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A capillary is narrower than an arteriole, yet arterioles offer the greatest resistance in the network. Why?
ACapillary walls secrete nitric oxide continuously
BCapillary beds have a far greater total cross sectional area
CCapillary blood carries a much lower viscosity
DCapillary length exceeds arteriolar length many times
Answer & Solution
Correct answer: B. Capillary beds have a far greater total cross sectional area
1. Resistance depends on the radius of an individual vessel, but the pressure a whole class of vessels imposes depends on the class in aggregate.
2. Any single capillary is narrower than any single arteriole.
3. Capillaries are so numerous, though, that their total cross sectional area far exceeds that of any other vessel class.
4. Blood entering that huge combined channel meets less aggregate opposition, and its velocity falls, giving time for exchange.
5. The steepest pressure drop and the greatest resistance therefore fall on the arterioles.
6. This is why arteriolar vasoconstriction and vasodilation dominate the regulation of blood pressure.
7. Viscosity is a property of the blood itself and does not differ between the two sites.
_Source: OpenStax Anatomy and Physiology (CC BY 4.0), Ch 20 "The Cardiovascular System: Blood Vessels and Circulation", section 20.2 Blood Flow, Blood Pressure, and Resistance_
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