HUMAN BIOLOGY

CirCulation: the heart and blood vessels 131

hoW does a blood vessel’s structure suit its

particular function in the cardiovascular

system?

• Arteries have thick, elastic walls and are the main pipelines for
  oxygenated blood. Smooth muscle in arterioles allows them
  to dilate and constrict. Arterioles function as control points for
  blood flow and blood pressure.


• Capillaries are where substances diffuse between the blood and
  extracellular fluid in tissues.


• Blood moves back to the heart through venules and veins.
  Valves in veins prevent the backflow of blood due to gravity.

taKe-hoMe Message

baroreceptor reflex

Automatic response by

sensors (in the carotid

arteries) sensitive to

changes in arterial blood

pressure. Brain centers that

receive the signals order the

response.

capillary bed An

interlacing network of

capillaries.

carotid arteries Arteries

that service the head and

neck and have blood pres-

sure sensors associated

with them.

pulse Pressure surge that

may be felt in arteries near

the body surface when a

heart ventricle contracts.

vasoconstriction

Narrowing of a blood

vessel’s diameter.

vasodilation Enlargement

of a blood vessel’s

diameter.

the heart to beat faster and con-

tract more forcefully. Neural signals

also cause the smooth muscle of

arterioles to contract. The result is

vasoconstriction, a narrowing of

the vessel diameter. In some parts

of the body arterioles have receptors

for hormones that trigger vasocon-

striction or vasodilation, thus help-

ing to maintain blood pressure.

Carotid arteries in the neck, in

the arch of the aorta, and elsewhere

contain pressure sensors called

baroreceptors. In what is called a

baroreceptor reflex, the sensors

monitor changes in mean arte-

rial pressure (“mean” 5 the mid-

point) and send signals to centers

in the brain. The brain centers use

this infor mation to coordinate the

rate and strength of heartbeats with

changes in the diameter of arterioles

and veins. The baroreceptor reflex

thus helps keep blood pressure

within normal limits in the face of sudden changes, such as

when you jump up from a chair.

venules and veins return blood to the heart

Capillaries merge into venules, or “little veins,” which in
turn merge into large-diameter veins. Venules function a
little like capillaries, in that some solutes diffuse across
their relatively thin walls (Figure 7.13D).
Veins are large-diameter, low-resistance transport tubes
to the heart (Figure 7.13E). Their valves prevent backflow.
When blood starts moving backward due to gravity, it
pushes the valves closed. Unlike an arterial wall, a vein
wall can bulge quite a bit under pressure. Thus veins are
reservoirs for variable volumes of blood. Together, the
veins of an adult can hold up to 50 to 60 percent of the total
blood volume.
When a person’s blood must circulate faster (for
instance, during exercise), the smooth muscle in veins con-
tracts. The wall stiffens, the vein bulges less, and venous
pressure rises—so more blood flows to the heart. Venous
pressure also rises when contracting skeletal muscle—
especially in the legs and abdomen—bulges against
adjacent veins. This muscle activity helps return blood
through the venous system (Figure 7.15).
Obesity, pregnancy, and other factors can weaken
venous valves. The walls of a varicose vein have become
overstretched because, over time, weak valves have allowed
blood to pool there.

Many vessels have roles in homeostatic
mechanisms that help control blood
pressure

Some arteries, all arterioles, and even veins have roles in
homeostatic mechanisms that help maintain adequate
blood pressure over time. Centers in the brain monitor rest-
ing blood pressure. When the pressure rises abnormally,
they order slower, less forceful heart contractions. They
also order smooth muscle in arterioles to relax. The result
is vasodilation—an enlargement (dilation) of the ves-
sel diameter. On the other hand, when the centers detect
an abnormal decrease in blood pressure, they command

F i g u r e 7.15 Contracting skeletal muscles help keep blood in veins flowing toward the heart. A Skeletal

muscles nestle against veins. B Contracting muscles provide a “push” that keeps blood flowing forward.

C When skeletal muscles relax, valves in the vein shut—preventing backflow. (© Cengage Learning)

blood flow to heart

valve

open

valve

closed

valve

closed

valve

closed

ABvenous valve C

Biophoto Associates/Science Source

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