In cardiac physiology, preload is the volume of blood present in a ventricle of the heart, after passive filling and atrial contraction. If the chamber is not mentioned, it is usually assumed to be the left ventricle.
Preload is theoretically most accurately described as the initial stretching of cardiac myocytes prior to contraction. This cannot be measured in vivo and therefore other measurements are used as estimates. Estimation is inaccurate, for example in a chronically dilated ventricle new sarcomeres may have formed in the heart muscle allowing the relaxed ventricle to appear enlarged.
Preload is affected by venous blood pressure and the rate of venous return. These are affected by venous tone and volume of circulating blood.