Baseline data are recorded with the subjects in the supine position breathing with normal tidal volume without PEEP and without LBNP applied. Data are then recorded with LBNP applied in two steps (40 and 15 mmHg). Recordings are continued 5 minutes after releasing the negative pressure. Data are collected in total in four different series; normal tidal volume without PEEP; normal tidal volume with PEEP 5cm H2O; three doubled tidal volume without PEEP; three doubled tidal volume with PEEP 5 cm H20. Respiratory rate is not controlled. The order of the four series is randomized. Between the series a 20 minute period of rest is allowed to the subjects. This period is also necessary to restore steady state between provocations.