The Cardiovascular System: Blood Vessels
, such as the
of the heart and the
dural venous sinuses
of the brain, are highly specialized, ﬂattened
veins with extremely thin walls composed only of endothelium.
Tey are supported by the tissues that surround them, rather
than by any additional tunics. Te dural venous sinuses, which
receive cerebrospinal ﬂuid and blood draining from the brain,
are reinforced by the tough dura mater that covers the brain
Blood vessels form special interconnections called
sēz; “coming together”). Most or-
gans receive blood from more than one arterial branch, and
arteries supplying the same territory oFen merge, forming
. Tese anastomoses provide alternate path-
, for blood to reach a given body
region. If one branch is cut or blocked by a clot, the collateral
channel can oFen provide suﬃcient blood to the area.
Arterial anastomoses occur around joints, where active
movement may hinder blood ﬂow through one channel. Tey
are also common in abdominal organs, the heart, and the brain
(for example, the
cerebral arterial circle
in ±igure 19.22d on
p. 727). Arteries that supply the retina, kidneys, and spleen
either do not anastomose or have a poorly developed collateral
circulation. If their blood ﬂow is interrupted, cells supplied by
such vessels die.
Te metarteriole–thoroughfare channel shunts of capillary
beds that connect arterioles and venules are examples of
. Veins interconnect much more freely
than arteries, and
are common. (You may
be able to see venous anastomoses through the skin on the dor-
sum of your hand.) Because venous anastomoses are abundant,
an occluded vein rarely blocks blood ﬂow or leads to tissue
Tere is relatively little smooth muscle or elastin in the tunica
media, which is poorly developed and tends to be thin even in
the largest veins. Te tunica externa is the heaviest wall layer.
Consisting of thick longitudinal bundles of collagen ﬁbers and
elastic networks, it is oFen several times thicker than the tu-
nica media. In the largest veins—the venae cavae, which return
blood directly to the heart—longitudinal bands of smooth mus-
cle make the tunica externa even thicker.
With their large lumens and thin walls, veins can accommo-
date a fairly large blood volume. Veins are called
because they can hold up to 65%
of the body’s blood supply at any time
. Even so,
they are normally only partially ﬁlled.
Te walls of veins can be much thinner than arterial walls
without danger of bursting because the blood pressure in veins
is low. However, the low-pressure condition demands several
structural adaptations to ensure that veins return blood to the
heart at the same rate it was pumped into the circulation. One
such adaptation is their large-diameter lumens, which oﬀer rela-
tively little resistance to blood ﬂow.
Another adaptation is valves that prevent blood from ﬂowing
, formed from folds of the tunica in-
tima, resemble the semilunar valves of the heart in both struc-
ture and function (see ±igure 19.1). Venous valves are most
abundant in the veins of the limbs, where gravity opposes the
upward ﬂow of blood. Tey are usually absent in veins of the
thoracic and abdominal body cavities.
Te eﬀectiveness of venous valves is demonstrated by this
simple experiment: Hang one hand by your side until the blood
vessels on its dorsal aspect distend with blood. Next place two
ﬁngertips against one of the distended veins, and pressing
ﬁrmly, move the superior ﬁnger proximally along the vein and
then release that ﬁnger. Te vein will remain collapsed (ﬂat)
despite the pull of gravity. ±inally, remove your distal ﬁngertip
and watch the vein reﬁll with blood.
are veins that are tortuous and dilated because
of incompetent (leaky) valves. More than 15% of adults suﬀer
from varicose veins, usually in the lower limbs.
Several factors contribute, including heredity and conditions
that hinder venous return, such as prolonged standing in one
position, obesity, or pregnancy. Both the “potbelly” of an over-
weight person and the enlarged uterus of a pregnant woman ex-
ert downward pressure on vessels of the groin, restricting return
of blood to the heart. Consequently, blood pools in the lower
limbs, and with time, the valves weaken and the venous walls
stretch and become ﬂoppy. Superﬁcial veins, which receive lit-
tle support from surrounding tissues, are especially susceptible.
Elevated venous pressure can also cause varicose veins. ±or
example, straining to deliver a baby or have a bowel movement
raises intra-abdominal pressure, preventing blood from drain-
ing from anal veins. Te resulting varicosities in the anal veins
and arterioles 15%
and venules 60%
Relative proportion of blood volume throughout
the cardiovascular system.
The systemic veins are called
capacitance vessels because they are distensible and contain a large
proportion of the blood volume. Pulmonary blood vessels supply the
lungs; systemic blood vessels supply the rest of the body.