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 pulsepulse,
alternate expansion and contraction of artery walls as heart action varies blood volume within the arteries. Artery walls are elastic. Hence they become distended by increased blood volume during systole, or contraction of the heart.
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). 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 systemcirculatory system,
group of organs that transport blood and the substances it carries to and from all parts of the body. The circulatory system can be considered as composed of two parts: the systemic circulation, which serves the body as a whole except for the lungs, and the
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). 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 hypertensionhypertension
or high blood pressure,
elevated blood pressure resulting from an increase in the amount of blood pumped by the heart or from increased resistance to the flow of blood through the small arterial blood vessels (arterioles).
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, 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 diureticsdiuretic
, drug used to increase urine formation and output. Diuretics are prescribed for the treatment of edema (the accumulation of excess fluids in the tissues of the body), which is often the result of underlying disease of the kidneys, liver, lungs, or heart (e.g.
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, beta-blockersbeta-blocker
or beta-adrenergic blocking agent
, drug that reduces the symptoms connected with hypertension, cardiac arrhythmias, angina pectoris, migraine headaches, and other disorders related to the sympathetic nervous system.
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, calcium-channel blockerscalcium-channel blocker,
any of a class of drugs used in treating hypertension, angina pectoris, and certain arrhythmias. They prevent the calcium ions needed for muscle contraction from entering the cells of smooth and cardiac muscle.
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, or ACE inhibitorsACE inhibitor
or angiotensin-converting enzyme inhibitor
, drug used to reduce elevated blood pressure (see hypertension), to treat congestive heart failure, and to alleviate strain on hearts damaged as a result of a heart attack (see infarction).
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. Low blood pressure (hypotension) has not been studied as extensively as high blood pressure. If not caused by disease or injury, it is generally considered to be a benign or even advantageous condition; however, studies have linked hypotension with feelings of tiredness or faintness and minor psychiatric conditions in some people.


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

Blood Pressure


the hydrodynamic pressure of blood in the blood vessels; it arises as a result of the work of the heart, which pumps the blood into the vascular system, and the resistance of the vessels.

The level of the blood pressure in the arteries, veins, and capillaries varies and is a measure of the body’s functional condition. Arterial pressure undergoes rhythmic variations, increasing during contraction of the heart (systole) and decreasing during the period of its relaxation (diastole). Each new portion of blood discharged by the heart distends the elastic walls of the aorta and central arteries. During the cardiac pause the distended walls of the arteries collapse and push the blood through the arterioles, capillaries, and veins. In man and in many other mammals, the maximum (systolic) pressure is about 120 millimeters of mercury (mm Hg), and the minimum (diastolic) pressure is about 70 mm Hg. The difference between these two values (the amplitude- of pressure change with each heart contraction) is called the pulse pressure. Under physical or emotional stress a short-term increase in arterial pressure occurs, which represents a physiological adaptive reaction. Arterial pressure may be measured by the direct method—that is, insertion into the vessel of a cannula that is connected by a tube with a manometer (the Englishman S. Hales first measured arterial pressure in this way in 1733)—or by an indirect method—that is, by a sphygmomanometer. In man, the arterial blood pressure is usually measured on the arm above the elbow; the value thus determined corresponds to the blood pressure only in that artery and not the entire body. However, the figures obtained make it possible to form an opinion about the blood pressure of the person being examined.

Figure 1. Blood pressure in various parts of the circulatory system. The broken line denotes the mean of the systolic and diastolic pressures. Venous pressure near the heart drops below zero (below atmospheric pressure).

When the blood passes through the capillaries, the blood pressure drops from approximately 40 mm Hg at the arteriole endings to 10 mm Hg at the place where the capillaries shade into the venules. This decrease in blood pressure is caused by the friction of the blood against the walls of the small vessels; it maintains the flow of blood in them. The level of capillary pressure depends on the tonus of the arterioles and on venous pressure and to a considerable degree determines the conditions of the exchange of substances between the blood and the tissues. In the veins a further drop in blood pressure occurs; at the ostia venae cavae blood pressure drops below atmospheric pressure, which is due to the suction of the negative pressure in the thorax (see Figure 1). Venous pressure is measured by the direct method —insertion into the vein of a needle connected to a manometer. The level and variations in blood pressure act on the baroreceptors of the vascular system; thus, neural and humoral reactions occur, which are directed toward maintaining the blood pressure at the level characteristic for the given organism and toward self-regulation of blood circulation.


In man, the average arterial blood pressure is 115–125 mm Hg in systole (maximum) and 70–80 mm Hg in diastole (minimum). The average levels change with age.

Table 1. Arterial pressure (mm Hg)
Age (in years)Systolic pressureDiastolic pressure
16-20. . . . . . . . . .100-12070-80
20-40. . . . . . . . . .120-13070-80
40-60. . . . . . . . . .up to 140up to 90
over 60. . . . . . . . . .up to 150up to 90

Blood pressure that is persistently higher than the indicated figures, hypertension, may be a symptom of a number of diseases (hypertensive disease, nephritis). Low blood pressure, hypotension, may be physiological, may accompany a number of pathological states, or may be a separate disease.


Val’dman, V. A. Venoznoe davlenie i venoznyi tonus, 2nd ed. Leningrad, 1947.
Kositskii, G. I. Zvukovoi metod issledovaniia arterial’nogo davleniia. Moscow, 1959.
Savitskii, N. N. Biofizicheskie osnovy krovoobrashcheniia i klinicheskie metody izucheniia gemodinamiki, 2nd ed. Leningrad, 1963.

blood pressure

[′bləd ‚presh·ər]
Pressure exerted by blood on the walls of the blood vessels.

blood pressure

the pressure exerted by the blood on the inner walls of the arteries, being relative to the elasticity and diameter of the vessels and the force of the heartbeat
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