Ejection fraction (EF) is defined by the ratio of end-systolic volume (ESV) and end-diastolic volume (EDV). The resulting fraction is a dimensionless number whose interpretation is ambiguous and most likely misleading. Despite this limitation, EF is widely accepted as a clinical marker of cardiac function. In this article we analyze the role of ESV, a fundamental variable of ventricular mechanics, compared with the popular EF. Common physiology-based mathematics can explain a simple association between EF and ESV. This concept is illustrated by a detailed analysis of the information obtained from angiocardiography, echocardiography and cardiac magnetic resonance studies. EF versus ESV produces a non-linear curve. For a small ESV, the EF approaches 100%, while for a large ESV, the EF gradually decreases toward zero. This elemental relationship is commonly observed in innervated natural hearts. Thus, the popularity of EF mostly derives from a fortuitous connection with the pivotal variable ESV. Alongside this finding, we unfold historical events that facilitated the emergence of EF as a result of serendipity. Our physiology-based approach denounces the circumstantial theories invoked to justify the importance of EF as an index of cardiac function, which are critically discussed. EF appears to be nothing more than a blessing in disguise. For this reason, we propose the ESV as a more logical metric for the analysis of ventricular function.