Twelve healthy volunteers will be studied. Mild hypovolemia will be induced by 12 hours fasting from food and drinks. At 8.30 AM the study protocol will start. The studied subjects will lie calm supine in a ICU bed able to provide passive head up and head down tilt. They will be connected to a Siemens SC9000 monitor showing their ECG waves and to a Nexfin (BMEYE) monitor for continuous noninvasive blood pressure (NBP) assessment. Both waves (ECG and NBP) will be recorded on a laptop PC through analogic/digital input/output converter (PowerLab 8/35, ADinstruments). The studied subjects will undergo to a sequence of (1) spontaneous breathing at 15 degrees head up, (2) spontaneous breathing at 15 degrees head down, (3) noninvasive ventilation at 15 degrees head up, and (4) noninvasive ventilation at 15 degrees head down. The sequence 1-2-3-4 will be randomized. After this four phases, a fluid challenge of ringer acetate 15ml*kg will be intravenously administered and the a sequence 1-2-3-4 will be repeated after a new randomization. Noninvasive ventilation will be provided with a facial mask with ventilatory setting: Psupp 8 cmH2O, PEEP 5 cmH2O, FiO2 0.28. Psupp will be decreased by 2 cmH2O steps if the inspiratory tidal volume will be >10ml*kg. During both spontaneous breathing and noninvasive ventilation the subjects will breathe following a metronome at 18bpm.