Human Physiology, 14th edition (2016)

480 Chapter 14

decrease in cuff pressure. When the cuff pressure is equal to

the systolic pressure, the first Korotkoff sound is heard as

blood passes in a turbulent flow through the constricted open-

ing of the artery.

Korotkoff sounds will continue to be heard at every systole

as long as the cuff pressure remains greater than the diastolic

pressure. When the cuff pressure becomes equal to or less than

the diastolic pressure, the sounds disappear because the artery

remains open and laminar flow resumes ( fig. 14.31 ). The last

Korotkoff sound thus occurs when the cuff pressure is equal

to the diastolic pressure.

Different phases in the measurement of blood pressure are

identified on the basis of the quality of the Korotkoff sounds

( fig. 14.32 ). In some people, the Korotkoff sounds do not dis-

appear even when the cuff pressure is reduced to zero (zero

pressure means that it is equal to atmospheric pressure). In

these cases—and often routinely—the onset of muffling of the

sounds (phase 4 in fig. 14.32 ) is used as an indication of dia-

stolic pressure rather than the onset of silence (phase 5).

The average arterial blood pressure in the systemic circula-

tion is 120/80 mmHg, whereas the average pulmonary arterial

blood pressure is only 22/8 mmHg. Because of the Frank-

Starling relationship, the cardiac output from the right ven-

tricle into the pulmonary circulation is matched to that of the

greater than the systolic pressure, the artery will be closed off
and silent during both diastole and systole. Turbulent flow, and
the sounds produced by the artery as a result of this flow, can
occur only when the cuff pressure is greater than the diastolic
pressure (to constrict the artery during diastole) but less than
the systolic pressure. The constriction can then partially open
at each systole and allow turbulent blood flow.
Let’s say that a person has a systolic pressure of 120 mmHg
and a diastolic pressure of 80 mmHg. When the cuff pressure
is between 80 and 120 mmHg, the artery will be closed dur-
ing diastole and open during systole. As the artery begins to
open with every systole, turbulent flow of blood through the
constriction will create sounds that are known as the sounds of
Korotkoff, as shown in figure  14.30. These are usually “tap-
ping” sounds because the artery becomes constricted, blood
flow stops, and silence is restored with every diastole. It should
be understood that the sounds of Korotkoff are not “lub-dub”
sounds produced by closing of the heart valves (those sounds
can be heard only on the chest, not on the brachial artery).
Initially, the cuff is usually inflated to produce a pressure
greater than the systolic pressure, so that the artery is pinched
off and silent. The pressure in the cuff is read from an attached
meter called a sphygmomanometer. A valve is then turned
to allow the release of air from the cuff, causing a gradual

Figure 14.30 The blood flow and Korotkoff sounds during a blood pressure measurement. When the cuff pressure
is above the systolic pressure, the artery is constricted. When the cuff pressure is below the diastolic pressure, the artery is open and
flow is laminar. When the cuff pressure is between the diastolic and systolic pressure, blood flow is turbulent and the Korotkoff sounds
are heard with each systole.

Last Korotkoff

sounds

Cuff pressure = 80

Diastolic pressure

= 80 mmHg

First Korotkoff

sounds

Cuff pressure = 120

Systolic pressure

= 120 mmHg

No sounds

Cuff pressure = 140

Sounds at

every systole

Cuff pressure = 100

Blood pressure = 120/80