Microgravity and the Plasma Volume                                                                                                                                                   SPACEFLIGHT 52: 235, 2010. 

Have you ever wondered why astronauts have so much difficulty standing after return from a space mission? In addition to weakness from loss of skeletal muscle there is a loss of plasma volume which constitutes about one-third of the extracellular fluid volume.  For example there was a reduction of the plasma volume by about 10% after a 9 day mission (SLS-1). (1)

In addition with space flight there is invariably a reduction in the thirst mechanism and water balance becomes negative within the first 2 days of flight (1) Since water is an anti-oxidant because of the hydrogen ions it contains, this invariable dehydration portends space-flight oxidative stress, with in turn vulnerability to significant injuries to the lining of the blood vessels (endothelium). I have addressed these issues with numerous papers.  (www.femsinspace.com)

It is indeed remarkable that rats subjected to swimming, while attached to a weight, for only 4 hours (2) and rats subjected to space flight after just 12 days (3) show identical pathology in the heart muscle. In both cases there are projections extending from the capillaries. These projections have been shown to be associated with gaps in the walls of the capillaries- conducive to reductions of plasma volume. (4)  

This reduction of plasma volume ( resistant to increased water intake ) represents  a serious  complication of microgravity because  invariably it triggers  reductions  of  2 vessel dilators and clot busters : atrial natriuretic peptide (A.N.P) and  in turn nitric oxide. (N.O.)  There is no solution to this life-threatening vascular complication other than the combination of magnesium therapy necessary for the synthesis and release of ANP and N.O and also gene therapy. (5, 6) Pharmaceuticals can’t be used because ultimately, in the presence of microgravity, they can not be sufficiently metabolized or excreted.

But why is A.N.P   invariably reduced, as for example, by as much as 40% after just 12 days in space? (1) Because the upper chambers of the heart can sense the reduced plasma volume and automatically reduce the release of A.N.P; otherwise all those subjected to microgravity would die soon after lift-off.  The heart prevents this but at a great price.  

     William J. Rowe M.D.  FBIS
     ( www.femsinspace.com)




  1. Leach Huntoon CS,  Grigoriev AL, Natochin YV. Fluid and electrolyte regulation in spaceflight. Science and Technology Series  94. Am Astronaut. Society, San Diego, 1998.
  2. 2. Kondalenko VF, Sergeev YP. Changes in the capillary ultrastructure of the rat myocardium following nonrepetitive physical exertion.   Byulleten Eksperimental noi  Biologii I  Meditsiny 78 :  88-91, 1974.

3.  Philpott DE, Popova IA, Kato K, et al. Morphological and biochemical examination of Cosmos 1887 rat heart tissue : Part 1-ultrastructure. FASEB J. 4: 73-78, 1990.

4. Mc Donald DM, Thurston G, Baluk P. Endothelial gaps as sites for plasma leakage in inflammation. Microcirculation  6: 7-22, 1999.

5. Rowe WJ.  Extraordinary hypertension after a lunar mission.  Am J. Med  122; e1, 2009.

6.Rowe WJ.  Potential renovascular hypertension, space missions, and the role of magnesium. Intern J Nephrol and renovascular Dis. 2 : 51-57, 2009.