The Impact to Human Physiology Outside of the Earth Gravitational Field


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As Neil Armstrong once said “Mystery creates wonder and wonder is the basis of man’s desire to understand”. The great unknown of space travel has intrigued and captivated people for generations and in recent years’ development in technologies have allowed more and more exploration into a world outside of our own. However, it’s not all plain sailing and there are many things to take in to consideration before setting off outside of the stratosphere. For example how the human body copes with being in space, what exactly happens to your body in space, and what are the risks?

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Different Factors

There are several areas related to travel and residence in space that are all currently being researched. The risks all vary depending on different factors including how long the period in space, the distance from the earth and exposure to radiation. For example a six month trip to the international space station will have a different set of risks in comparison to a three year round trip to mars. Distance from the Earth is a real problem not only because the way the body reacts to zero gravity but also the time taken to travel vast distances to other planets can span a large part of the space traveller’s life.

Three Gravity Fields

Human beings have evolved to survive perfectly within the gravitational field of the Earth. On the surface of Mars humans would experience just one third of the Earth’s gravity and the return home would have to involve a lot of rehabilitation (Williams, 2016). During the voyage to get to Mars in the first place humans would encounter three different gravity fields. Even though that sounds easy enough the transition from one gravity field to another is not. It has an effect on your spatial orientation, balance, locomotion, hand-eye coordination and suffer motion sickness.


NASA has learned that without the optimum amount of gravity acting upon the human body, bones lose vital minerals. Density also drops at a rate of over 1% per month (known as Spaceflight osteopenia). Which is very similar to the rate of bone density depletion in elderly people on earth. The most worrying part of this is that on return to Earth, the damage that would have been caused to an individual’s bones may be irreversible, even if proper rehabilitation techniques are employed. This in turn leads to a much greater risk of osteoporosis-related fractures later in life (Abadie, Lloyd and Shelhamer, 2018).


The lack of gravitational force acting on the body also causes muscle wastage or atrophy. Gravity also informs muscles on how strong they need to be. In zero gravity the human body will perceive that it doesn’t need the muscles anymore as they are not working against any gravitational force. In particular muscles used in the fight against gravity, like the ones in the calves and spine, used to maintain an upright posture can lose around 20% of their mass if not exercised. In zero gravity muscle wastage can be as high as 5% a week (, 2001) this can also increase the chance of tendonitis and fat accumulation. The muscles of the human heart are also hugely affected by the lack of gravity. When astronauts spend long periods at zero gravity, their hearts can actually become more spherical and lose muscle mass just like the rest of the muscles in the body. This is obviously not desirable and could lead to any number of cardiac problems. The spherical shape (caused by lack of gravity) of the heart may indicate the heart functioning less efficiently, however, the condition appears to be temporary as astronauts hearts have returned to their original shape after they returned to Earth. Long-term effects of this condition are unknown (Lewis, 2014).

Cardiovascular System

Space travel also wreaks havoc with the blood circulation in human beings. The cardiovascular system is designed to pump blood around the body against the force of gravity, which normally pulls blood towards the feet. Raising blood pressure, giving astronauts red, puffed faces and can lead to damage of eyesight. (BBC Guides, 2018). In human’s usual upright position, gravity determines a continuous pattern of fluid distribution with higher arterial pressure in the feet (200 mmHg) and lower in the head (70 mmHg) relative to the heart at (100 mmHg). In space however, due to the lack of gravity, blood is free to move up to the chest and head. This redistribution of blood toward the head causes altered responses of baroreceptor (used to monitor blood pressure and volume), nervous and endocrine systems (Katkov and Chestukhin, 1980).


The brain is also subject to adverse conditions whilst in space. Microgravity has also been suggested as an instigator in the neurological dysfunction related to space travel. Gravity and changes in gravitational influence have been observed to have an effect on cells at a cytoarchitectural level. The spatial relationships of intracellular organelles and cytoskeletal structures are influenced by gravity. This in turn affects biochemical and biosynthetic pathways. This can have a negative effect on the transportation of proteins, RNA transcription and even the replication of DNA itself (Sarkar et al. 2006).

A study of 15 astronauts who had been on missions involving about six months of orbital freefall. Found that tissues at the back of the eyes that surrounding the optic nerves were swollen and warped or distorted. They would remain like this for a few weeks after their return to earth. This change could explain why nearly fifty percent of travellers who have spent long periods of time in space are more likely to develop problems with their eyesight (Letzser, 2018). As previously mentioned the raised blood pressure in the head, from the lack of gravity, also has a negative impact on eyesight. According to NASA, about two litres of fluid shifts from an astronaut’s legs toward their head while in space. Ultimately putting the brain under more pressure and affecting the eyes and vision (Strauss, 2016).

Preparation for Space Travel

In an effort to combat the negative impact the lack of gravity has upon the human body. Astronauts are required to exercise their bodies for at least 2 hours per 24 hour period. However, exercise in space is not as beneficial as it is on Earth, due to the microgravity. For example, lifting 100kg on Earth is much more strenuous than it would be to lift 100kg in space. Therefore exercise equipment needs to be specially designed (NASA, 2018). An example of this is a treadmill that’s uses elasticated straps to pull the user down to the treadmill, allowing the user to ‘run’. Resistance training is the most effective type of exercise in microgravity, due to the lack of gravity to work against, other types of force to work against need to be introduced. This works for muscle tissue and skeletal strength it also exercises the heart and cardiovascular functions. As on Earth Biophosphates are also used in partnership with exercising to help reduce space flight induced bone loss. They work by slowing the rate at which cells which break down bone (osteoclasts) work, which in turn allows the bone building cells (osteoblasts) to work more effectively (Harding, 2018).

Artificial Gravity

On a cytoarchitectural level however, is where the problems for human space travel still remain. Even though it is possible to create “artificial gravity” by spinning the space craft or station. Due to the spinning, centrifugal force acts to pull inhabitants to the outside of the craft. In theory this process could be used to simulate gravity, however it wouldn’t act in the same way because large coriolis forces would be present. An effect whereby a mass moving in a rotating system experiences a force acting perpendicular to the direction of motion and to the axis of rotation (, 2018). The presence of this force would then lead for things to fall down with a curve, rather than a straight down like they would on Earth. So even though the presence of gravity could be imitated, the effect it would of have on a cellular level would be different to that of the gravity on Earth. It is widely accepted that the only way to create gravity is the presence of immense mass. Even moving to another gravitational filed wouldn’t always work in the long run as the gravitational strength of another planet, for example mars, is different to Earths (because of its smaller mass). Which in turn still leads to a difference in organelle positioning within cells and the distribution of bodily fluids for example.


So as our space travelling technologies continue to become more and more advanced. The one thing holding us back from travelling to the far reaches of our solar system may in fact be the human body itself. Not purely because of the fact our lifespans are relatively short in comparison to the time needed to reach other planets. Human beings just aren’t designed to be in space but as Buzz Aldrin said “Exploration is wired into our brains. If we can see the horizon we want to know what’s beyond.” With NASA planning to attempt Mars landings around 2030 who knows what lies in store for human space exploration.

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