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What slows blood down
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Blood does not run through open tubes: something opposes it the whole way, and that is called peripheral vascular resistance. It is, quite simply, the opposition the circuit offers to the passage of blood.
That resistance is not fixed. It rises when the vessels constrict and falls when they dilate, and the place where it is really decided is not the aorta. While blood travels through the aorta and the large arteries, pressure barely changes; the drop happens when it reaches the arterioles, the tiny arteries at the end of the route. They are the ones in charge.
And they are in charge for a surprising geometric reason: what counts is the diameter of the vessel, and it counts raised to the fourth power. If the vessel opens up, resistance does not fall a little; it plummets. It is a hose: narrowing it slightly makes flow far more difficult.
In the end the two things come together in a single calculation. Blood pressure is cardiac output, that is, the blood the heart pumps per minute, multiplied by that resistance.
In brief
Peripheral vascular resistance is the opposition the circulatory system offers to blood flow.
It rises when vessels constrict and falls when they dilate.
Pressure barely falls in the aorta and plummets in the arterioles, the main regulators of resistance.
Diameter rules: when a vessel dilates, resistance falls with the fourth power of that change.
A hose: narrowing it slightly makes flow far more costly.
Blood pressure is cardiac output multiplied by that resistance.
Sources (1)
- National Center for Biotechnology Information, StatPearls PublishingPhysiology, Peripheral Vascular Resistance, StatPearlsPublished
This page explains the topic in general terms; it does not tell anyone what treatment to follow.
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