696
UNIT 4
Maintenance of the Body
19
Muscular Arteries
Distally the elastic arteries give way to the
muscular arteries
,
which deliver blood to specific body organs (and so are some-
times called
distributing arteries
). Muscular arteries account for
most of the named arteries studied in the anatomy laboratory.
Teir internal diameter ranges from that of a little finger to that
of a pencil lead.
Proportionately, muscular arteries have the thickest tunica
media of all vessels. Teir tunica media contains relatively more
smooth muscle and less elastic tissue than do elastic arteries
(±able 19.1). For this reason, they are more active in vasocon-
striction and less distensible (capable of stretching). In muscu-
lar arteries, however, there
is
an
elastic membrane
on each face
of the tunica media.
Arterioles
Te smallest of the arteries,
arterioles
have a lumen diameter
ranging from 0.3 mm down to 10 μm. Larger arterioles have
all three tunics, but their tunica media is chiefly smooth mus-
cle with a few scattered elastic fibers. Smaller arterioles, which
lead into the capillary beds, are little more than a single layer
of smooth muscle cells spiraling around the endothelial lining.
As we will describe shortly, minute-to-minute blood flow
into the capillary beds is determined by arteriolar diameter,
which varies in response to changing neural, hormonal, and
local chemical influences. When arterioles constrict, the tissues
served are largely bypassed. When arterioles dilate, blood flow
into the local capillaries increases dramatically.
Capillaries
Describe the structure and function of a capillary bed.
Te microscopic
capillaries
are the smallest blood vessels. Teir
exceedingly thin walls consist of just a thin tunica intima (see
Figure 19.1b). In some cases, one endothelial cell forms the en-
tire circumference of the capillary wall. At strategic locations
along the outer surface of some capillaries are spider-shaped
pericytes
, smooth muscle–like cells that stabilize the capillary
wall and help control capillary permeability
(Figure 19.3a).
Average capillary length is 1 mm and average lumen diam-
eter is 8–10 μm, just large enough for red blood cells to slip
through in single file. Most tissues have a rich capillary sup-
ply, but there are exceptions. ±endons and ligaments are poorly
vascularized. Cartilage and epithelia lack capillaries, but receive
nutrients from blood vessels in nearby connective tissues, and
the avascular cornea and lens of the eye receive nutrients from
the aqueous humor.
If we compare arteries and arterioles to expressways and
roads, capillaries are the back alleys and driveways that pro-
vide direct access to nearly every cell in the body. Given their
location and thin walls, capillaries are ideally suited for their
role—exchange of materials (gases, nutrients, hormones, and so
on) between the blood and the interstitial fluid (Figure 19.2 and
±able 19.1). We describe these exchanges later in this chapter.
Here, we focus on capillary structure.
Table 19.1
Summary of Blood Vessel Anatomy
VESSEL TYPE/
ILLUSTRATION*
 
AVERAGE
LUMEN
DIAMETER (D)
AND WALL
THICKNESS (T)
 
RELATIVE TISSUE
MAKEUP
Elastic artery
D: 1.5 cm
T: 1.0 mm
 
Muscular artery
D: 6.0 mm
T: 1.0 mm
 
Arteriole
D: 37.0 μm
T: 6.0 μm
 
Capillary
D: 9.0 μm
T: 0.5 μm
 
Venule
D: 20.0 μm
T: 1.0 μm
 
Vein
D: 5.0 mm
T: 0.5 mm
*Size relationships are not proportional. Smaller vessels are drawn
relatively larger so detail can be seen. See column 2 for actual
dimensions.
E
ndothelium
E
lastic
T
issues
S
mooth Muscles
Fibrous (Collagenous)
T
issues
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