A dip in blood pressure (BP) in response to head-up tilt (HUT) or active standing up might be due to rapid pooling in the veins below the heart (preload) or muscle activation-induced drop in systemic vascular resistance (afterload). We hypothesized that in the cardiovascular response to passive HUT, where in contrast to active standing little BP dip is observed, features affecting the preload play a key role. We developed a baroreflex model combined with a lumped parameter model of the circulation, including visco-elastic stress relaxation of the systemic veins. Cardiac contraction is modeled using the varying elastance concept. Gravity affects not only the systemic but also the pulmonary circulation. In accordance with the experimental results, model simulations do not show a BP dip upon HUT; the tilt back response is also realistic. Assuming venous capacities to be steady state values, the introduction of stress relaxation initially reduces venous pooling. The resulting time course of venous pooling is comparable to measured impedance changes. When venous pressure-volume dynamics are neglected, rapid venous pooling (completed within 30 seconds) leads to a drop in BP. The direct effect of gravity on the pulmonary circulation influences the BP response in the first ~5 seconds after HUT and tilt back. Concluding, the initial BP response to HUT is mainly determined by the response of the venous system. The time course of lower body pooling is essential in understanding the response to passive HUT.