Due to the low gravity, cosmic radiation and alien living conditions, spending a long period of time in space can severely harm any living organism. As space missions are becoming longer and more people are being sent into space, it is only a matter of time before there are serious health effects for astronauts such as musculoskeletal problems, concerns with the cardiovascular system and psychological issues.
Peak bone mass is achieved in early adulthood and decreases with age as our cells become less efficient at remodelling and maintaining our bones. Our bones are constantly being eroded away in bone resorption then remade in bone remodelling before hardening in mineralisation. To keep our bones strong and healthy the rate at which cells replenish bone must be the same or higher than the rate it is removed, otherwise there would be a net loss.
In space, bone deterioration occurs at a rate of 1-1.5 % a month. This may not sound too significant but the average ISS space mission lasts six months, meaning on average astronauts can lose up to nearly 10 % of their entire bone mass from a single mission!
Why does this happen?
Well, as you’ve probably already guessed, without gravity there’s very little strain on our bones because we no longer need to bear any weight, and since the easiest way to get around is floating we’re doing hardly any physically activity. This means the usual micro-cracks that occur when our bones experience strain do not form.
Micro-cracks are necessary to stimulate bone remodelling and to induce our body to generate more cells to do this job (osteoblasts) from bone marrow stem cells. Therefore, if these cracks don’t occur our bones will naturally erode away since the bone replenishing cells are not stimulated nor are they being made as frequently, yet bone eroding cells (osteoclasts) are still going strong.
The overall effect is that bones become weak as the rate of removal will exceed the rate of remodelling. This is very similar to the common bone resorption disease called osteoporosis. This disease results in a decreased density of bones causing them to become brittle and susceptible to fractures.
A bone has a similar structure to a sponge - imagine a healthy bone as a sponge with hundreds of tiny holes, and a bone from an individual with osteoporosis as a sponge with much larger holes. The sponge with larger holes is much easier to compress and tear compared to the more dense healthy sponge which is much more robust.
Furthermore, in addition to the lack of strain as an explanation for bone deterioration, fluid redistribution is another factor guilty of weakening the bones of an astronaut.
The lack of gravity causes the blood pressure of an individual to equalise; for example, on Earth when upright your blood gathers at your feet due to gravity however, when there is no gravity blood circulates freely so more blood collects at the head than usual. The brain interprets this increase in blood pressure as an increase in total fluid volume in the body so, to counteract this, the kidneys will absorb less water and there would be more urination.
Imagine your body as a half filled water bottle; when on Earth gravity pulls the water down so you can accurately measure how much water is in there however, if there was no gravity the water would float around freely so it’s now much harder to you to determine exactly how much water is in the bottle.
The body is trying to reduce its blood pressure and fluid volume by expelling as much water as is needed to return to normal levels. However, this loss of water raises the sodium content of the blood beyond normal levels, since the body doesn’t actually have too much water it’s just that the brain thinks it does.
Now that the body has high levels of sodium in its blood, this sodium will outcompete calcium for resorption in the kidney to return to the blood. As a consequence, sodium levels remain high and the body’s calcium content drops as it’s excreted in urine.
To restore its blood calcium levels back to normal the body will have to deplete some of its stores for example, by eroding bone for its calcium causing demineralisation meaning the bones become soft and weak.
To make matters worse, some hormones are also altered in space including parathyroid hormones which help to control levels of calcium and vitamin D within the bone and blood.
If vitamin D levels cannot be regulated and levels drop it will mean that less calcium can be absorbed into the blood from the intestines, and therefore even more bone will be eroded and demineralised in an attempt to replenish blood calcium levels. The result of this is known as osteomalacia which is softening of the bones caused by a deficiency in vitamin D.
So, obviously this weakening and deterioration of bones isn’t a good thing and we must try to prevent this as much as we can, but how?
Unfortunately, at this moment in time bone deterioration in space cannot be prevented altogether but there are methods that are proven to be effective at minimising it. For example, due to the lack of gravity it’s clearly very difficult to exercise and put enough strain on our bones to cause micro-cracks in space. However, given the right specialised equipment, such as a treadmill and cycle ergometer, it is possible.
Furthermore, an effective way to prevent bone demineralisation is to try returning fluid distribution to that similar of on Earth using a LBNP (lower body negative pressure) device, and supplements can be taken to avoid vitamin and mineral deficiencies.
A number of other countermeasures have and are still being investigated by scientists such as the use of bone formation agents. These stimulate the replication and inhibit the death of bone remodelling cells with the aim of encouraging as much bone regeneration as possible.
In conclusion, technically our astronauts are wasting away in space since their bones are deteriorating significantly, and although we can’t prevent it completely there are methods of minimising this deterioration. So, the question is, would you want to be an astronaut knowing some of the effects microgravity has on your body?
CLUNIE, G., and KEEN, R.W. (eds.) (2014). Osteoporosis. 2nd ed. New York: Oxford University Press.
PLANEL, H. (2004). Space and Life: An Introduction to Space Biology and Medicine. Washington DC: CRC Press.
SCHNEIDER, D.L. (2011). The Complete Book of Bone Health. New York: Prometheus Books.
WOTRING, V.E. (2011). Evidence Report: Risk of Therapeutic Failure Due to Ineffectiveness of Medication. Human Research Program Evidence. Houston, Texas: Johnson Space Centre.
**Image by Bradley Dunn on Unsplash**
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