CHAPTER 5 • VASCULAR FUNCTION
107
Vascular Tone
Under
normal
physiologic
conditions,
changes in the diameter of precapillary resist-
ance vessels (small arteries and arterioles)
represent the
most important mechanism
for regulating systemic vascular resistance.
Resistance vessels are normally in a partially
constricted state that is referred to as the vas-
cular tone of the vessel. This tone is gener-
ated by smooth muscle contraction within
the wall of the blood vessel. From this par-
tially constricted state, a vessel can constrict
further and thereby increase resistance, or it
can dilate by smooth muscle relaxation and
thereby decrease resistance. Venous vessels
likewise possess a level of vascular tone.
Extrinsic and intrinsic mechanisms deter-
mine the degree of smooth muscle activation
(Fig. 5.12). Extrinsic mechanisms, such as
sympathetic nerves and circulating hormones,
originate
outside
of the
organ
or
tissue.
Intrinsic mechanisms originate from within
the blood vessel or the tissue surrounding
the vessel. Examples of intrinsic mechanisms
include endothelial—derived factors, smooth
muscle
myogenic
tone,
locally
produced
hormones, and tissue metabolites. Some of
these extrinsic and intrinsic factors promote
vasoconstriction
(e.g., sympathetic nerves,
angiotensin II, and endothelin—1), whereas
Extrinsic
Intrinsic
Arteriole
Tissue
others promote smooth muscle relaxation and
vascular dilation (e.g., endothelial—derived
nitric oxide and tissue metabolites such as
adenosine and hydrogen ion). Therefore, at
any given time, vasoconstrictor and vasodi-
lator influences are competing to determine
the vascular tone. The extrinsic and intrin-
sic mechanisms regulating vascular tone are
described in more detail in Chapters 6 and 7.
In general, the vasoconstrictor mechanisms
are important for maintaining systemic vascu-
lar resistance and arterial pressure, whereas
vasodilator mechanisms regulate blood flow
within organs. For example, if the body needs
to maintain arterial blood pressure when a
person stands up, vasoconstrictor mecha-
nisms (primarily sympathetic adrenergic) are
activated to constrict resistance vessels and
increase systemic vascular resistance. If an
organ requires more blood flow and oxygen
delivery (e.g., exercising muscle), vasodila-
tor mechanisms will predominate and over-
ride
vasoconstrictor
influences.
Therefore,
the competition between vasoconstrictor and
vasodilator influences can be thought of as
competition between maintenance of arterial
blood pressure and organ perfusion.
VENOUS BLOOD PRESSURE
Venous
pressure
is
a
general
term
that
represents the average blood pressure within
the venous compartment. A more specific term,
central venous pressure (CVP), describes the
blood pressure in the thoracic vena cava near
the right atrium. This pressure is important
because it determines the filling pressure of
the right ventricle, and thereby determines
ventricular stroke volume through the Frank—
Starling mechanism as discussed in Chapter 4.
Constriction
Dilation
■ FIGURE 5.12 Vascular tone. The state of vessel
tone is determined by the balance between constric-
tor and dilator influences. Extrinsic influences origi-
nate outside of the tissue, whereas intrinsic influences
originate from the vessel or surrounding tissue.
Venous Blood Volume
and Compliance
Several factors influence CVP: cardiac output,
respiratory activity, contraction of skeletal mus-
cles (particularly leg and abdominal muscles),
sympathetic vasoconstrictor tone, and gravi-
tational forces. All of these factors ultimately
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