An arteriole constricts so that its radius decreases to half its original value. Assuming the pressure gradient remains unchanged, what happens to blood flow through that arteriole?
AFlow decreases to one-half of the original
BFlow decreases to one-quarter of the original
CFlow decreases to one-sixteenth of the original
DFlow decreases to one-eighth of the original
Poiseuille's law states that flow is proportional to r⁴. If radius halves (r → r/2), flow changes by (1/2)⁴ = 1/16. This fourth-power relationship is the key insight: small changes in arteriolar radius produce enormous changes in resistance and flow. A linear relationship (option A) would make arteriolar tone a far weaker control mechanism. This 16-fold resistance increase explains why arterioles — not arteries or capillaries — are the body's primary tool for redirecting blood between organs.
Question 2 Multiple Choice
During vigorous exercise, blood flow must increase simultaneously to working muscles and to skin (for cooling). What vascular arrangement makes this possible without necessarily starving other organs?
AOrgans are arranged in series, so increasing cardiac output directs more blood through all organs sequentially
BOrgans are arranged in parallel off the aorta, so each can independently lower its arteriolar resistance and draw more flow
CVenous capacitance vessels constrict to reduce total blood volume, increasing pressure to all organs equally
DArterioles dilate simultaneously in all organs including the gut, distributing the increase evenly
Organs are arranged in parallel off the aorta, each receiving blood at nearly the same arterial input pressure. Each organ independently controls its own arteriolar tone, setting its own resistance and thus its own share of cardiac output. The heart increases output in response to increased venous return (Frank-Starling). A series arrangement (option A) would mean that increasing flow to muscles necessarily reduces flow to all organs downstream — making simultaneous multi-organ demands impossible to meet independently.
Question 3 True / False
Veins function primarily as passive conduits that transport blood back to the heart and play no active role in cardiovascular regulation.
TTrue
FFalse
Answer: False
Veins are capacitance vessels that hold approximately 65% of total blood volume at rest and can actively constrict under sympathetic stimulation. This constriction reduces 'unstressed volume,' shifting blood toward the heart and increasing venous return. Greater venous return stretches the ventricles, increasing the force of the next contraction via the Frank-Starling mechanism — directly boosting cardiac output. Treating veins as mere passive pipes misses their essential role as an adjustable blood reservoir during exercise, hemorrhage, or postural changes.
Question 4 True / False
In the systemic circulation, recruiting additional parallel vascular beds (e.g., opening up more capillary beds in exercising muscle) decreases total peripheral resistance.
TTrue
FFalse
Answer: True
In a parallel circuit, total resistance is determined by the sum of conductances (1/R_total = 1/R₁ + 1/R₂ + ...). Adding a new parallel pathway always adds conductance and therefore lowers total resistance. When exercising muscles dilate their arterioles, they add high-conductance pathways, reducing total peripheral resistance even as individual organ resistances fall. This is why vigorous exercise can simultaneously increase cardiac output and lower blood pressure in trained individuals.
Question 5 Short Answer
Explain why arterioles — rather than arteries or capillaries — serve as the primary resistance vessels and the body's main control point for distributing blood flow between organs.
Think about your answer, then reveal below.
Model answer: Arterioles have abundant smooth muscle in their walls that can rapidly contract or relax, producing meaningful fractional changes in vessel radius. Because Poiseuille's law makes resistance proportional to 1/r⁴, even modest changes in arteriolar radius (e.g., halving radius increases resistance 16-fold) allow precise, powerful control over organ blood flow. Arteries are too large for the same fractional radius change to produce large effects; capillaries lack smooth muscle and cannot actively constrict. Arterioles sit at the entry point to each organ's microcirculation, where sympathetic signals and local metabolic cues converge to fine-tune flow distribution.
The r⁴ relationship is the quantitative foundation for all qualitative claims about arterioles as resistance vessels. Any complete answer should connect the fourth-power sensitivity to the practical control function, not just assert that arterioles have smooth muscle.