Fluid and Electrolyte Homeostasis

Objectives

The exposure to microgravity has long been known to affect the body's fluid and electrolyte balance. The headward fluid shift upon entering microgravity result in increased excretion of fluids and electrolytes, which reaches its maximum after the second day in space and is maintained at this reduced level throughout the flight. This loss of total body water (TBW) is accompanied by a redistribution of fluids in the body: the amount of intracellular fluid, extracellular fluid and blood plasma volume (PV) all change. In addition, a change in the fluid electrolyte levels, such as sodium or potassium, was observed in previous missions.

Although the short-term effects of microgravity on fluid and electrolyte homeostasis within the body are well known from the Shuttle program, the long-term effects were not defined. The results from this investigation provided better understanding on the mechanics of fluid and electrolyte regulation and the knowledge is used to develop countermeasures.

Shuttle-Mir Missions
Mir-18

Approach
Fluid and electrolyte balance in the body is regulated by several systems. The kidneys play the most important role in the regulation of fluid and electrolyte excretion and retention. There also many endocrine and circulatory factors which regulate fluid homeostasis.

To assess the impact of extended duration space flight on fluid and electrolyte homeostasis, blood, urine and saliva samples were collected from the three participating astronauts. The specimen were assayed for several biochemical and endocrine analytes to determine electrolyte balance, kidney function and levels of regulatory hormones in the blood stream.

Total body water, plasma volume, and extracellular fluid volume were measured to determine water distribution in the body. To measure total body water, the astronauts ingested a known dose of labeled water (H₂ ¹⁸O), followed by a seven hour period of urine and saliva sampling. Extracellular fluid volume was studied using a bromide solution as the tracer. Plasma volume was measured pre- and postflight using the carbon monoxide rebreathing method.

Results
Some of the objectives of this experiment were compromised during the mission, due to a failure of one of the freezers, which resulted in the loss of the biospecimen collected early inflight. Overall the observed decreases in plasma and extracellular fluid volume were similar to changes found during 14 day Shuttle flights. The changes in fluid volume that occur early during space flight appeared to remain throughout the long-term mission. This indicated that these are not transient effects, but rather reflect an adaptation to space flight which occurs within the first days to weeks of flight.

Earth Benefits
The results from this investigation provided better understanding on the mechanics of fluid and electrolyte regulation. This knowledge is used to develop countermeasures that will reduce the risk for the astronauts reentering gravity after long-duration stays in microgravity.

Publications
None available at this time

Principal Investigators
Helen Lane
NASA/Johnson Space Center

Anatoly Griegoriev
Institute of Biomedical Problems

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Curator: Julie Oliveaux Responsible NASA Official: John Uri

Page last updated: 07/16/1999