Body fluid regulation in µ-gravity differs from that on Earth: an overview

Abstract

Similar to the response to central hypervolemic conditions on Earth, the shift of blood volume from the legs to the upper part of the body in astronauts entering µ-gravity should, in accordance with the HenryGauer mechanism, mediate diuresis and natriuresis. However, fluid balance and kidney function experiments during various space missions resulted in the surprising observation that the responses qualitatively differ from those observed during simulations of hypervolemia on Earth. There is some evidence that the attenuated responses of the kidney while entering weightlessness, and also later during space flight, may be caused by augmented fluid distribution to extravascular compartments compared to conditions on Earth. A functional decoupling of the kidney may also contribute to the observation that renal responses during exposure to µ-gravity are consistently weaker than those during simulation experiments before space flight. Deficits in body mass after landing have always been interpreted as an indication of absolute fluid loss early during space missions. However, recent data suggest that body mass changes during space flight are rather the consequences of hypocaloric nutrition and can be overcome by improved nutrition schemes. Finally, sodium-retaining humoral systems are activated during space flight and may contribute to a new steady-state of metabolic balances with a pronounced increase in body sodium compared to respective conditions on Earth. A revision of the classical "µ-gravity fluid shift" scheme is required.

Metabolic and cardiovascular responses during sub-maximal exercise in humans after 14 days of head-down tilt bed rest and inactivity

“…In addition, the effects of the muscular metaboreflex on blood pressure and heart rate are enhanced after space flight both during isometric contractions (Fu et al 2002) and during dynamic exercise (Iellamo et al 2006). Therefore, since the modification of blood distribution obtained during HDTBR overlaps that observed in real microgravity (Convertino 1996), we may hypothesise that a larger and more effective metaboreflex may have helped maintain a normal pressure response during exercise also in the present condition, although the mechanisms underlying the loss of body fluid may be different in the two conditions (Drummer et al 2000).…”

Physiological, Pharmacokinetic, and Pharmacodynamic Changes in Space

“…This was measured mainly by the reduction in plasma norepinephrine levels observed in these models 42,43,48–51 . However, renal plasma flow (RPF), glomerular filtration rate (GFR), and urine production were shown to be unchanged in space, 39,52,53 contrary to previous data from simulated microgravity models that suggested increased GFR and diuresis 35 . Instead of increased natriuresis, an increased sodium reabsorption was observed in flight, 31 resulting in a positive sodium balance (Figure 2).…”

From space to Earth: advances in human physiology from 20 years of bed rest studies (1986–2006)

“…But, several space missions have shown that conclusions drawn from HDBR studies are not totally transferable to space. It has been shown in the MIR-92-, D2-, SLS-1/SLS-2 missions that no diuresis occurred in astronauts when entering microgravity (Drummer et al 1993;Drummer et al 2000a;Leach et al 1996). Pre-mission treatment like sauna sessions, decreased fluid intake and application of diuretics might support these findings of balanced fluid homeostasis at the beginning of spaceflight.…”