One of the major issues in space explorations is the negative effect of prolonged exposure to very low gravity (i.e. a condition often referred to as microgravity or weightlessness) on astronauts' health. In this unusual condition the human body undergoes an adaptation process, often referred to as "deconditioning", that leads to significant loss of bone and muscle mass, hormonal changes, as well as modifications of important cardiovascular parameters. While the new physiological state is adequate to the microgravity environment, it results unsuitable upon return on earth. Hence after the mission astronauts experience various health-related problems and must undertake an intense rehabilitation program. Cardiovascular deconditioning originates mainly from a shift of blood from the lower extremities to the upper part of the body caused by the absence of gravity. In the long term (i.e. days) this causes a loss of blood and body fluid. In the short term (i.e. minutes) it elicits modifications to arterial blood pressure and heart rate.