In brief, blood vessels have three functions.
Blood vessels form a closed system of tubes that carries
blood away from the heart (in arteries), transports
through the
tissues of the body (in arterioles,
capillaries, and venules) and then returns it to the
heart (in veins).
Exchange of substances between the blood and body
tissue cells occurs as blood flows through the capillaries.
Nutrients and oxygen diffuse from the blood through
interstitial fluid into tissue cells. Waste products,
including carbon dioxide, diffuses from tissue cells
through interstitial fluid into the blood.
Let's start by looking at arteries.
Arteries and Arterioles Are
Thick-Walled Vessels
The walls of arteries and arterioles are thick and have
three layers of tissues, or tunics. These three layers sur-
round the lumen, or hollow area through which the blood
flows (Figure 11.11). Under control of the autonomic
nervous system and/or local chemicals, smooth muscles in
the arterial walls can contract and narrow the lumen (va-
soconstriction) or relax and enlarge the lumen (vasodi-
lation). Vasoconstriction and vasodilation are important
in maintaining blood pressure and controlling blood flow,
as we shall discuss later in the chapter.
The major systemic arteries are shown in Figure
11.12 on the next page. The aorta is the largest artery of
the body, with a diameter of 2-3 cm (about 1
in.). Its four
principal divisions are as follows.
ascending aorta
emerges from the left ventricle
posterior to the pulmonary trunk. The ascending aorta
gives off two coronary artery branches that supply the
myocardium of the heart.
arch o f the aorta
is formed when the ascending aorta
branches to the left. The arch of the aorta descends
and ends at the level of the intervertebral disc between
the fourth and fifth thoracic vertebrae.
thoracic aorta
is the section of the aorta between the
arch of the aorta and the diaphragm.
a b d o m in a l aorta
is the section of the aorta between
the diaphragm and the common iliac arteries, which
carry blood to the lower limbs.
S tru ctu re o f an a rte ry • Figure 11.11
Lumen—the hollow space
through which blood flows
Tunica intima:
• Simple squamous endothelium lines the lumen.
• Basement membrane anchors the endothelium.
• Inner elastic lamina allows the lining to stretch.
Tunica media:
• Smooth muscle
controls the diameter
of the lumen.
• Elastic tissue
allows the tunica
media to stretch.
Tunica externa is
composed of elastic
fibers and collagen
fibers that make it
tough and stretchy.
Transverse section through an artery
Internal elastic
Tunica externa
Lumen with
blood cells
Tunica media
Blood Vessels Are the Body's Plumbing 321
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