Hello again all! I finally give you the continuation story to the post “Gravity and the Sensation of Weightlessness” (https://odysseyofadreamer.wordpress.com/2011/04/10/gravity-and-the-sensation-of-weightlessness/). There are many physiological changes that astronauts experience in space, so if the title caught your attention, read on! We recently discussed the misconceptions that exist concerning weightlessness in earth orbit. It is not a lack of gravity, but the astronauts being in a state of free fall with only the non-contact force of gravity acting upon the astronauts. (See the above link for more detail.)
Now, let’s discuss the physiological effects on the astronauts. I am sure we are all aware that sometimes astronauts feel a bit queasy during the first few days of a spaceflight (ask Russell Schweickart, Apollo 9, who was so ill that his EVA was postponed, after almost being cancelled completely). Now let’s look at what those are and why.
SPACE SICKNESS
Space sickness (Space Adaptation Syndrome/SAS or Space Motion Sickness/SMS) refers to the nausea, vomiting, vertigo, lethargy, headaches, and sense of malaise that is experienced by about half of all astronauts. This was less reported in the days of Mercury and Gemini, leading to the conclusion that perhaps astronauts did not experience this problem until they had more room to move and float about during Apollo, Skylab, and the Shuttle programs. It is not so much weightlessness itself that causes this syndrome, but instead how the body adapts to changing gravitational force. Interestingly enough, susceptibility to motion sickness on earth does not correlate to experiencing SAS. Symptoms generally last from 24-72 hours (this is why EVA is not scheduled for the first 72 hours of the mission.) Sometimes astronauts use medications such as promethazine and scopolamine, but these medications also have side effects (drowsiness, dry mouth, and other anticholinergic side effects). It is also helpful in not exacerbating symptoms for the first 72 hours if astronauts try to remain vertical, refraining from flipping/turning rapidly, move slowly, and trying to keep their head position fixed as much as possible.
FLUID REDISTRIBUTION
Upon experiencing weightlessness in earth orbit, the normal hydrostatic pressure gradient of the venous vasculature is lost. (Higher mean arterial pressure in the lower extremities decreases towards the upper body, which normally prevents blood from pooling in the lower half of the body.) Intravascular volume migrates from the lower extremities towards the head, which is visually noticeable soon after weightlessness begins as facial edema, swelling of neck veins, decreased size of the lower extremities. Low pressure baroreceptors in the heart perceive this shift in volume as a hypervolemic state. The body will then promote diuresis and decrease thirst, by changing the secretion of hormones from the kidney, thus affecting electrolyte balance. This fluid redistribution contributes to several other physiological phenomenon experienced by astronauts, such as nasal congestion, space motion sickness, and electrolyte imbalances. Electrolyte imbalance is a concern in that arrhythmias can result. There have been cases of a crew member experiencing sustained ventricular bigeminy during EVA, 16 PVCs a minute during shuttle reentry, and a 5 beat run of ventricular tachycardia during Skylab. (Note: I was unable to find the overall incidence of arrhythmia during spaceflight; one source said uncommon, though I also read that EKG changes are common.) Care is taken to screen astronaut candidates for existing cardiovascular problems.
NASAL CONGESTION
This sensation is experienced by astronauts very shortly after weightlessness is experienced. As fluid redistribution occurs, the superficial vasculature swells, and capillary permeability increases, astronauts experience fullness in the sinus cavities lasting from a couple of hours to a couple of days. During Skylab flights, astronauts reported improvement in these symptoms from cycle exercise, as this would promote fluid return to the lower extremities.
SLEEP DISTURBANCE
Decreased sleep and fatigue have been reported by astronauts, and one survey reported that up to 50% of astronauts used sleep medications at some point during their mission. Circadian rhythm, the normal 24 hour physiological cycle that regulates processes such as sleeping/waking and temperature, is disrupted during spaceflight. Astronauts experience a sunrise or sunset every 45 minutes, rather than the normal triggers that maintain circadian rhythm, sunset in the evening and sunrise in the morning. Astronauts have also reported feeling either too warm or too cold, as well as noises or other disturbances. For a couple of weeks prior to their scheduled flight, astronauts are put on a strict sleep/wake schedule, in order to help them sleep on the schedule necessary for the flight workload.
INCREASED FLATULENCE
Though the exact reason may be undetermined, astronauts may also experience the feeling that their stomach is displaced toward their diaphragm.
CHANGES IN IMMUNITY
Astronauts experience weakened immunity in space, thus explaining why quarantine before spaceflight is so important. Various components involved in the immune response, such as T cells are decreased, leading to increased susceptibility to germs, including viruses in the body that may normally be suppressed on earth.
MUSCLE ATROPHY (including changes in cardiac muscle)
Muscle deconditioning is a major concern, particularly for astronauts that spend extended amounts of time in space. This includes skeletal muscle and cardiac muscle. As discussed before, on earth gravity is “felt” because of the opposing normal force. To stand up on earth, we have to overcome the normal force, which is not the case in space. The same applies with cardiac muscle, as the heart does not have to work as hard against resistance, thus deconditioning results from decreased demand. Astronauts stick to a specific exercise routine before, during and after spaceflight in order to decrease adverse effects on the musculoskeletal system.
OSTEOPOROSIS
Our skeleton is necessary for structural support, calcium metabolism, production of blood components in the marrow, and protection of internal organs. Cells called osteoblasts build new bone, while osteoclasts break down old bone; the balance turnover of these processes maintains the skeleton so that no net bone loss exists. Activity of osteoblasts is not stimulated during spaceflight, thus disrupting the cycle of bone turnover. (This provides another concern that hypercalcemia resulting from breakdown of bone along with decreased fluid intake can promote the formation of kidney stones.) Astronauts can lose 1-2% of their bone density each month if preventative measures are not taken, which is greater than two times the rate of bone lose in a normal adult over an entire year. Unlike older people affected with osteoporosis on earth where osteoporosis is irreversible, astronauts may regain almost all of their bone density with therapy. Much research is still being done in this area, which will benefit patients suffering from osteoporosis. Astronauts are required to participate in a very specific exercise routine during spaceflight help guard against disuse osteoporosis.
In conclusion, there are a variety of physiological changes that astronauts experience during spaceflight, this being merely a brief overview. While the majority of these effects are temporary, much research is being done to protect astronauts against more serious concerns, such as osteoporosis and radiation exposure. There are many measures taken to protect and maintain the health of astronauts before, during and after spaceflight. Research that is done on these phenomenon can be (and has been) used to help those afflicted with similar conditions back on earth. Candidates for the Astronaut Program are carefully screened to identify conditions that could be dangerous for them during spaceflight.
Resources:
M Barret, SL Pool. “Principles of Clinical Medicine for Spaceflight.”
G Clément, MF Reschke. “Neuroscience in Space.”
http://www.nasa.gov/audience/foreducators/postsecondary/features/F_Bones_in_Space.html
http://www.nasa.gov/exploration/humanresearch/areas_study/physiology/index.html
Odyssey of a Dreamer 
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