SPACE PHYSIOLOGY
We have seen that the volume of fluid in the body and the fluid pressures inside the veins and arteries are well-regulated by the various components of the renal/endocrine system. On Earth, gravity affects the distribution of fluids inside the body by pulling the various body fluids down towards the feet. Upon entry into space, the virtual absence of gravity causes these fluids to redistribute upwards towards the chest and the head. This perceived increase in fluid volume in the upper part of the body causes multiple physiologic changes in the kidneys and associated fluid-regulating hormones, in the cardiovascular system, and in the red blood system. We have already covered this "headward fluid shift" in detail, particularly in terms of the cardiovascular and erythropoeitic changes that take place. Now, we will discuss how your kidneys, working with certain hormones, adapt to space flight and regulate the levels of fluids and electrolytes in the body. In fact, it turns out that changes in the body's fluid volumes begin on the launch pad even before space flight.Pretend you are an astronaut preparing for launch. You are strapped into your seat in the shuttle at least three hours before launch; you are wearing your Launch and Entry Suit (LES) with an attached helmet and visor. The LES is a pressurized space suit that provides pure oxygen for breathing in the event of a bailout at high altitude. In addition, the LES is a buoyant suit (it floats). You are also wearing a parachute. With all of these safety measures in place, you are still not protected from the fluid elimination function that your body will soon begin!
Figure 8. The launch position in the space shuttle.As you are seated with your head back and your legs and feet inclined upward, the fluids in your body are already beginning to redistribute. While sitting in the launch position, your body begins to respond to the increased volume of fluid that it senses in the upper part of the body and it begins to tell you so. As part of this response, you feel as though you must go to the bathroom, but, of course, that is impossible given your situation. At this point, it is time to be thankful for the "diaper" that all astronauts must wear during launch. It is the finest and most absorbent diaper ever made! This is important, because you stand to lose up to about a liter of urine.
All astronauts wear these diapers and all astronauts experience the need to urinate during this early pre-launch phase. Even if the astronauts attempt to dehydrate themselves on the day prior to launch (which many of them do), the fact that the kidneys work very hard to maintain the production of about one milliter of urine per minute under normal Earthbound circumstances means that the astronaut's bodies will continue to produce at least a minimum amount of urine. And the fact that the body is oriented in the head-down, legs-up launch position means that, as fluids are pulled upwards by gravity, the body will respond by attempting to eliminate what it detects as a "flood" of fluid in the chest, resulting in even more urine production. All of this results in the need to urinate. Besides, as you probably have experienced, during times when you are excited and nervous (which you can imagine the astronauts feel before launch), the need to urinate is exacerbated! Primarily, however, the "upward" fluid shift created by the astronaut's launch position is responsible for the size of the response that occurs.
Once you arrive in space, you will notice that, even though you continue to slowly eliminate more fluid, you do not feel thirsty. Scientists believe that the kidneys and endocrine glands ad just the fluid-regulating hormones to stimulate the excretion of salt and minerals that will help to diminish your desire to drink. Later in the mission, your kidneys and hormones establish new "space-normal" levels of salts, minerals, and hormones appropriate for the reduced fluid volume.
The renal/endocrine system works in conjunction with the cardiovascular system. Experiments that are carried out to study the heart and blood system are naturally linked to experiments that study thekidney's blood cleansing system and the release of fluid-regulatinghormones and salts. This makes sense and further underscores our need to look at the body as an integrated system. For the remainder of this chapter, we will examine a comprehensive space flight experiment that was designed by Dr. Carolyn Leach from the NASA Johnson Space Center in Houston, Texas, to investigate how the kidneys and the associated electrolyte and hormone balances in the body contribute to the adaptation of the body to space flight. This investigation measured:
- changes in kidney function, including glomerular filtration rate (GFR) and plasma flow through the kidney;
- changes in body fluid volumes, including plasma volume (PV), extracellular fluid volume (ECV), and total body water (TBW);
- hormone, electrolyte, and other chemical changes;
- body mass measurements and food and water intake.
Hypothesis 1
Adaptation of kidney and electrolyte fluid-regulating functions in microgravity occurs in two stages: an acute, rapid stage (hours to days) and an adaptive, longer-term stage (days to weeks).
Hypothesis 2
The overall renal/endocrine adaptation process in space is primarily a general response to a headward shift of body fluids.
Remember to keep these hypotheses in mind as we review some of the important measurement sets that were actually carried out during Dr. Leach's experiment in space. We will return to them at the conclusion of our examination of Dr. Leach's experiment.