Cardiovascular Physiology
389
Because the MAP is identical throughout the body, differences
in fl ows between organs depend entirely on the relative resis-
tances of their respective arterioles. Arterioles contain smooth
muscle, which can either relax and cause the vessel radius to
increase
(vasodilation)
or contract and decrease the vessel
radius
(vasoconstriction).
Thus the pattern of blood-fl ow dis-
tribution depends upon the degree of arteriolar smooth muscle
contraction within each organ and tissue. Look back at Figure
12–3, which illustrates the distribution of blood fl ows at rest;
these are due to differing resistances in the various locations.
Such distribution can change greatly—as during exercise, for
example—by changing the various resistances.
How can resistance be changed? Arteriolar smooth muscle
possesses a large degree of spontaneous activity (that is, contrac-
tion independent of any neural, hormonal, or paracrine input).
This spontaneous contractile activity is called
intrinsic tone
(also called basal tone). It sets a baseline level of contraction
that can be increased or decreased by external signals, such as
neurotransmitters. These signals act by inducing changes in the
muscle cells’ cytosolic calcium concentration (see Chapter 9 for
a description of excitation-contraction coupling in smooth mus-
cle). An increase in contractile force above the vessel’s intrinsic
tone causes vasoconstriction, whereas a decrease in contractile
force causes vasodilation. The mechanisms controlling vasocon-
striction and vasodilation in arterioles fall into two general cat-
egories: (1) local controls, and (2) extrinsic (or refl ex) controls.
Local Controls
The term
local controls
denotes mechanisms independent
of nerves or hormones by which organs and tissues alter their
own arteriolar resistances, thereby self-regulating their blood
fl ows. This includes changes caused by autocrine/paracrine
agents. This self-regulation includes the phenomena of active
hyperemia, fl ow autoregulation, reactive hyperemia, and local
response to injury.
Active Hyperemia
Most organs and tissues manifest an increased blood flow
(hyperemia)
when their metabolic activity is increased
(
Figure 12–34a
); this is termed
active hyperemia.
For exam-
ple, the blood fl
ow to exercising ske
letal muscle increases
in direct proportion to the increased activity of the muscle.
Active hyperemia is the direct result of arteriolar dilation in
the more active organ or tissue.
The factors that cause arteriolar smooth muscle to relax
in active hyperemia are local chemical changes in the extracel-
lular fl uid surrounding the arterioles. These result from the
increased metabolic activity in the cells near the arterioles.
The relative contributions of the different factors implicated
vary, depending upon the organs involved and on the duration
of the increased activity. Thus, we will list, but not attempt to
quantify, the local chemical changes that occur in the extra-
cellular fl
uid.
Perhaps the most obvious change that occurs when tis-
sues become more active is a decrease in the local concentra-
tion of oxygen, which is used in the production of ATP by
oxidative phosphorylation. A number of other chemical fac-
tors
increase
when metabolism exceeds blood fl
ow, including:
1.
carbon dioxide, an end product of oxidative metabolism;
2.
hydrogen ions (decrease in pH), for example, from
lactic acid;
3.
adenosine, a breakdown product of ATP;
4.
potassium ions, accumulated from repeated action
potential repolarization;
5.
eicosanoids, breakdown products of membrane
phospholipids;
6.
osmolarity, from the breakdown of high-molecular-
weight substances;
7.
bradykinin,
a peptide generated locally from a
circulating protein called
kininogen
by the action of
an enzyme,
kallikrein,
secreted by active gland cells;
and
8.
nitric oxide,
a gas released by endothelial cells that
acts on the immediately adjacent vascular smooth
muscle. Its action will be discussed further in an
upcoming section.
Local changes in all these chemical factors have been
shown to cause arteriolar dilation under controlled experi-
mental conditions, and they all probably contribute to the
Active hyperemia
Flow autoregulation
(a)
(b)
Restoration
of blood
flow toward
normal
in organ
Arteriolar
dilation
in organ
O
2
,
metabolites,
vessel-wall
stretch
in organ
Blood flow
to organ
Blood flow
to organ
Arteriolar
dilation
in organ
O
2
,
metabolites
in organ
interstitial fluid
Metabolic
activity
of organ
Arterial
pressure
in organ
Begin
Begin
Figure 12–34
Local control of organ blood
fl ow in response to (a) increases
in metabolic activity, and (b)
decreases in blood pressure.
Decreases in metabolic activity or
increases in blood pressure would
produce changes opposite those
shown here.
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