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In the book Endurance: A year in space, a lifetime discovery, there is a paragraph as follows:

Then I hear Terry’s voice, breaking in midsentence, as if he were here with me: “. . . pills for the fluid loading protocol, Anton? Or did you leave them on station?”

I don't understand what is fluid loading protocol? I can understand it is some kind of medicine that astronauts have to take, but I don't know exactly what it is and what is it for.

Please explain it to me.

Thanks

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Fluid loading was a protocol designed to ease shuttle astronauts' readjustment to Earth gravity.

One of the most important changes negatively impacting flight operations and crew safety is landing day orthostatic intolerance. Astronauts who have orthostatic intolerance (literally, the inability to remain standing upright) cannot maintain adequate arterial blood pressure and have decreased brain blood levels when upright, and they experience light-headedness and perhaps even fainting. This may impair their ability to stand up and egress the vehicle after landing, and even to pilot the vehicle while seated upright as apparent gravity increases from weightlessness to 1.6 g during atmospheric re-entry.

....

Starting about 2 hours before landing, astronauts ingest about 1 liter (0.58 oz) of water along with salt tablets. Subsequent refinements to enhance palatability and tolerance include the addition of sweeteners and substitution of bouillon solutions.

The reasons for this condition are not well understood but are linked to the body's physiological responses to free fall.

While orthostatic intolerance is perhaps the most comprehensively studied cardiovascular effect of spaceflight, the mechanisms are not well understood.

Source

The Flight Rules contain more rationale about the protocol

Oral fluid loading with a salt/water mixture similar to body fluids has been shown to significantly reduce detrimental heart rate and blood pressure changes during orthostatic stress post flight. Basing the amount of salt and water (in a proper ratio) or other approved solution on the crewmember’s body mass helps to assure the fluid loading is adequate for larger crewmembers and possibly not too much for smaller crewmembers. ASTROADE ® and other tested isotonic solutions provide plasma volume expansion similar to salt tablets/water and may be easier for some crewmembers to ingest. Flight Surgeons must approve these alternate solutions for individual use prior to flight.

The basis for initiating fluid loading 1 hour prior to TIG includes the amount of time it takes for fluid to leave the stomach, enter the intestine, and be absorbed to affect the plasma volume. A plot for gastric emptying (i.e., the time to leave the stomach and enter the intestine) showed that it takes approximately 60 minutes for nearly all of an isotonic solution consumed to enter the intestine (approximately 20 minutes for 80 percent, approximately 40 minutes for 95 percent, approximately 60 minutes for 98 percent, and approximately 80 minutes for 100 percent). Furthermore, it takes approximately 2 hours from initiation of fluid loading for the change in plasma volume to plateau (at approximately 5 percent). Since landing occurs 1 hour after TIG, change in plasma volume will peak at landing if fluid loading is initiated 1 hour prior to TIG.

From a wave-off perspective, the desire is to delay fluid loading as close to TIG as possible since weather wave-offs often occur just minutes prior to TIG. However, consuming too much too fast can cause gastric discomfort and vomiting.

If fluid loading is initiated 1 hour prior to TIG but deorbit is subsequently delayed, the impact is crew discomfort and increased WCS usage . Although this is undesirable, it is not a safety issue. However, if fluid loading is delayed and the reduction in the change in plasma volume at landing is significant, then crewmembers may feel faint or dizzy upon return to 1g and may not be able to egress unassisted. This can be a safety concern for crewmembers involved with landing the vehicle and for all crewmembers in an emergency egress scenario.

and the table of how much to drink enter image description here

Source

This page from the Deorbit Preparation Checklist shows the protocol in the timeline.

enter image description here

Source

Acronymology:

EI - Entry Interface, 400k feet, the point at which atmospheric effects become noticeable

PMC - Private Medical Conference, a discussion between a crewmember and a Mission Control flight surgeon on an encrypted comm loop

TIG - Time of Ignition (for the deorbit burn)

WCS - Waste Collection System, the shuttle toilet

Note: Presumably ISS astronauts have similar protocols when they return to Earth but NASA has chosen not to publish ISS Flight Rules.

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    $\begingroup$ I am missing the medical reasons for that procedure. As far as I know, in zero gravity fluids are shifted from the human legs to the torso. The body reacts as if there is too much water within the body and the kidneys are producing more urine to reduce blood volume. To be prepared for the return to gravity, the blood volume must be increased. When gravity shifts some of the blood to the legs again, there must be enough blood volume remaining in the torso and head. Without fluid loading, there will be less blood volume in the body than necessary for living in gravity after landing. $\endgroup$ – Uwe Jul 20 '18 at 14:53
  • $\begingroup$ There's a writeup in the first reference I linked that, although I know little about medical stuff, appears to me to say that the cause is not well understood. A couple of theories are discussed. I'll add a line about the causation, thanks. $\endgroup$ – Organic Marble Jul 20 '18 at 15:11
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    $\begingroup$ See your link on page 384 with a graphic about "blood volume changes during spaceflight". The blood shift between legs and torso is explained there. $\endgroup$ – Uwe Jul 20 '18 at 15:23
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    $\begingroup$ Of interest is the fact that, on Space Shuttle missions, salt tablets and related substances were considered critical consumables by Mission Control. During STS-113 in particular, said supply was a concern due to the numerous landing opportunity wave-offs endured during the mission. When asked by Mission Control how many salt tablets the crew still had on board, the mission commander, Jim Wetherbee, responded along the lines of, "Don't worry, we've got enough salt tablets up here to float a battleship!" $\endgroup$ – Digger Aug 20 '18 at 15:10

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