THE SPACE FLIGHT INVESTIGATION

Before experiments were carried out in space, scientists did not know the mechanisms that cause changes in fluid and electrolyte balance during space flight. Did the kidney actually change the way it operates in space? Did hormones influence fluid retention or fluid elimination in space in the same way that they do on Earth? Are electrolyte concentrations maintained at the same levels in space as they are on Earth? These are only a few of the many complicated questions that Dr. Leach's study was designed to examine. But before we begin our discussion of some of her results, let's see how her study was designed to eliminate certain confounding factors, or factors that have interfered in the past with the scientific examination of fluid and other dynamics of the body in space.

On many space flight missions, astronauts have experienced space motion sickness, a malady similar in some respects to motion sickness on Earth, which can decrease their desire to eat or drink. You know how it feels if you have ever felt queasy and sick to your stomach; your normal diet of both solids and liquids is interrupted. Also, you may have heard from the doctor or your parents that when you actually get to the point of vomiting, you have become very dehydrated. These same things can happen early in a space flight: such space motion sickness can be a problem for the scientist who is interested in understanding normal fluid intake and output dynamics of space flight. A related problem can occur when the astronauts take certain medications to help reduce the effects of space motion sickness. Such medication introduces chemicals into the body that can alter the results of an experiment. It was fortunate that Dr. Leach's experimental results were obtained from astronauts who did not experience significant space motion sickness. Because the astronauts did not report sickness as being a problem, they did not have to use medication.

Another factor that should be understood in interpreting body fluid volumes and electrolyte levels is the dietary change that occurs in space. In order for scientific results to be accurately interpreted, essentially all nutrients and any other chemicals taken into the body from the food and beverages that the astronauts ingest must be well documented. Diet, and its effect on renal/endocrine function, can be monitored quite closely by the use of a diet log, which is similar to a diary that one keeps to record everything one eats and drinks. As an example, if you eat a lot of salty popcorn at the movies without drinking anything, your glomerular filtration rate (GFR) is likely to decrease so that water is preserved in your system to dilute the salt. You will probably not be interrupted during your movie to go to the bathroom! On the other hand, if you drink a large soda during the movie without eating something salty, the GFR is likely to increase in response to the need to eliminate an excess of fluid in the body. You know what this means! Of course, these examples are simplified. In reality, the effects of diet on GFR and other renal/endocrine functions are complicated and involve hormonal and other chemical interactions occurring at many levels.

Another factor that, in the past, has interfered with the study of fluid regulation in space is preflight self-imposed dehydration. This means that during the immediate preflight period, certain astronauts have been known to stop drinking fluids so that they do not have to urinate as often after all, you can imagine how uncomfortable a very wet diaper can be! (Remember from a previous section, it was mentioned that all astronauts wear a diaper because of the long wait on the launch pad before they finally take off.) Previous renal/endocrine results have probably been altered by this artificial attempt to influence fluid dynamics. In a well-designed scientific study of the body, only natural events should be taking place so that natural events can be measured. For Dr. Leach's study, the astronauts were instructed to hydrate themselves normally before the flight and during the flight. That is, they were told to hydrate themselves ad libitum (a Latin term for "in accordance with one's wishes"!. They were asked to drink freely if they became thirsty.

Figure 11. Astronauts "fluid load" before returning home after a mission. This ingestion of extra fluid is necessary to counteract some of the physiological effects of a sudden return to gravity.

Finally, for the results that you are going to see, the postflight fluid intake level was artificially influenced, but for a very good reason. The astronauts were asked to fluid load, which means that they drank about two liters of saline solution just before reentering the Earth's atmosphere (Figure 11). Saline is a water and salt, combination; the salt is added so that the body will retain the water. This fluid loading approach seems to be in contradiction to the maintenance of "natural" effects that scientists normally want to measure. The difference here is that, while in space, the astronauts have established a "space-normal" fluid level in the body that is lower than their "Earth-normal" fluid level. These space-normal fluid levels, while fine for the space environment, create problems for the astronauts when they return to the clutches of gravity here on Earth). Astronauts experience orthostatic intolerance (the inability to maintain a standing posture), and they can become very weak for a period of time. We discussed this problem in the cardiovascular chapter. Therefore, fluid loading is considered a countermeasure (counter = reverse, measure = effort), which is an effort taken to counteract or reverse these unfavorable effects. Dr. Leach measured certain renal/endocrine parameters in astronauts that had not fluid loaded and in astronauts who had. Therefore, she was able to compare and determine how well this countermeasure works.

Preflight, inflight, and postflight portions of Dr. Leach's experiment were all done on two space missions. We will review two measurement sets from one of those missions to compare certain aspects of renal/endocrine behavior before, during, and after space flight. For each measurement set, we will include information about why the measurements are important, the equipment or technique used to make the measurement, the expected results, and the actual results. These measurement sets were designed to measure changes in:

• glomerular filtration rate (GFR) and renal plasma flow (RPF); and
• hormone (ADH and aldosterone) and electrolyte levels (Na⁺).