698
UNIT 4
Maintenance of the Body
19
sphincter
, surrounds the root of each true capillary at the metar-
teriole and acts as a valve to regulate blood flow into the capillary.
Blood flowing through a terminal arteriole may go either
through the true capillaries or through the shunt. When the
precapillary sphincters are relaxed (open), as in Figure 19.4a,
blood flows through the true capillaries and takes part in ex-
changes with tissue cells. When the sphincters are contracted
(closed), as in Figure 19.4b, blood flows through the shunts and
bypasses the tissue cells.
Local chemical conditions and arteriolar vasomotor nerve
fibers regulate the amount of blood entering a capillary bed. A
bed may be flooded with blood or almost completely bypassed,
depending on conditions in the body or in that specific organ.
For example, suppose you have just eaten and are sitting relaxed,
listening to your favorite musical group. Food is being digested,
and blood is circulating freely through the true capillaries of
your gastrointestinal organs to receive the breakdown products
of digestion. Between meals, however, most of these same capil-
lary pathways are closed.
To take another example, when you exercise vigorously, blood
is rerouted from your digestive organs (food or no food) to the
capillary beds of your skeletal muscles where it is more immedi-
ately needed. ±is rerouting helps explain why vigorous exercise
right a²er a meal can cause indigestion or abdominal cramps.
Venous System
Describe the structure and function of veins, and explain
how veins differ from arteries.
Veins carry blood from the capillary beds toward the heart.
Along the route, the diameter of successive venous vessels in-
creases, and their walls gradually thicken as they progress from
venules to larger and larger veins.
Venules
Capillaries unite to form
venules
, which range from 8 to 100 μm
in diameter. ±e smallest venules, the
postcapillary venules
, con-
sist entirely of endothelium around which pericytes congregate.
Postcapillary venules are extremely porous (more like capillar-
ies than veins in this way), and fluid and white blood cells move
easily from the bloodstream through their walls. Indeed, a well-
recognized sign of inflammation is adhesion of white blood
cells to the postcapillary venule endothelium, followed by their
migration through the wall into the inflamed tissue.
Larger venules have one or two layers of smooth muscle cells
(a scanty tunica media) and a thin tunica externa as well.
Veins
Venules join to form
veins
. Veins usually have three distinct tunics,
but their walls are always thinner and their lumens larger than
those of corresponding arteries (see Figure 19.1 and Table 19.1).
Consequently, in histological preparations, veins are usually col-
lapsed and their lumens appear slitlike.
Capillary Beds
Capillaries do not function independently. Instead they form in-
terweaving networks called
capillary beds
. ±e flow of blood from
an arteriole to a venule—that is, through a capillary bed—is called
the
microcirculation
. In most body regions, a capillary bed
consists of two types of vessels: (1) a
vascular shunt
(metarteriole–
thoroughfare channel), a short vessel that directly connects the
arteriole and venule at opposite ends of the bed, and (2)
true capil-
laries
, the actual
exchange vessels
(Figure 19.4)
.
±e
terminal arteriole
feeding the bed leads into a
metar-
teriole
(a vessel structurally intermediate between an arteriole
and a capillary), which is continuous with the
thoroughfare
channel
(intermediate between a capillary and a venule). ±e
thoroughfare channel, in turn, joins the
postcapillary venule
that drains the bed.
±e
true capillaries
number 10 to 100 per capillary bed,
depending on the organ or tissues served. ±ey usually branch
off the metarteriole (proximal end of the shunt) and return to
the thoroughfare channel (the distal end), but occasionally they
spring from the terminal arteriole and empty directly into the
venule. A cuff of smooth muscle fibers, called a
precapillary
(a) Sphincters open
—blood flows through true capillaries.
(b) Sphincters closed
—blood flows through metarteriole – thoroughfare
channel and bypasses true capillaries.
Precapillary sphincters
Metarteriole
Vascular shunt
Terminal arteriole
Postcapillary venule
Terminal arteriole
Postcapillary venule
Thoroughfare
channel
True
capillaries
Figure 19.4
Anatomy of a capillary bed.
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