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.
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.
The rewritten readings were written for this dataset.
More to read
What determines cardiac output
Cardiac output measures blood pumped each minute: stroke volume multiplied by heart rate. During exercise, increases in both can raise output severalfold. Neural and hormonal signals regulate rate; ventricular filling and resistance influence stroke volume.
Filling and the next contraction
A heartbeat proceeds from filling to contraction and then ejection. Filling stretches ventricular muscle, establishing preload. Within physiological limits, greater preload generally increases contraction force through the Frank-Starling relationship. Preload is not the only influence: more force at unchanged preload and afterload indicates increased contractility.
Balancing the heart's two pumps
Circulation supplies every tissue while maintaining suitable pressures. The heart's right pump sends blood through the lungs; its left pump supplies the body. These circuits are connected in series, so their average outputs must balance. Changes in filling volumes and pressures help maintain that balance through the connected circulation.
Learn: The circulatory system review
Blood circulation is one loop run by two pumps in series. The review follows blood from body to right heart to lungs to left heart and back out. It shows how filling, squeezing, and vessel tone each shape flow. With that map in mind, preload and afterload stop being jargon and become visible as filling before the squeeze and load against it.
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2 · Watch
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Where it sits
Optional connections
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.