Maintenance of the Body
Distally the elastic arteries give way to the
which deliver blood to speciﬁc body organs (and so are some-
). 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
on each face
of the tunica media.
Te smallest of the arteries,
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.
Describe the structure and function of a capillary bed.
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
, smooth muscle–like cells that stabilize the capillary
wall and help control capillary permeability
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.
Summary of Blood Vessel Anatomy
D: 1.5 cm
T: 1.0 mm
D: 6.0 mm
T: 1.0 mm
D: 37.0 μm
T: 6.0 μm
D: 9.0 μm
T: 0.5 μm
D: 20.0 μm
T: 1.0 μm
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