Human Physiology, 14th edition (2016)

476 Chapter 14

sympathoadrenal system. Increased activity of the sympa-

thoadrenal system can raise blood pressure by stimulating

vasoconstriction of arterioles (thereby raising total peripheral

resistance) and by promoting an increased cardiac output.

Sympathetic stimulation can also affect blood volume indi-

rectly, by stimulating constriction of renal blood vessels and

thus reducing urine output.

Blood pressure is measured in units of millimeters of

mercury (mmHg). When this measurement is taken, the blood

pushes on one surface of a U-shaped column of mercury while

the atmosphere pushes on the other surface (see chapter 16,

fig. 16.18). If the blood pressure were equal to the atmospheric

pressure, the measurement would be 0 mmHg. A mean arte-

rial pressure of 100 mmHg indicates that the blood pressure is

100 mmHg higher than the atmospheric pressure. Instruments

used to measure blood pressure, called sphygmomanometers,

contain mercury or are spring-loaded devices that are cali-

brated against mercurial instruments.

capillary wall.) The blood pressure upstream of the arterioles—
in the medium and large arteries—is correspondingly increased
( fig. 14.24 ).
The total cross-sectional area of capillaries is greater (due
to their large number) than the cross-sectional areas of the
arteries and arterioles ( fig.  14.25 ), further reducing the capil-
lary blood pressure and flow. Thus, although each capillary is
much narrower than each arteriole, the capillary beds served
by arterioles do not provide as great a resistance to blood flow
as do the arterioles.
Variations in the diameter of arterioles as a result of vaso-
constriction and vasodilation thus affect blood flow through
capillaries and, simultaneously, the arterial blood pressure
“upstream” from the capillaries. In this way, an increase in
total peripheral resistance due to vasoconstriction of arteri-
oles can raise arterial blood pressure. Blood pressure can also
be raised by an increase in the cardiac output. This may be
due to elevations in cardiac rate or in stroke volume, which
in turn are affected by other factors. The most important vari-
ables affecting blood pressure are thus the cardiac rate, stroke
volume (determined primarily by the blood volume ), and
total peripheral resistance. An increase in any of these, if not
compensated for by a decrease in another variable, will result
in an increased blood pressure.

Arterial Cardiac Total

blood ~ output 3 peripheral

pressure resistance

Cardiac Stroke

rate volume Vasocontriction

Blood pressure can be regulated by the kidneys, which
control blood volume and thus stroke volume, and by the

Figure 14.24 The effect of vasoconstriction on
blood pressure. A constriction increases blood pressure
upstream (analogous to the arterial pressure) and decreases
pressure downstream (analogous to capillary and venous
pressure).

Increased

pressure

Decreased

pressure

Constriction

Figure 14.25 The relationship between blood

pressure and the cross-sectional area of vessels. As blood

passes from the aorta to the smaller arteries, arterioles, and

capillaries, the cross-sectional area increases as the pressure

decreases.

5,000

4,000

3,000

2,000

1,000

0

Area (c

2 )

0

20

40

60

80

100

120

Pressure (mmHg)

Capillaries

Arterioles Venules

Aorta Arteries Veins Vena cava