Assessment of fluid responsiveness includes respiratory maneuvers that involve the interaction between heart, lung and abdominal compartments affecting venous return and, in particular, inferior vena cava hemodynamics. A multiscale zero-dimensional and one-dimensional closed-loop model of the human circulation is used here as a tool to investigate the transient hemodynamic responses to forced breathing movements, as the Valsalva maneuver, where large flow accelerations and volume redistribution are produced, leading to instantaneous and large variations of the Froude or equivalent Speed Index number. In order to provide a versatile and realistic framework, intra-thoracic and intra-abdominal pressures are generated here by a ventilatory model of the lungs driven by the muscular activity in the chest and in the abdomen. This model also provides the opportunity to account for the Starling-resistor like behavior of the pulmonary circulation generated by the interaction between flow in capillaries, pleural pressure and alveolar pressure. Computational results are compared with cross sectional areas derived from computed tomographies and registered values of pressure obtained during spontaneous breathing conditions and the performance of the Valsalva maneuver.