Preload, Afterload and the Frank-Starling Relationship
A heart that fills more stretches more and ejects more, which is how the two sides keep matched output without any signal between them. Output is set by filling, by the resistance the heart pumps against and by contractility.
What a learner can do afterwards
- Calculates cardiac output from stroke volume and rate, and predicts the effect of a change in filling.
- Separates a change in preload from a change in contractility on a stated curve.
- Explains how the right and left sides stay matched without direct communication.
1 · Read
Think of the pump as shipping parcels each minute. Output equals what one beat ejects multiplied by the beats per minute, and exercise raises both. Filling stretches the wall before the squeeze, and that stretch decides the next ejection: more in, more out. Nerves and hormones tune the rate on top.
Now learn to read the curve in two moves. Extra filling slides you along the curve you already sit on, while a change in strength shifts the whole curve up or down. So the same filling with a bigger beat means strength itself changed. Filling moves along, strength moves the line.
Next, meet the load fought during ejection. It is the pressure the chamber must beat to push blood out, and higher pressure trims each beat. Filling before the squeeze and load against it are the two forces shaping every ejection you predict.
Finally, see how the two sides stay matched with no hotline between them. Right serves the lungs and left serves the body, sitting in series so each side feeds the other. If one side shipped more, blood would pile up and pressures would shift to rebalance them. Output tracks input on its own.
You track filling, load fought, and strength to predict what each beat ejects.
2 · Watch
Take it off screen
Where it sits
Where this leads
Jobs that lean on this skill. Follow one to see everything it is built on.
8 questions wait behind this lesson, each with its answer explained. Every answer feeds the sky: stars light as they are learned, and dim when it is time to come back.