Résumé : A total of five studies have been included in this work. The first two studies describe cardiovascular response changes to (simulated) microgravity in healthy subjects. In a third study, the circulatory response to standing is discussed in five cosmonauts before and after short-duration spaceflight. The last two studies deal with failing adaptive mechanisms of orthostatic blood-pressure control in patients who are prone to head-up tilt induced syncope. The team of Professor A. E. Aubert (F. Beckers, K. Couckuyt and J. Liu) from the Laboratory of Experimental Cardiology of the University Hospital in Leuven was involved in the completion of this thesis. Part of the work has been coordinated by Dr. W. Wieling (J. Gisolf and J.M. Karemaker) of the Academic Medical Center in Amsterdam and by Professor H. Ector (T. Reybrouck) of the Division of Clinical Cardiology in Leuven. © 2008 Elsevier Masson SAS. All rights reserved.