During intense exercise, sympathetic nervous system activity increases systemically, yet blood flow to the working muscles rises dramatically. What explains this apparent contradiction?
ASympathetic signals are blocked from reaching exercising muscles during physical activity
BLocal metabolic vasodilators in active muscle override sympathetic vasoconstriction, while constriction is maintained in resting tissues
CThe heart's increased output alone is sufficient to force more blood into active muscles despite vasoconstriction
DExercising muscles release norepinephrine, which binds beta receptors to cause vasodilation
This is the key insight of vascular control: metabolic autoregulation dominates neural control locally. Accumulation of adenosine, H⁺, CO₂, and K⁺ in active muscle directly relaxes arteriolar smooth muscle, overriding sympathetic constriction. Meanwhile, sympathetic tone is maintained in inactive tissues, redistributing cardiac output toward where metabolic demand is highest. The heart's increased output is also a factor, but the redistribution relies on differential vascular resistance, not pressure alone.
Question 2 Multiple Choice
A patient's blood flow to an ischemic limb is restored after a brief occlusion. Immediately after flow resumes, the limb flushes and becomes hyperemic. What mechanism explains this reactive hyperemia?
ASympathetic nerves detect the occlusion and send a reflex signal to dilate downstream arterioles
BEndothelin-1 released during ischemia causes massive vasodilation once flow resumes
CMetabolites that accumulated during ischemia (adenosine, H⁺, CO₂) cause intense local vasodilation when flow is restored
DIncreased venous pressure during occlusion dilates collateral arterioles through the Bayliss myogenic response
Reactive hyperemia is a direct demonstration of metabolic autoregulation. During occlusion, active tissues continue consuming oxygen and generating metabolic byproducts (adenosine, CO₂, H⁺, K⁺) that cannot be washed away. These accumulate and produce strong vasodilatory signals on arteriolar smooth muscle. When flow is restored, those signals are already maximal, causing intense vasodilation until the metabolites are cleared. Sympathetic and endothelial mechanisms play secondary roles here; the primary driver is local metabolite buildup.
Question 3 True / False
Nitric oxide (NO) released by endothelial cells causes vasodilation by directly relaxing arterial smooth muscle.
TTrue
FFalse
Answer: True
True. Endothelial cells release NO in response to shear stress from blood flow and chemical stimuli like acetylcholine. NO diffuses across to the underlying smooth muscle where it activates guanylyl cyclase, producing cGMP, which ultimately triggers smooth muscle relaxation and vasodilation. This is the molecular basis of endothelial vasodilatory control and the target of drugs like nitrates used in angina.
Question 4 True / False
Because the sympathetic nervous system controls arteriolar tone, interrupting nerve supply to a tissue will cause its blood vessels to constrict and reduce local blood flow.
TTrue
FFalse
Answer: False
False. Removing sympathetic tone causes vasodilation, not constriction. Sympathetic activation releases norepinephrine and causes vasoconstriction; the resting state is one of moderate sympathetic tone, and denervation removes this constrictor influence. Furthermore, metabolic autoregulation operates independently of neural input — even a completely denervated tissue can match blood flow to metabolic demand through local accumulation of vasodilatory metabolites. Neural control modulates tone globally; local metabolic control dominates when demand changes.
Question 5 Short Answer
Why does a small change in arteriolar radius produce such a large change in blood flow? What law governs this, and what are its physiological consequences?
Think about your answer, then reveal below.
Model answer: Poiseuille's law states that flow is proportional to the fourth power of the vessel radius. Doubling the radius increases flow 16-fold; halving the radius reduces flow to 1/16 of its former value. This means arterioles — the primary resistance vessels — can exert enormous control over tissue perfusion with relatively small changes in smooth muscle tone. It explains why arteriolar control is so important: tiny adjustments in diameter dramatically redirect blood flow, making arterioles the primary regulators of tissue perfusion distribution.
The fourth-power relationship makes the arteriole the most powerful lever in the cardiovascular system for distributing flow. A modest sympathetic vasoconstriction or metabolic vasodilation translates into massive changes in local blood delivery. This is why arteriolar tone — not cardiac output alone — determines how blood is distributed among organs, and why arterial blood pressure can remain stable even as perfusion patterns shift dramatically during exercise, stress, or disease.