PHYS201 · Lecture 6Cardiac output and what sets it
Slide 4
Preload and the Frank-Starling law
A larger EDV gives a larger stroke volume.
Stroke volumeEnd-diastolic volume
Slide 6
Afterload
The load the ventricle must overcome to eject.
Wall stress = P × r ÷ 2h
Slide 7
Contractility (inotropy)
More calcium, a higher curve.
Stroke volumeEnd-diastolic volume
Lesson filesSlidesHandout
PHYS201 · Unit 2The heart and circulation
4 · The cardiac cycle31 min
5 · Heart sounds and murmurs24 min
6 · Cardiac output and what sets it27 min
6 · Cardiac output and what sets it27 min
7 · Blood pressure and its control29 min
8 · Exercise and the circulation33 min
9 · The microcirculation26 min
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Within limits, the heart pumps out whatever volume is returned to it. So on the graph you have end-diastolic volume along the x axis, stroke volume up the y axis, and the curve rises steeply at first and then flattens off at high volumes.
A little bit of history, because I like it. Otto Frank was a German physiologist working on frog hearts in the eighteen nineties, and Ernest Starling was in London about twenty years later with his heart-lung preparation in dogs. They never worked together.
How does a larger end-diastolic volume give a larger stroke volume without extra calcium?
In the lecturer's words
“Preload is the stretch on the ventricular muscle cells at the end of diastole, just before contraction.”
Quiz, question 3
Afterload rises and nothing else changes. What happens to stroke volume?
Not quite. That describes more preload, and here filling has not changed.
Correct. The ventricle empties less well against the higher pressure, so end-systolic volume rises.
Not quite. Heart rate changes the output per minute, not the volume of one beat.
Flashcard, 6 of 9
What does more contractility do to the Frank-Starling curve?
Show answer
It lifts the ventricle onto a higher curve. More preload only moves it along the same one.
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