Cardiovascular deconditioning: role of blood volume and sympathetic neurohormones

Studies of prolonged bed rest and water immersion have amply demonstrated that serious deconditioning of certain gravity-oriented biologic systems occurs with extended exposures to environments in which the necessity for gravity compensation has been reduced. The precise physiologic mechanisms responsible for the loss of orthostatic tolerance following such studies are unknown, but may be related to diminished blood volume, decreased muscle or tissue pressure in the extremities or to functional alterations in the sympathetic nervous system. In view of the fact that noradrenaline is the principal agent for maintaining vascular tone the author has focused his attention on the sympathetic nervous system as being the system most likely responsible for the loss of orthostatic tolerance following such experiments. The contributory effects of diminished circulating blood volume resulting from such environments are also discussed.