714
UNIT 4
Maintenance of the Body
19
However, systemic adjustments mediated by the vasomo-
tor center must also occur to ensure that blood delivery to the
muscles is both faster and more abundant. During exercise,
sympathetic nervous system activity increases. Norepinephrine
released from sympathetic nerve endings causes vasoconstric-
tion of the vessels of blood reservoirs such as the digestive
viscera and skin, diverting blood away from these regions tem-
porarily and ensuring that more blood reaches the muscles.
In skeletal muscles, the sympathetic nervous system and lo-
cal metabolic controls have opposing effects on arteriolar di-
ameter. During exercise, local controls
override
sympathetic
vasoconstriction. Consequently, blood flow to skeletal muscles
can increase tenfold or more during physical activity, as you saw
in Figure 19.13, and virtually all capillaries in the active muscles
open to accommodate the increased flow.
Epinephrine acting at beta (β) adrenergic receptors and ace-
tylcholine acting at cholinergic receptors were once thought to
contribute to arteriolar dilation during exercise. However, these
appear to have little physiological importance in controlling hu-
man skeletal muscle blood flow.
Without question, strenuous exercise is one of the most de-
manding conditions the cardiovascular system faces. Ultimately,
the major factor determining how long muscles can contract
vigorously is the ability of the cardiovascular system to deliver
adequate oxygen and nutrients and remove waste products.
The Brain
Blood flow to the brain averages 750 ml/min and is maintained
at a relatively constant level. Constant cerebral blood flow is
necessary because neurons are totally intolerant of ischemia.
Also, the brain is unable to store essential nutrients despite be-
ing the most metabolically active organ in the body.
Cerebral blood flow is regulated by one of the body’s most
precise autoregulatory systems and is tailored to local neuro-
nal need. For example, when you make a fist with your right
hand, the neurons in the le± cerebral motor cortex controlling
O
2
CO
2
H
+
K
+
Intrinsic mechanisms
(autoregulation)
• Metabolic or
my
ogenic control
s
• Distribute blood flow to individual
organs and tissues as needed
Metabolic
• Prostaglandins
A
denosine
• Nitric oxide
S
ympathetic tone
Neuronal
Extrinsic mechanisms
Vasodilators
Vasoconstrictors
• Neuronal or hormonal control
s
• Maintain mean arterial pressure
(MAP)
• Redistribute blood during exercise
and thermoregulation
Hormonal
• Atrial natriuretic peptide
Myogenic
• Stretch
Metabolic
• Endothelins
S
ympathetic tone
Neuronal
Hormonal
• Angiotensin II
• Antidiuretic hormone
• Epinephrine
• Norepinephrine
Figure 19.15
Intrinsic and extrinsic control of arteriolar smooth muscle in the
systemic circulation.
Epinephrine and norepinephrine constrict arteriolar smooth muscle by
acting at
a
-adrenergic receptors.
b
-adrenergic receptors (causing vasodilation) are present in
arterioles supplying skeletal and heart muscle, but their physiological relevance is minimal.
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