[The motor structures responsible for venous flow].

There are numerous forces which propel the blood within the veins: --At tissue venule level the residual arteriolar pressure especially intervenes. --At peripheral vein level, the dominant forces are those which lower the transmural pressure, either by decreasing intravascular pressure (gravity, aspiration from neighbouring veins), or by increasing the extravascular pressure creating a compression effect by squeezing of the flexible vessel between muscles and a rigid aponeurosis. --At central vein level, the principal role is ensured by the thoraco-abdominal mechanics, whereby contraction of the diaphragm produces compression and aspiration. The right heart intervenes, when required, and directs blood towards the pulmonary circulation. The result of the simultaneous, successive application of these forces, differs depending on the organ. In addition, the flexibility of the vessel varies according to neuro-hormonal control. Finally, the rheological properties of the fluid and the degree of backflow must be taken into account. Thus, one gets a double flow (hydraulic transfer, propagated waves) which varies according to requirements. The energy expenditure due to flow is a function of the characteristics and volume of the liquid displaced. The complexity of such a flow has the advantage of adaptability but the inconvenience of fragility.