A.  During Diastole the greater the end diastolic volume entering the heart  then the greater the amount of blood ejected during diastole then the greater  the  volume of blood ejected during systolic contraction (stroke volume).

II. Concepts.
    A. When the heart fills with excess blood the force of the contractions increases.
        1.  Causes an increase in the load experienced by each muscle fibre due to the extraneous blood entering the heart. 
        2. Stretching of the muscle fibres increases the affinity of troponin C for calcium
            a. Causes a greater number of cross-bridges to form within the muscle fibres
            b.  Increases the contractile force of the cardiac muscle. 

    B.  This preload is related to the  stretch on the individual fibers is related to the end-diastolic volume of the ventricle.

III.  Ionotropic effect.
    A.  Increased afterload or contractility, as in the administration of a positive  inotropic agent, would shift the line up, increasing stroke volume.
    B.  Decreased afterload or contractility, as in class  myocardial ischemia, would shift the line down, decreasing stroke volume.