584
UNIT FOUR
Lumen
of arteriole
Red blood cell
(b)
(a)
Red blood cell
FIGURE 15.35
±Vasodilation±and±
vasoconstriction. (
a
) Relaxation
of smooth muscle in the arteriole
wall produces dilation, whereas (
b
)
contraction of the smooth muscle
causes constriction (
a
and
b
1,500
×
).
Temporal a.
Carotid a.
Brachial a.
Radial a.
Dorsalis pedis a.
Posterior tibial a.
Popliteal a.
Femoral a.
Facial a.
FIGURE 15.33
Sites where an arterial pulse is most easily detected.
(
a.
stands for artery.)
Blood volume
increases
Heart rate
increases
Stroke volume
increases
Blood pressure increases
Blood viscosity
increases
Peripheral resistance
increases
FIGURE 15.34
Some of the factors that inF
uence arterial blood
pressure.
Arterial walls are elastic, so when the ventricles dis-
charge a surge of blood, arteries swell. Almost immediately,
the elastic tissues recoil, and the vessel walls press against
the blood inside. This action helps force the blood onward
against the peripheral resistance in arterioles and capillar-
ies. Recoiling of the arteries maintains blood pressure during
diastole. If there were no elasticity in the arterial walls, blood
pressure would fall to zero between ventricular contractions.
Elastic recoil also converts the intermittent fl
ow of blood,
characteristic of the arterial system, into a more continuous
movement through the capillaries.
Viscosity
The
viscosity
(vis-kos
ı˘-te) of a fl uid is a physical property
that derives from the ease with which its molecules flow
past one another. The greater the viscosity, the greater the
resistance to fl
ow.
resistance by constricting these vessels. Blood tends to back
up into the arteries supplying the arterioles, and the arte-
rial pressure rises. Dilation of the arterioles has the opposite
effect—peripheral resistance lessens, and the arterial blood
pressure drops in response
(f
g. 15.35)
.
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