Other factors influencing stroke volume, and therefore cardiac output, are preload and afterload work requirements placed on the heart. Preload is a measure of the amount of blood filling the ventricles just prior to contraction. When the volume of blood filling the ventricles increases, the heart responds by contracting more strongly, pumping out bod to keep pace. Conversely, a decreased preload causes a weaker contraction. This phenomenon ,known as Starling’s law of heart, ensure that blood does not get backed up in the venous circulation. However, with aging comes change in the elasticity and smooth muscle of veins and subsequent dilutions, or varicosities, of veins .This increases the capacity of veins to hold blood and decreases the rate of venous return to the heart, ultimately causing a decreased preload and cardiac output.