Maintenance of the Body
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
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.
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
• Metabolic or
• Distribute blood flow to individual
organs and tissues as needed
• Nitric oxide
• Neuronal or hormonal control
• Maintain mean arterial pressure
• Redistribute blood during exercise
• Atrial natriuretic peptide
• Angiotensin II
• Antidiuretic hormone
Intrinsic and extrinsic control of arteriolar smooth muscle in the
Epinephrine and norepinephrine constrict arteriolar smooth muscle by
-adrenergic receptors (causing vasodilation) are present in
arterioles supplying skeletal and heart muscle, but their physiological relevance is minimal.