Human Physiology, 14th edition (2016)

Cardiac Output, Blood Flow, and Blood Pressure 481

left ventricle into the systemic circulation. Since the cardiac

outputs are the same, the lower pulmonary blood pressure must

be caused by a lower peripheral resistance in the pulmonary

circulation. Because the right ventricle pumps blood against

a lower resistance, it has a lighter workload and its walls are

thinner than those of the left ventricle.

Pulse Pressure and Mean

Arterial Pressure

When someone “takes a pulse,” he or she palpates an artery

(for example, the radial artery) and feels the expansion of the

artery occur in response to the beating of the heart; the pulse

rate is thus a measure of the cardiac rate. The expansion of the

artery with each pulse occurs as a result of the rise in blood

pressure within the artery as the artery receives the volume of

blood ejected from the left ventricle.

Because the pulse is produced by the rise in pressure from

diastolic to systolic levels, the difference between these two

pressures is known as the pulse pressure. A person with a

blood pressure of 120/80 (systolic/diastolic) would therefore

have a pulse pressure of 40 mmHg.

Pulse pressure^5 systolic pressure^2 diastolic pressure

At diastole in this example, the aortic pressure equals

80 mmHg. When the left ventricle contracts, the intraventricu-

lar pressure rises above 80 mmHg and ejection begins. As a

result, the amount of blood in the aorta increases by the amount

ejected from the left ventricle (the stroke volume). Due to the

increase in volume, there is an increase in blood pressure. The

Figure 14.31 The indirect, or auscultatory, method of blood pressure measurement. The first Korotkoff sound is

heard when the cuff pressure is equal to the systolic blood pressure, and the last sound is heard when the cuff pressure is equal to the

diastolic pressure. The dashed line indicates the cuff pressure.

Cuff pressure No flow

120

mmHg

80

mmHg

Sound

Turbulent flow Laminar flow

Systole

Blood pressure

Diastole

Artery

silent

Artery

silent

First

sound

Last

sound

Figure 14.32 The five phases of blood pressure

measurement. Not all phases are heard in all people. The cuff

pressure is indicated by the falling dashed black line.

130

120

110

100

90

80

70

60

Cuff pressure (mmHg)

1

Snapping

sounds

2

Murmurs

(^3) Thumping (^4) Muffling (^5) Silence
Silence
14 mmHg
Blood flows during
systole only
(turbulent flow)
systole and diastoleBlood flows during
(laminar flow)
No blood
flows
5 mmHg
5 mmHg
20 mmHg
Relative intensity of sounds