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The Frank Starling law appears to be a trivial observation:

stroke volume is directly related to the end diastolic volume

In everyday language, as more blood fills the heart, the heart pumps more blood out.

Can someone point out whether there are some inaccuracies in the above description because I don't understand why every article on FS law requires upward 20 pages. Even youtube video on this topic is 8 minutes long.

What is the big idea here that I am missing?

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This is the short answer from wikipedia :) :

As the heart fills with more blood than usual, the force of cardiac muscular contractions increases.[2] This is a result of an increase in the load experienced by each muscle fiber due to the extra blood load entering the heart. The stretching of the muscle fibers augments cardiac muscle contraction by increasing the calcium sensitivity of the myofibrils, [3] causing a greater number of actin-myosin cross-bridges to form within the muscle fibers. The force that any single cardiac muscle fiber generates is proportional to the initial sarcomere length (known as preload), and the stretch on the individual fibers is related to the end-diastolic volume of the left and right ventricles.

In the human heart, maximal force is generated with an initial sarcomere length of 2.2 micrometers, a length which is rarely exceeded in the normal heart. Initial lengths larger or smaller than this optimal value will decrease the force the muscle can achieve. For larger sarcomere lengths, this is the result of less overlap of the thin and thick filaments; for smaller sarcomere lengths, the cause is the decreased sensitivity for calcium by the myofilaments.

in one sentence, it is a response to tension.

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