Questions: Vascular Tone and Resistance Regulation

Question 1

An arteriole's radius is reduced by half (from r to r/2) due to smooth muscle contraction. According to Poiseuille's law, what happens to the resistance through that vessel?

• A. Resistance doubles — it is proportional to 1/r, so halving r doubles resistance.
• B. Resistance quadruples — it is proportional to 1/r², so halving r quadruples resistance.
• C. Resistance increases 16-fold — it is proportional to 1/r⁴, so halving r raises resistance by a factor of 2⁴ = 16.
• D. Resistance increases 8-fold — the combined effects of radius and length changes produce an 8× increase.

Answer: (C)

Poiseuille's law states R ∝ 1/r⁴. If r → r/2, then 1/(r/2)⁴ = 16/r⁴, so resistance increases 16-fold. This fourth-power relationship is the key insight of the topic — it explains why arterioles are such powerful regulators of blood flow. A modest 20% reduction in arteriolar diameter roughly doubles resistance. This extreme sensitivity means the body can produce large changes in vascular resistance and therefore blood flow distribution through small adjustments in arteriolar tone. No other cardiovascular variable has this kind of leverage, which is why arterioles — not large arteries — are the primary site of resistance regulation.

Question 2

During vigorous exercise, sympathetic nervous system activation increases dramatically. Yet blood flow in working skeletal muscle increases substantially rather than decreasing. What explains this apparent contradiction?

• A. Skeletal muscle lacks sympathetic innervation, so its arterioles are not subject to sympathetic vasoconstriction during exercise.
• B. Local metabolic vasodilation (from CO₂, H⁺, K⁺, adenosine released by active muscle) overrides sympathetic vasoconstriction in working muscle, while sympathetic constriction dominates in less metabolically active vascular beds.
• C. The heart's increased cardiac output during exercise dilates arterioles throughout the body due to increased perfusion pressure.
• D. Epinephrine released during exercise binds β₂-receptors in skeletal muscle arterioles, directly overriding local sympathetic α₁-mediated constriction.

Answer: (B)

This is metabolic autoregulation in action — and it illustrates the three-tier hierarchy of vascular control. Sympathetic activation provides systemic constriction across most beds. But in working skeletal muscle, local metabolic products (CO₂, H⁺, K⁺, adenosine, lactate) accumulate in proportion to metabolic rate and act directly on arteriolar smooth muscle to cause vasodilation. This local signal is strong enough to override sympathetic tone. Meanwhile, less metabolically active beds (gut, kidneys) remain under sympathetic constriction. The result is redistribution of cardiac output to where it's needed, without requiring proportional increases in total cardiac output.

Question 3

Metabolic autoregulation allows working skeletal muscle arterioles to vasodilate in response to local tissue metabolites, even when global sympathetic tone is elevated, thus directing blood flow to tissues with the greatest metabolic demand.

• T. True
• F. False

Answer: True

Metabolic autoregulation is the local mechanism by which blood flow matches tissue demand independently of neural or hormonal control. When a tissue increases its metabolic rate, the resulting accumulation of CO₂, H⁺, K⁺, adenosine, and other metabolites acts directly on local arteriolar smooth muscle to cause relaxation and vasodilation. This occurs even in the presence of elevated sympathetic tone — the local metabolic signal is sufficient to overcome sympathetic vasoconstriction. This allows the cardiovascular system to redirect blood flow to active tissues without requiring neural commands to specify which tissues are working hardest.

Question 4

Nitric oxide (NO) produced by endothelial cells causes vasoconstriction by directly activating smooth muscle contraction.

• T. True
• F. False

Answer: False

Nitric oxide is a potent vasodilator, not a vasoconstrictor. When increased blood flow increases shear stress on endothelial cells, they produce NO, which diffuses into adjacent smooth muscle cells and activates guanylyl cyclase, raising cGMP levels and causing smooth muscle relaxation (vasodilation). This flow-mediated dilation matches vessel caliber to blood flow demand. The potent vasoconstrictor produced by endothelial cells is endothelin-1 — the functional opposite of NO. NO is also the mechanism by which nitroglycerin causes vasodilation in angina treatment, and it is the target of phosphodiesterase inhibitors (like sildenafil) that prevent cGMP breakdown.

Question 5

Explain why arterioles, rather than larger arteries, are the primary site of vascular resistance regulation, and what physiological consequences follow from Poiseuille's law.

Model answer: Arterioles are the primary resistance vessels because Poiseuille's law (R ∝ 1/r⁴) gives small vessels extreme sensitivity: small changes in arteriolar diameter produce large changes in resistance and therefore blood flow. Large arteries have wide lumens and low resistance regardless of moderate constriction; their high baseline radius means the 1/r⁴ relationship gives them less regulatory leverage. Arterioles, being small, sit in the steepest part of the fourth-power curve — a 20% reduction in diameter roughly doubles resistance. Physiologically, this allows the body to redirect blood flow with precision: constricting arterioles in one bed while relaxing them in another redistributes cardiac output without requiring large changes in total output or heart rate.

The r⁴ dependence is also why arterial diseases like arteriosclerosis that modestly reduce arteriolar lumen diameter have disproportionately large effects on resistance and tissue perfusion. And it is why drugs that target vascular smooth muscle (vasodilators like calcium channel blockers or ACE inhibitors) are effective antihypertensives — small reductions in arteriolar tone, applied across the vascular system, produce large reductions in total peripheral resistance and blood pressure.