CHAPTER 7 • ORGAN BLOOD FLOW
167
Phasic
Contractions
0
20
40
60
80
Time (sec)
Sustained
Contraction
Time (sec)
■ FIGURE 7.14 Skeletal muscle active hyperemia following phasic and sustained (tetanic) contractions.
The to p panel shows that phasic contractions cause flow to decrease during contraction and increase dur-
ing relaxation, although the net effect is an increase in flow during contraction. When contractions cease,
a further increase in flow occurs because mechanical compression of the vasculature is removed. The
bottom panel shows that sustained, tetanic contractions generate high intramuscular forces that com -
press the vasculature and reduce flow. When contraction ceases, a large hyperemia follows.
prolonged hypoperfusion of muscle caused
by intense sympathetic activation eventually
leads to vasodilator mechanisms dominating
over the sympathetic vasoconstriction, lead-
ing to sympathetic escape and partial restora-
tion of blood flow.
Evidence exists, at least in nonprimate spe-
cies such as cats and dogs, for sympathetic
cholinergic innervation
of skeletal muscle
resistance vessels. The neurotransmitter for
these fibers is acetylcholine, which binds to
muscarinic receptors to produce vasodilation.
This branch of the autonomic nervous sys-
tem has little or no influence on blood flow
under resting conditions; however, activation
of these fibers in anticipation of exercise and
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