Preload can be defined as the initial stretching of the cardiac myocytes prior to contraction.
Related to the sarcomere length.
Indices of preload are the ventricular end-diastolic volume or pressure.
When venous return is increased, the end-diastolic pressure and volume of the ventricle are increased, which stretches the sarcomeres, increasing their preload.
With hypovolemia there is less ventricular filling and therefore shorter sacromere lengths,reducing preload.
Changes in ventricular preload affects ventricular stroke volume by the Frank-Starling mechanism.
Increased preload increases stroke volume, whereas decreased preload decreases stroke volume by altering the force of contraction of the cardiac muscle.