According to this model, centred on the importance of venous return, Pmsf, and not the arterial pressure, propels flow towards the right atrium. To explain this, one must consider that veins and venules represent a blood bulk, of large size and of high compliance.
Magder and De Varennes nicely compared the cardiovascular system to a bathtub, filled by a tap and emptied through a hole located in the side of the tub [12] (Fig. 1). The flow out of the tub, likened to venous return, is only determined by the hydrostatic pressure of water above the hole, not by the pressure coming out of the tap, which is likened to cardiac output, because it is low compared to the tub volume. In the “bathtub model”, the venous system plays the role of the tub, and the tap, i.e. cardiac output, only fills the bathtub [12]. The role of the hydrostatic pressure that propels water out of the tub is played by the pressure generated by the elastic recoil of the venous system. The volume that stretches the elastic veins provides the potential energy that generates flow. However, as any elastic structure, e.g. a balloon, veins have a resting volume which does not stress their elastic walls and creates no pressure. This volume which does not contribute to venous flow is called the unstressed blood volume. In the Magder and De Varennes bathtub model, it corresponds to the water volume below the hole. Any amount of blood added to the venous reservoir will produce tension in its walls and increase the intravascular pressure. This part of venous blood (water volume above the hole) is the stressed blood volume (Fig. 1).
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The venous system is large: in an adult human, it contains approximately 70% of the total blood volume, i.e. 3.5 L [13], with about 70% in the splanchnic vascular bed [14, 15]. Moreover, venous compliance is 40 times greater than arterial compliance [16]. In humans under normal conditions, the unstressed blood volume is approximately 70% of the total blood volume [12]. It represents a large reserve of paramount importance that may be recruited by an α-adrenergic-mediated venous constriction [17, 18]. This venoconstriction moves part of the unstressed blood volume to the stressed blood volume [15, 19].
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