164
CARDIOVASCULAR PHYSIOLOGY CONCEPTS
Arterial Blood Partial Pressure
(mm Hg)
■ FIGURE 7.12 Effects of arterial partial pressure of oxygen and carbon dioxide on cerebral blood flow. An
arterial partial pressure of oxygen
(PO2)
of <50 mm Hg (norm al value is about 95 mm Hg) causes cerebral
vasodilation and increased flow. A reduction in arterial partial pressure of carbon dioxide
(PCO2)
below
its normal value of 40 mm Hg decreases flow, whereas PCO2 values >40 mm Hg increase flow. Therefore,
cerebral blood flow is more sensitive to changes from normal arterial PCO2 values than from normal arte-
rial PO2 values.
flow. Maximal sympathetic activation increases
cerebral vascular resistance by no more than
20% to 30%, in contrast to an approximately
500% increase occurring in skeletal muscle.
The reason, in part, for the weak sympathetic
response by the cerebral vasculature is that
metabolic mechanisms are dominant in regu-
lating flow; therefore, functional sympatholy-
sis occurs during sympathetic activation. This
is crucial to preserve normal brain function;
otherwise, every time a person stands up or
exercises, both of which cause sympathetic
activation, cerebral perfusion would decrease.
Therefore, baroreceptor reflexes have little
influence on cerebral blood flow. Sympathetic
activation shifts the autoregulatory curve to
the right, similar to what occurs with chronic
hypertension.
In recent years, we have learned that neu-
ropeptides originating in the brain signifi-
cantly influence cerebral vascular tone, and
they may be involved in producing head-
aches (e.g., migraine and cluster headaches)
and
cerebral
vascular
vasospasm
during
strokes.
Parasympathetic cholinergic fibers
innervating the cerebral vasculature release
nitric oxide and vasoactive intestinal poly-
peptide (VIP). These substances, along with
acetylcholine,
produce
localized
vasodila-
tion. Other nerves appear to release the local
vasodilators calcitonin gene-related peptide
(CGRP) and substance P. Sympathetic adr-
energic nerves can release neuropeptide-Y
(NPY) in addition to norepinephrine, which
causes localized vasoconstriction.
Vascular
and neuronal sources of endothelin-1 can also
produce vasoconstriction within the brain.
Skeletal Muscle Circulation
The primary function of skeletal muscle is to
contract and generate mechanical forces to
provide support to the skeleton and produce
movement of joints. This mechanical activ-
ity consumes large amounts of energy and
therefore requires delivery of considerable
amounts of oxygen and substrates, as well
as the efficient removal of metabolic waste
products. Both oxygen delivery and metabolic
waste removal functions are performed by the
circulation.
MICROVASCULAR ORGANIZATION
IN SKELETAL MUSCLE
The
circulation within
skeletal
muscle
is
highly organized (Fig. 7.13). Arterioles give
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