Veins • Figure 11.15
Although veins have a structure similar to arteries, they have larger
diameters, thinner walls, and less smooth muscle and elastic tissue.
Structure of
Valves are inward folds of the tunica
intima that prevent the backflow of
blood. They are found in many veins,
especially those in the limbs.
Tunica Intima:
• Simple squamous endothelium
lines the lumen.
• Basement membrane anchors the
There is no internal elastic lamina in
veins. Also, the tunica intima is
thinner in veins compared to arteries.
The tunica media consists of smooth
muscle, which controls the diameter
of the lumen. Also, the tunica media
has no elastic lamina and is thinner
than in arteries.
The tunica externa is composed
of elastic fibers and collagen fibers
that make it tough and stretchy.
The tunica externa is the thickest
part of the vein.
The lumen is wider in veins
than in corresponding arteries.
Veins and Venules Are Thin-Walled
Blood Collectors
Once blood has passed through arterioles and capillaries,
it reaches the venules and veins. As you will see later in
this chapter, blood pressure drops tremendously over this
distance. So the venules and veins are low-pressure ves-
sels. This difference in pressure can be seen in the blood
leaving a cut vessel. Blood flows from a cut vein slowly and
evenly, but it gushes out of a cut artery in rapid spurts.
When a blood sample is needed, it is usually collected from
a vein because pressure is low in veins and because veins
are close to the skin surface.
Structurally, veins and venules are similar to their arte-
rial counterparts, with three layers, or tunics, but they have
thinner walls and larger lumens (Figure 11.15a). These
features make veins very expandable. Many veins have
valves in them to prevent blood from flowing backward. All
veins of the lower limbs have valves, which are more numer-
ous than in veins of the upper limbs. (Arteries do not have
valves.) In people with weak venous valves, gravity forces
blood backward through the valve. This increases venous
blood pressure, which pushes the vein's wall outward. After
repeated overloading, the walls lose their elasticity and be-
come stretched and flabby, a condition called
varicose veins.
Because veins are low-pressure vessels, blood tends
to pool in them, which is why veins are the largest blood
reservoir. To get adequate volumes of blood flowing
back to the heart, veins depend on the continuous flow
of blood through the cardiovascular system caused by
contractions of the heart. In addition, contractions of
the skeletal muscles and respiratory muscles help out
by directing the blood in the veins back to the heart.
Let's discuss these two pumps in more detail.
The skeletal muscle pump acts in the following way
to move blood back to the heart.
While you are standing at rest, as in Figure 11.15b,
both the venous valve closer to the heart and the one
farthest from the heart in this part of the leg are open,
326 CHAPTER 11
The Cardiovascular System: Heart, Blood Vessels, and Circulation
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