure	Central venous pressure/right atrial pressure → Right ventricular pressure → Pulmonary artery pressure → Pulmonary wedge pressure/left atrial pressure → Left ventricular pressure → Aortic pressure

Other	Ventricular remodeling

Vascular system/
Hemodynamics

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vascular navs: anat/physio/dev, noncongen/systemic vasculitis/congen/neoplasia, symptoms+signs/eponymous, proc

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