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Stroke volume
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Stroke volume is simply the blood the heart sends out with each beat.
And it is never all of the blood that was inside. The ventricle fills, contracts, and there is always some left over. That is why stroke volume is a subtraction: what was there at the end of filling minus what was left after the contraction.
What does that number depend on? Three things. Preload, which is how much the ventricle filled before it contracted. Contractility, which is the muscle's own strength. And afterload, the resistance it has to overcome to push the blood out, which relates to blood pressure and which, unlike the other two, lets less blood out the higher it gets.
Think about how a stretched rubber band snaps back harder than a slack one: the more the ventricle fills, the more its muscle stretches and the harder it contracts. That is the Frank-Starling law.
In brief
Stroke volume is the volume of blood the heart ejects with each beat.
Not all of it leaves: it is the volume at the end of filling minus what remains after contraction.
It depends on preload, contractility and afterload, which relates to blood pressure and works in the opposite direction.
The more it fills, the more its muscle stretches and the harder it contracts: the Frank-Starling law.
A stretched rubber band snaps back harder than a slack one.
Sources (1)
- National Center for Biotechnology Information, StatPearls PublishingPhysiology, Cardiac Output, StatPearlsPublished
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