Biology of Disease

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Traditionally, blood pressure is measured using a manual

sphygmomanometer and stethoscope (Greek, sphygmos,

pulse), as illustrated in Figure 14.10 (A)–(D).There are also

electronic instruments that can be used (Figure 14.9 (B)). With a

sphygmomanometer, an inflatable cuff connected to a pressure

gauge, in this case a mercury manometer, is placed around the

upper arm over the brachial artery and inflated (Figure 14.10 (B)).

A stethoscope is placed on the skin above the brachial artery so

that blood sounds may be heard. When the pressure in the cuff is

greater than the systolic pressure the arteries are compressed and

no brachial pulse can be heard. The cuff pressure is then slowly

reduced and when it falls to that of the systolic pressure, clear

sounds (Korotkoff sounds) can be heard due to the turbulence

generated as the blood flows through the partially occluded artery

(Figure 14.10 (B)–(C)). As the pressure further reduces the sounds

become suddenly muffled and then disappear completely. The

pressure at this point is generally taken as the diastolic pressure

(Figure 14.10 (D)).

BOX 14.1 Measuring blood pressure

Figure 14.10 Panels (A)

to (D) schematically show

the use of a manual

sphygmomanometer and

stethoscope to measure blood

pressure.See text for details.

Pressure in

cuff below

120 mmHg

120

Sounds audible

using stethoscope

Pressure in

cuff above

120 mmHg

120

Cuff inflated

Pressure in

cuff below

70 mmHg

70

B)

C)

D)

Sounds cease

A)

Blood pressure

120/70 mm Hg

Brachial

Artery