Right Ventricular Preload
65% of systemic blood is present in the venous system. The systemic veins are an important vascular reservoir, capable of storing blood volume for later use in times of stress – active venoconstriction may add as much as 1L of blood to the circulation [Barash, PG. Clinical Anesthesia, 5th ed. (Philadelphia), p. 883, 2006], with additional mobilization possible when venous pressure falls due to hypovolemia.
Venous return (VR) is critical as it must equal cardiac output at steady state. Importantly, venous return is reduced by increases in venous resistance, which, unlike in the arterial system, occur primarily in large vessels (ex. IVC, SVC) [Barash, PG. Clinical Anesthesia, 5th ed. (Philadelphia), p. 883, 2006] and may be mediated by alpha-agonists.