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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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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. hypertension, hypotension.

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Oxford Food & Nutrition Dictionary:

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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.

Oxford Food & Fitness Dictionary:

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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.

Oxford Companion to the Body:

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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

Gale 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

Oxford Dictionary of Sports Science & Medicine:

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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.

Answer of the Day:

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Flying Low

So you think that extensive flying in airplanes is bad for you? Well, it turns out just living near the airport is also damaging to your health. Recent studies have shown that the noise of planes flying overhead immediately boosts the blood pressure of those who hear it — even while they're sleeping. And the louder the noise, the higher the blood pressure will go. British researchers found a marked increase in a person's blood pressure when (s)he was exposed to noise levels exceeding 35 decibels. The noise could be from passing aircraft, street traffic and even nearby snoring.

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From our Archives: Today's Highlights, February 23, 2008

Columbia Encyclopedia:

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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).

Dictionary of Cultural Literacy: Health:

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.)

Wiley Dictionary of Flavors:

Blood Pressure

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The internal force exerted on the walls of the blood vessels measured in millimeters of mercury. Systolic pressure is that which is measure by the pumping of the heartbeat, and the diastolic pressure is that measured between beats or the resting pressure. Blood pressure health is of great concern as high blood pressure or hypertension is one of the greatest causes of secondary potentially disastrous health consequences including stroke, heart disease, and other complications. Certain herbs and nutraceuticals are considered useful in the control of high blood pressure as are flavor volatiles themselves. See Aromatherapy, Nutraceuticals, Herbs, Volatiles, Brain.

Saunders Veterinary Dictionary:

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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.

Mosby's Dental Dictionary:

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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.

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For a list of words related to blood pressure, see:

Diagnostic Terminology - blood pressure: pressure of blood in main arteries
Physiology - blood pressure: pressure exerted by blood on walls of vessels
Procedures - blood pressure: measurement of pressure exerted by blood against walls of main arteries

Bradford's Crossword Solver's Dictionary:

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See crossword solutions for the clue Blood-pressure.

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For information about high blood pressure, see Hypertension.

Blood pressure
Diagnostics
A sphygmomanometer, a device used for measuring arterial pressure.
MeSH	D001795

Blood pressure (BP) is the pressure exerted by circulating blood upon the walls of blood vessels, and is one of the principal vital signs. When used without further specification, "blood pressure" usually refers to the arterial pressure of the systemic circulation. During each heartbeat, blood pressure varies between a maximum (systolic) and a minimum (diastolic) pressure.^[1] The blood pressure in the circulation is principally due to the pumping action of the heart.^[2] Differences in mean blood pressure are responsible for blood flow from one location to another in the circulation. The rate of mean blood flow depends on the resistance to flow presented by the blood vessels. Mean blood pressure decreases as the circulating blood moves away from the heart through arteries, capillaries and veins due to viscous losses of energy. Mean blood pressure drops over the whole circulation, although most of the fall occurs along the small arteries and arterioles.^[3] Gravity affects blood pressure via hydrostatic forces (e.g., during standing) and valves in veins, breathing, and pumping from contraction of skeletal muscles also influence blood pressure in veins.^[2]

The measurement blood pressure without further specification usually refers to the systemic arterial pressure measured at a person's upper arm and is a measure of the pressure in the brachial artery, major artery in the upper arm. A person’s blood pressure is usually expressed in terms of the systolic pressure over diastolic pressure and is measured in millimetres of mercury (mmHg), for example 140/90.

Systemic arterial blood pressure

Classification

Classification of blood pressure for adults
according to the American Heart Association^[4]
Category	systolic, mmHg	diastolic, mmHg
Hypotension	< 90	< 60
Desirable	90–119	60–79
Prehypertension	120–139	or 80–89
Stage 1 Hypertension	140–159	or 90–99
Stage 2 Hypertension	160–179	or 100–109
Hypertensive Crisis	≥ 180	or ≥ 110

The table on the right shows the classification of blood pressure adopted by the American Heart Association for adults who are 18 years and older.^[4] It assumes the values are a result of averaging blood pressure readings measured at two or more visits to the doctor.^[5]^[6]

In the UK, blood pressures are usually categorised into three groups: low (90/60 or lower), high (140/90 or higher), and normal (values above 90/60 and below 130/80). ^[7]^[8]

Normal

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 human subjects with no known history of hypertension, an average blood pressure of 112/64 mmHg was found,^[9] which are currently classified as desirable or "normal" values. Normal values fluctuate through the 24-hour cycle, with highest readings in the afternoons and lowest readings at night.^[10]^[11]

Various factors, such as age and gender influence average values, influence a person's average blood pressure and variations. In children, the normal ranges are lower than for adults and depend on height.^[12] As adults age, systolic pressure tends to rise and diastolic tends to fall.^[13] In the elderly, blood pressure tends to be above the normal adult range,^[14] largely because of reduced flexibility of the arteries. Also, an individual's blood pressure varies with exercise, emotional reactions, sleep, digestion and time of day.

Differences between left and right arm blood pressure 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 consistent difference greater than 10 mmHg which may need further investigation, e.g. for obstructive arterial disease.^[15]^[16]

The risk of cardiovascular disease increases progressively above 115/75 mmHg.^[17] 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. Regarding hypotension, in practice blood pressure is considered too low only if noticeable symptoms are present.^[18]

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.

Average blood pressure in (mmHg):

1 year	6–9 years	adults
95/65	100/65	110/65 – 140/90

Physiology

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.^[19]

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 mean arterial pressure, assuming no reduction in stroke volume or central venous return.
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.^[20]^[21] ^[22]
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), blood viscosity, 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 blood pressure. 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, anemia (low red blood cell concentration), reduces viscosity, whereas increased red blood cell concentration increases viscosity. It had been thought that aspirin and related "blood thinner" drugs decreased the viscosity of blood, but instead studies found^[23] that they act by reducing the tendency of the blood to clot.

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 blood pressure from moment to moment.

Moreover, blood pressure is the result of cardiac output increased by peripheral resistance: blood pressure = cardiac output X peripheral resistance. As a result, an abnormal change in blood pressure is often an indication of a problem affecting the heart's output, the blood vessels' resistance, or both. Thus, knowing the patient's blood pressure is critical to assess any pathology related to output and resistance.

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),^[24]

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

MAP can be approximately determined from measurements of the systolic pressure $\! P_{\text{sys}}$ and the diastolic pressure $\! P_{\text{dias}}$ while there is a normal resting heart rate,^[24]

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

Pulse pressure

Curve of the arterial pressure during one cardiac cycle

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.^[25] The loss of arterial compliance that occurs with aging explains the elevated pulse pressures found in elderly patients.

The pulse pressure can be simply calculated from the difference of the measured systolic and diastolic pressures,^[25]

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

Arm–leg gradient

The arm–leg (blood pressure) gradient is the difference between the blood pressure measured in the arms and that measured in the legs. It is normally less than 10 mmHg,^[26] but may be increased in e.g. coarctation of the aorta.^[26]

Vascular resistance

The larger arteries, including all large enough to see without magnification, are conduits with low vascular resistance (assuming no advanced atherosclerotic changes) with high flow rates that generate only small drops in pressure. The smaller arteries and arterioles have higher resistance, and confer the main drop in blood pressure along the circulatory system.

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. When the reflected wave meets the next outbound pressure wave, the pressure inside the vessel rises higher than the 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,^[27]^[28]^[29] 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, but the following mechanisms of regulating arterial pressure have been well-characterized:

Baroreceptor reflex: Baroreceptors in the high pressure receptor zones (mainly in the aortic arch and carotid sinus) detect changes in arterial pressure. These baroreceptors send signals ultimately to the medulla of the brain stem, specifically to the Rostral ventrolateral medulla (RVLM). 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.^[30]
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.
Baroreceptors in low pressure receptor zones (mainly in the venae cavae and the pulmonary veins, and in the atria) result in feedback by regulating the secretion of antidiuretic hormone (ADH/Vasopressin), renin and aldosterone. The resultant increase in blood volume results an increased cardiac output by the Frank–Starling law of the heart, in turn increasing arterial blood 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 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.

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.^[31] Blood pressure values are generally reported in millimetres of mercury (mmHg), though aneroid and electronic devices do not use mercury.

For each heartbeat, blood pressure 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 120 mmHg systolic and 80 mmHg diastolic (written as 120/80 mmHg, and spoken [in the US and UK] as "one-twenty over eighty").

Systolic and diastolic arterial blood pressures 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, blood pressure is one of the four main vital signs routinely monitored by medical professionals and healthcare providers.^[32]

Measuring pressure invasively, by penetrating the arterial wall to take the measurement, is much less common and usually restricted to a hospital setting.

Noninvasive

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

Palpation

A minimum systolic value can be roughly estimated by palpation, most often used in emergency situations, but should be used with caution.^[33] It has been estimated that, using 50% percentiles, carotid, femoral and radial pulses are present in patients with a systolic blood pressure > 70 mmHg, carotid and femoral pulses alone in patients with systolic blood pressure of > 50 mmHg, and only a carotid pulse in patients with a systolic blood pressure of > 40 mmHg.^[33]

A more accurate value of systolic blood pressure can be obtained with a sphygmomanometer and palpating the radial pulse.^[34] The diastolic blood pressure cannot be estimated by this method.^[35] The American Heart Association recommends that palpation be used to get an estimate before using the auscultatory method.

Auscultatory

Auscultatory method aneroid sphygmomanometer with stethoscope

Mercury manometer

The auscultatory method (from the Latin word 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, considered the gold standard, 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 blood pressure. The cuff pressure is further released until no sound can be heard (fifth Korotkoff sound), at the diastolic arterial pressure.

The auscultatory method is the predominant method of clinical measurement.^[36]

Oscillometric

The oscillometric method was first demonstrated in 1876 and involves the observation of oscillations in the sphygmomanometer cuff pressure^[37] which are caused by the oscillations of blood flow, i.e., the pulse.^[38] 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 reduced 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; 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, which 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, blood pressure measurements taken in a doctor's office may not correctly characterize their typical blood pressure.^[39] In up to 25% of patients, the office measurement is higher than their typical blood pressure. This type of error is called white-coat hypertension (WCH) and can result from anxiety related to an examination by a health care professional.^[40] The misdiagnosis of hypertension for these patients can result in needless and possibly harmful medication. WCH can be reduced (but not eliminated) with automated blood pressure measurements over 15 to 20 minutes in a quiet part of the office or clinic.^[41]

Debate continues regarding the significance of this effect.^{[citation needed]} Some reactive patients will react to many other stimuli throughout their daily lives and require treatment. In some cases a lower blood pressure reading occurs at the doctor's office.^[42]

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 sleep, home monitoring could be used for these purposes instead of ambulatory blood pressure monitoring.^[43] Home monitoring may be used to improve hypertension management and to monitor the effects of lifestyle changes and medication related to blood pressure.^[5] Compared to ambulatory blood pressure measurements, home monitoring has been found to be an effective and lower cost alternative,^[43]^[44]^[45] but ambulatory monitoring is more accurate than both clinic and home monitoring in diagnosing hypertension. Ambulatory monitoring is recommended for most patients before the start of antihypertensive drugs.^[46]

Aside from the white-coat effect, blood pressure 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 blood pressure in affected patients were based on readings in a clinical environment.

When measuring blood pressure, 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 blood pressure readings; 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 blood pressure 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.^[47]

Since blood pressure 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 blood pressure 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

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).

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.

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 blood pressure to drive its blood through the fetal circulation.

The blood pressure in the fetal aorta is approximately 30 mmHg at 20 weeks of gestation, and increases to approximately 45 mmHg at 40 weeks of gestation.^[48]
The average blood pressure for full-term infants:
Systolic 65–95 mm Hg
Diastolic 30–60 mm Hg^[49]

Blood pressure is the measurement of force that is applied to the walls of the blood vessels as the heart pumps blood throughout the body.^[50] The human circulatory system is 400,000 miles long, and the magnitude of blood pressure is not uniform in all the blood vessels in the human body. The blood pressure is determined by the diameter, flexibility and the amount of blood being pumped through the blood vessel.^[50] Blood pressure is also affected by other factors including exercise, stress level, diet and sleep.

The average normal blood pressure in the brachial artery, which is the next direct artery from the aorta after the subclavian artery, is 120mmHg/80mmHg. Blood pressure readings are measured in millimeters of mercury (mmHg) using sphygmomanometer. Two pressures are measured and recorded namely as systolic and diastolic pressures. Systolic pressure reading is the first reading, which represents the maximum exerted pressure on the vessels when the heart contracts, while the diastolic pressure, the second reading, represents the minimum pressure in the vessels when the heart relaxes.^[51] Other major arteries have similar levels of blood pressure recordings indicating very low disparities among major arteries. The innominate artery, the average reading is 110/70mmHg, the right subclavian artery averages 120/80 and the abdominal aorta is 110/70mmHg.^[52] The relatively uniform pressure in the arteries indicate that these blood vessels act as a pressure reservoir for fluids that are transported within them.

Pressure drops gradually as blood flows from the major arteries, through the arterioles, the capillaries until blood is pushed up back into the heart via the venules, the veins through the vena cava with the help of the muscles. At any given pressure drop, the flow rate is determined by the resistance to the blood flow. In the arteries, with the absence of diseases, there is very little or no resistance to blood. The vessel diameter is the most principal determinant to control resistance. Compared to other smaller vessels in the body, the artery has a much bigger diameter (4mm), therefore the resistance is low.^[52]

In addition, flow rate (Q) is also the product of the cross-sectional area of the vessel and the average velocity (Q=AV). Flow rate is directly proportional to the pressure drop in a tube or in this case a vessel. ∆P α Q. The relationship is further described by Poisseulle’s equation ∆P=8µlQ/πr^4.^[53] As evident in the Poisseulle’s equation, although flow rate is proportional to the pressure drop, there are other factors of blood vessels that contribute towards the difference in pressure drop in bifurcations of blood vessels. These include viscosity, length of the vessel, and radius of the vessel.

Factors that determine the flow’s resistance as described by Poiseuille’s relationship:

$\Delta P = \frac{8 \mu l Q}{\pi r^4}$

∆P: Pressure drop/gradient
µ: Viscosity
l: length of tube. In the case of vessels with infinitely long lengths, l is replaced with diameter of the vessel.
Q: flow rate of the blood in the vessel
r: radius of the vessel

Assuming steady, laminar flow in the vessel, the blood vessels behavior is similar to that of a pipe. For instance if p1 and p2 are pressures are at the ends of the tube, the pressure drop/gradient is:^[54]

$\frac{p_1 - p_2}{l} = \Delta P$

In the arterioles blood pressure is lower than in the major arteries. This is due to bifurcations, which cause a drop in pressure. The more bifurcations, the higher the total cross-sectional area, therefore the pressure across the surface drops. This is why the arterioles have the highest pressure-drop. The pressure drop of the arterioles is the product of flow rate and resistance: ∆P=Q xresistance. The high resistance observed in the arterioles, which factor largely in the ∆P is a result of a smaller radius of about 30 µm.^[55] The smaller the radius of a tube, the larger the resistance to fluid flow.

Immediately following the arterioles are the capillaries. Following the logic obvserved in the arterioles, we expect the blood pressure to be lower in the capillaries compared to the arterioles. Since pressure is a function of force per unit area, (P=F/A), the larger the surface area, the lesser the pressure when an external force acts on it. Though the radii of the capillaries are very small, the network of capillaries have the largest surface area in the vascular network. They are known to have the largest surface area (485mm) in the human vascular network. The larger the total cross-sectional area, the lower the mean velocity as well as the pressure.^[52]

Reynold’s number also affects the blood flow in capillaries. Due to its smaller radius and lowest velocity compared to other vessels, the Reynold’s number at the capillaries is very low, resulting in laminar instead of turbulent flow.^[56]

The Reynold’s number (denoted NR or Re) is a relationship that helps determine the behavior of a fluid in a tube, in this case blood in the vessel. The equation for this dimensionless relationship is written as:^[53]

$NR=\frac{\rho v L}{\mu}$

ρ: density of the blood
v: mean velocity of the blood
L: characteristic dimension of the vessel, in this case diameter
μ: viscosity of blood

The Reynold’s number is directly proportional to the velocity and diameter of the tube. Note that NR is directly proportional to the mean velocity as well as the diameter. A Reynold’s number of less than 2300 is laminar fluid flow, which is characterized by constant flow motion, whereas a value of over 4000, is represented as turbulent flow. Turbulent flow is characterized as chaotic and irregular flow.^[53]

Disorders

Disregulation disorders of blood pressure control include high blood pressure, blood pressure that is too low, and blood pressure that shows excessive or maladaptive fluctuation.

High

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.^[57]

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). If systolic blood pressure is elevated (>140) with a normal diastolic blood pressure (<90), it is called "isolated systolic hypertension" and may present a health concern.^[58]^[59]

For those with heart valve regurgitation, a change in its severity may be associated with a change in diastolic pressure. In a study of people with heart valve regurgitation that compared measurements 2 weeks apart for each person, there was an increased severity of aortic and mitral regurgitation when diastolic blood pressure increased, whereas when diastolic blood pressure decreased, there was a decreased severity.^[60]

Low

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.^[6]

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 blood pressure medicine
Hormonal abnormalities, such as Addison's disease
Eating disorders, particularly anorexia nervosa and bulimia

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 .

Fluctuating blood pressure

Normal fluctuation in blood pressure is adaptive and necessary. Fluctuations in pressure that are significantly greater than the norm are associated with greater white matter hyperintensity, a finding consistent with reduced local cerebral blood flow^[61] and a heightened risk of cerebrovascular disease.^[62] Within both high- and low-blood pressure groups, a greater degree of fluctuation was found to correlate with an increase in cerebrovascular disease compared to those with less variability, suggesting the consideration of the clinical management of blood pressure fluctuations, even among normotensive older adults.^[62] Older individuals and those who had received blood pressure medications were more likely to exhibit larger fluctuations in pressure.^[62]

At other sites

Site		Normal pressure range (in mmHg)^[63]
Central venous pressure		3–8
Right ventricular pressure	systolic	15–30
Right ventricular pressure	diastolic	3–8
Pulmonary artery pressure	systolic	15–30
Pulmonary artery pressure	diastolic	4–12
Pulmonary vein/ Pulmonary capillary wedge pressure		2–15
Left ventricular pressure	systolic	100–140
Left ventricular pressure	diastolic	3-12

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.

Systemic 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.

Variants of venous pressure include:

Central venous pressure, which is a good approximation of right atrial pressure,^[64] which is a major determinant of right ventricular end diastolic volume. (However, there can be exceptions in some cases.)^[65]
The jugular venous pressure (JVP) is the indirectly observed pressure over the venous system. It can be useful in the differentiation of different forms of heart and lung disease.
The portal venous pressure is the blood pressure in the portal vein. It is normally 5–10 mm Hg^[66]

Pulmonary pressure

Main article: Pulmonary artery pressure

Normally, the pressure in the pulmonary artery is about 15 mmHg at rest.^[67]

Increased blood pressure 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.^[68]

Relation to wall tension

Regardless of site, blood pressure is related to the wall tension of the vessel according to the Young–Laplace equation (assuming that the thickness of the vessel wall is very small as compared to the diameter of the lumen):

$\sigma_\theta = \dfrac{Pr}{t} \$

where

P is the blood pressure
t is the wall thickness
r is the inside radius of the cylinder.
$\sigma_\theta \!$ is the cylinder stress or "hoop stress".

For the thin-walled assumption to be valid the vessel must have a wall thickness of no more than about one-tenth (often cited as one twentieth) of its radius.

Components of cylinder stress.

The cylinder stress, in turn, is the average force exerted circumferentially (perpendicular both to the axis and to the radius of the object) in the cylinder wall, and can be described as:

$\sigma_\theta = \dfrac{F}{tl} \$

where:

F is the force exerted circumferentially on an area of the cylinder wall that has the following two lengths as sides:
t is the radial thickness of the cylinder
l is the axial length of the cylinder

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^ [1]^{[dead link]}
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^ ^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
^ ^a ^b Markham LW, Knecht SK, Daniels SR, Mays WA, Khoury PR, Knilans TK (November 2004). "Development of exercise-induced arm-leg blood pressure gradient and abnormal arterial compliance in patients with repaired coarctation of the aorta". Am. J. Cardiol. 94 (9): 1200–2. doi:10.1016/j.amjcard.2004.07.097. PMID 15518624.
^ 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 (1 July 1995). "Mechanical principles in arterial disease". Hypertension 26 (1): 2–9. doi:10.1161/01.HYP.26.1.2. 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
^ Booth, J (1977). "A short history of blood pressure measurement". Proceedings of the Royal Society of Medicine 70 (11): 793–9. PMC 1543468. PMID 341169. //www.pubmedcentral.nih.gov/articlerender.fcgi?tool=pmcentrez&artid=1543468.
^ "Vital Signs (Body Temperature, Pulse Rate, Respiration Rate, Blood Pressure)". OHSU Health Information. Oregon Health & Science University. http://www.ohsu.edu/xd/health/health-information/topic-by-id.cfm?ContentTypeId=85&ContentId=P00866. Retrieved 2010-04-16.
^ ^a ^b 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. PMC 27481. PMID 10987771. http://bmj.com/cgi/pmidlookup?view=long&pmid=10987771.
^ Interpretation - Blood Pressure - Vitals, University of Florida. Retrieved 2008-03-18.
^ G8 Secondary Survey, "Manitoba". Retrieved 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 et al (2005). "Anxiety and outcome expectations predict the white-coat effect". Blood Pressure Monitoring 10 (6): 317–9. doi:10.1097/00126097-200512000-00006. 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.
^ Niiranen, TJ, Kantola IM 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 et al (2007). "The Relative Utility of Home, Ambulatory, and Office Blood Pressures in the Prediction of End-Organ Damage". Am J Hypertens 20 (5): 476–82. doi:10.1016/j.amjhyper.2006.12.011. PMC 1931502. PMID 17485006. http://www.nature.com/ajh/journal/v20/n5/abs/ajh200783a.html. ^{[dead link]}
^ Kate Lovibond, Sue Jowett, Pelham Barton, Mark Caulfield et al. "Cost-effectiveness of options for the diagnosis of high blood pressure in primary care: a modelling study", The Lancet, 24 August 2011. Retrieved 24 August 2011.
^ National Heart, Lung and Blood Institute. Tips for having your blood pressure taken. http://www.nhlbi.nih.gov/hbp/detect/tips.htm.
^ 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). Foundations of Maternal-Newborn Nursing. (4th ed p.476). Philadelphia:Elsevier.
^ ^a ^b Dugdale, David. "Blood Pressure". http://www.nlm.nih.gov/medlineplus/ency/article/003398.htm. Retrieved 1 April 2011.
^ Klabunde, Richard. "Arterial Blood Pressure". http://www.cvphysiology.com/Blood%20Pressure/BP002.htm. Retrieved 31 March 2011.
^ ^a ^b ^c Fung, Yuan-cheng (1997). Biomechanics:Circulation. New York: Springer. p. 571. ISBN 0-387-94384-6.
^ ^a ^b ^c Munson; Young, Okiishi, Huebsch (2009). Fundamentals of Fluid Mechanics (Sixth ed.). New Jersey: John Wiley &Sons, Inc.. p. 725. ISBN 978-0-470-26284-9.
^ Womersley, J. R. (1955). "Method for the calculation of velocity, rate of flow and viscous drag in arteries when the pressure gradient is known". Journal of Physiology 127 (3): 553–563. PMC 1365740. PMID 14368548. //www.pubmedcentral.nih.gov/articlerender.fcgi?tool=pmcentrez&artid=1365740.
^ Sircar, Sabyasach (2008). Principles of Medical Physiology. India: vistasta Publishing. ISBN 978-1-58890-572-7.
^ Fung, Yuan-cheng; Zweifach, B.W. (1971). "Microcirculation: Mechanics of Blood Flow in Capillaries". Annual Review of Fluid Mechanics 3: 189–210. doi:10.1146/annurev.fl.03.010171.001201.
^ Textbook of Medical Physiology, 7th Ed., Guyton & Hall, Elsevier-Saunders, ISBN 0-7216-0240-1, page 220.
^ "Isolated systolic hypertension: A health concern? - MayoClinic.com". http://www.mayoclinic.com/health/hypertension/AN01113. Retrieved 2011-12-07.
^ "Clinical Management of Isolated Systolic Hypertension". http://web.archive.org/web/20081218160455/http://www.clevelandclinicmeded.com/medicalpubs/diseasemanagement/nephrology/isosystolic/isosystolic.htm. Retrieved 2011-12-07.
^ Gottdiener JS, Panza JA, St John Sutton M, Bannon P, Kushner H, Weissman NJ (July 2002). "Testing the test: The reliability of echocardiography in the sequential assessment of valvular regurgitation". American Heart Journal 144 (1): 115–21. doi:10.1067/mhj.2002.123139. PMID 12094197. http://www.medscape.com/viewarticle/439534. Retrieved 2010-06-30.
^ Thomas, A.J., Perry, R., Barber, R., Kalaria, R.N., O’Brien, J.T. (2002) "Pathologies and pathological mechanisms for white matter hyperintensities in depression," ' 'Ann N Y Acad Sci., 977:333–339.
^ ^a ^b ^c Brickman AM, Reitz C, Luchsinger JA, Manly JJ, Schupf N, Muraskin J, DeCarli C, Brown TR, Mayeux R. Long-term blood pressure fluctuation and cerebrovascular disease in an elderly cohort. Arch Neurol. 2010 May;67(5):564-9 PMID 20457955 PMCID PMC2917204 Free Full Text: http://www.ncbi.nlm.nih.gov/pmc/articles/PMC2917204/?tool=pubmed NIHMSID NIHMS216309 http://archneur.ama-assn.org/cgi/content/short/67/5/564
^ Table 30-1 in: Trudie A Goers; Washington University School of Medicine Department of Surgery; Klingensmith, Mary E; Li Ern Chen; Sean C Glasgow (2008). The Washington manual of surgery. Philadelphia: Wolters Kluwer Health/Lippincott Williams & Wilkins. ISBN 0-7817-7447-0.
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^ Tkachenko BI, Evlakhov VI, Poyasov IZ (2002). "Independence of changes in right atrial pressure and central venous pressure". Bull. Exp. Biol. Med. 134 (4): 318–20. doi:10.1023/A:1021931508946. PMID 12533747.
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^ 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 1-56053-420-6, ISBN 978-1-56053-420-4

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

Dimensions	Fractional shortening = (End-diastolic dimension – End-systolic dimension) / End-diastolic dimension

Interaction diagrams	Cardiac cycle · Wiggers diagram · Pressure volume diagram

Tropism	Chronotropic (Heart rate) · Dromotropic (Conduction velocity) · Inotropic (Contractility) · Bathmotropic (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

Blood flow	Compliance · Vascular resistance (Total peripheral resistance) · Pulse · Perfusion

Blood pressure	Pulse pressure (Systolic - Diastolic) · Mean arterial pressure Jugular venous pressure Portal venous pressure

Regulation of BP	Baroreflex · Kinin–kallikrein system · Renin–angiotensin system · Vasoconstrictors/Vasodilators · Autoregulation (Myogenic mechanism, Tubuloglomerular feedback, Cerebral autoregulation) · Paraganglia (Aortic body, Carotid body, Glomus cell)

M: HRT

anat/phys/devp

noco/cong/tumr, sysi/epon, injr

proc, drug (C1A/1B/1C/1D), blte

M: VAS

anat(a:h/u/t/a/l,v:h/u/t/a/l)/phys/devp/cell/prot

noco/syva/cong/lyvd/tumr, sysi/epon, injr

proc, drug(C2s+n/3/4/5/7/8/9)

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