relaxed ventricle to appear enlarged. Factors affecting preload Preload is affected by venous blood pressure and the rate of venous return. These are affected by venous tone and volume of circulating blood. Preload is related to the ventricular end-diastolic volume; a higher end-diastolic volume implies a higher preload. However, the relationship is not simple because of the restriction of the term preload to single myocytes. Preload can still be approximated by the inexpensive echocardiographic measurement end diastolic volume or EDV. Preload increases with exercise (slightly), increasing blood volume (overtransfusion, polycythemia) and neuroendocrine excitement (sympathetic tone). An arteriovenous fistula can increase preload. Preload is also affected by two main body "pumps": Respiratory pump - Intrapleural pressure decreases during inspiration and abdominal pressure increases, squeezing local abdominal veins, allowing thoracic veins to expand and increase blood flow towards the right atrium. Skeletal muscle pump - In the deep veins of the legs, surrounding muscles squeeze veins and pump blood back towards the heart. This occurs most notably in the legs. Once blood flows past valves it cannot flow backwards and therefore blood is “milked” towards the heart. See also Afterload Cardiac output Frank–Starling law of the heart Passive leg raising test Volume overload References External links Overview at cvphysiology.com Cardiovascular physiology