The Difference Between Preload, Afterload, Contractility, and Ejection Fraction
Preload (also known as end diastolic volume) is the force causing a stretch of the heart muscle chamber after receiving blood. Afterload is the force needed to expel this blood into the next compartment.
Contractility reflects the possible range of forces that can be generated for left ventricular (LV) contraction. In other words, it describes the strength and flexibility of the heart muscle. Contractility is primarily indexed by measuring the ejection fraction, the formula for which is:
Ejection fraction = SV/EDV,
where SV is the stroke volume, or the total volume of blood expelled from the LV after each contraction,
and EDV is the end diastolic volume, or the total volume of the LV during ventricular filling.
*Note: According to the Frank-Starling Law,
SV = EDV-ESV.
In other words, the difference between EDV and SV is the residual volume of the LV after systolic emptying of the chamber. This volume is the end systolic volume (ESV). So, ejection fraction is the fraction of the EDV that is expelled from the LV in a contraction.
Higher contractility allows a higher increase in stroke volume, which is necessary when more blood is needed to support strenuous activity. However, if the heart stretches too much, the myosin heads and actin filaments will not have enough overlapping area, and therefore cross-bridge cycling is impossible. (Visit this article to learn more about cross-bridge cycling.) The heart won't be able to produce a contraction to expel the blood in the chamber. So, regular heart contractions at rest do not utilize maximal stroke volume; rather, maximal contractility usage is reserved for strenuous activity, so that volume expansion when necessary is possible.
Further reading:
https://www.deltexmedical.com/decision_tree/preload-afterload-and-contractility/
https://www.ncbi.nlm.nih.gov/books/NBK493174/
https://www.ncbi.nlm.nih.gov/books/NBK470295/
https://www.sciencedirect.com/topics/medicine-and-dentistry/ejection-fraction
