Blood flow into capillary networks is regulated by
smooth muscle cells called precapillary sphincters,
found at the beginning of each network (see Fig. 13–1).
Precapillary sphincters are not regulated by the nerv-
ous system but rather constrict or dilate depending on
the needs of the tissues. Because there is not enough
blood in the body to fill all of the capillaries at once,
precapillary sphincters are usually slightly constricted.
In an active tissue that requires more oxygen, such as
exercising muscle, the precapillary sphincters dilate to
increase blood flow. These automatic responses ensure
that blood, the volume of which is constant, will circu-
late where it is needed most.
Some organs have another type of capillary called
sinusoids, which are larger and more permeable than
are other capillaries. The permeability of sinusoids
permits large substances such as proteins and blood
cells to enter or leave the blood. Sinusoids are found
in the red bone marrow and spleen, where blood cells
enter or leave the blood, and in organs such as the294 The Vascular System
BOX13–1 DISORDERS OF ARTERIES
The most common sites for aneurysm formation
are the cerebral arteries and the aorta, especi-
ally the abdominal aorta. Rupture of a cerebral
aneurysm is a possible cause of a cerebrovascular
accident (CVA). Rupture of an aortic aneurysm is
life-threatening and requires immediate corrective
surgery. The damaged portion of the artery is
removed and replaced with a graft. Such surgery
may also be performed when an aneurysm is found
before it ruptures.
Atherosclerosis—this condition has been men-
tioned previously; see Chapters 2 and 12.Arteriosclerosis—although commonly called
“hardening of the arteries,” arteriosclerosis really
means that the arteries lose their elasticity, and their
walls become weakened. Arteries carry blood under
high pressure, so deterioration of their walls is part
of the aging process.
Aneurysm—a weak portion of an arterial wall
may bulge out, forming a sac or bubble called an
aneurysm. Arteriosclerosis is a possible cause, but
some aneurysms are congenital. An aneurysm may
be present for many years without any symptoms
and may only be discovered during diagnostic pro-
cedures for some other purpose.BOX13–2 DISORDERS OF VEINS
to pool in the leg veins, stretching their walls. If the
veins become overly stretched, the valves within
them no longer close properly. These incompetent
valves no longer prevent backflow of blood, leading
to further pooling and even further stretching of
the walls of the veins. Varicose veins may cause dis-
comfort and cramping in the legs, or become even
more painful. Severe varicosities may be removed
surgically.
This condition may also develop during preg-
nancy, when the enlarged uterus presses against
the iliac veins and slows blood flow into the inferior
vena cava. Varicose veins of the anal canal are called
hemorrhoids, which may also be a result of preg-
nancy or of chronic constipation and straining to
defecate. Hemorrhoids that cause discomfort or
pain may also be removed surgically. Developments
in laser surgery have made this a simpler procedure
than it was in the past.Phlebitis—inflammation of a vein. This condition
is most common in the veins of the legs, because
they are subjected to great pressure as the blood is
returned to the heart against the force of gravity.
Often no specific cause can be determined, but
advancing age, obesity, and blood disorders may
be predisposing factors.
If a superficial vein is affected, the area may be
tender or painful, but blood flow is usually main-
tained because there are so many anastomoses
among these veins. Deep vein phlebitis is potentially
more serious, with the possibility of clot formation
(thrombophlebitis) and subsequent dislodging of
the clot to form an embolism.
Varicose veins—swollen and distended veins that
occur most often in the superficial veins of the legs.
This condition may develop in people who must sit
or stand in one place for long periods of time.
Without contraction of the leg muscles, blood tends