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By: Noopur Ranganathan
Remember that time you were upside down in a roller coaster and, for exactly one second, your body left contact with your seat? The moment of exhilaration that made waiting in the long queue for that ride completely worthwhile?
That was your moment of zero gravity!
Zero gravity can be defined as a state of weightlessness, a condition when the gravitational force is zero. A state of apparent weightlessness, zero gravity is best experienced in outer space. When a spacecraft orbits the earth, the acceleration of the spacecraft, known as the centrifugal force, counterbalances the earth's gravity and neutralizes the gravitational force. In other words, the gravitational pull felt by the spacecraft becomes equal to the centrifugal force exerted on the spacecraft. This cancels out the effect of both forces and results in a state of zero gravity. Astronauts experience weightlessness at this point and tend to float; the ultimate roller coaster thrill every little child dreams of.
Occasional trips to the amusement park for a gravity-defying ride will not harm anyone. However, it is important to note that extended exposure to zero gravity is dangerous to our health. Without the downward pull of gravity, our body cannot function properly.
The human body tends to relax in a state of weightlessness because it no longer fights the pull of gravity. This lack of the gravitational pull alleviates the mechanical strain otherwise endured by our skeletal system. Although the exact trigger is unknown, scientists believe that the reduced stress on bones may be responsible for progressive bone loss, a condition most commonly seen in patients confined to beds due to long term illnesses or old age. Lack of stress on the bones reduces the formation of bone building cells called osteoblast cells. Fewer bone building cells result in a loss of bone mass. While in space, the amount of weight the bones must support gets reduced to zero. While making any movements, the bones are not subjected to the same level of stress they would have otherwise endured while on earth. This breaks down calcium that is normally stored in bones, and releases it into the bloodstream. Increased levels of calcium in the blood, lead to a higher incidence of renal stones. The high level of calcium in the blood also reflects a high level of bone mass loss which makes the bones weak and increases the risk of fracture.
All organisms on earth have sensory and response systems with which they respond to internal and external stimuli. Besides the five main senses (smell, touch, sight, taste and hearing) we humans possess, there is one more powerful sense called the vestibular sense that enables our body to sense movements and use it to maintain balance. Walking a tightrope, pirouetting in a ballet performance, or twisting while diving, all showcase our vestibular system as it actively keeps track of the position of our arms and legs and enable us to perform these tricks without losing balance. This requires the precise integration of our body's sensory and response systems including visual, vestibular, somatosensory (pressure and stretch receptors in our skin), and auditory. Together, these senses constantly collect and interpret data from all over the body. However, loss of gravity negatively impacts this spatial perception. The dizziness or difficulty in walking we experience after spinning around in a circle, clearly demonstrate what happens when we lose this sense even for a few moments. The vestibular system, which is situated in the ears, is comprised of otolith organs and semicircular canals. We can sense the direction and speed of linear acceleration (speed changes without change in direction) due to the otolith organs while the semicircular canals allow us to sense the direction and speed of angular acceleration (speed changes along with change in direction). The vestibular apparatus in the human ears is designed to work with earth's gravity and provides sensory information about motion, equilibrium, and spatial orientation. Take gravity away and we can no longer figure out where the ground and ceiling are!
Gravity ensures that the blood in our body maintains an optimum blood pressure level. While standing, the blood pressure in our feet is as high as 200 mm Hg (millimeters of mercury). In the brain however, the pressure is only 60 to 80 mmHg. Take gravity away and the blood pressure equalizes around 100 mmHg throughout our body. Our face puffs up with fluid and our legs thin out because the fluid drains out. The shift to higher blood pressure in the head triggers an alarm that the body has too much blood. Increased blood pressure can make the blood vessels bleed. Optic nerves can swell and this can impair the vision. High blood pressure can lead to a stroke that can damage that area in the brain that processes images. Thus, gravity acts as an important force that helps to maintain the correct pressure in the right places in our body.
A living cell exchanges nutrients and wastes from its watery environment through the process of diffusion which is effective only over a specific distance. This limitation impacts the size of the cell. Nutrient absorption is therefore much more effective in smaller sized cells. Cell biologists Marina Feric and Clifford Brangwynne of Princeton University have proven that in order to optimize nutrient absorption in cells, gravity must play a significant role in determining the size of the cell. Their paper, published in the 2013 October issue of Nature Cell Biology, clearly shows how eukaryotic cells are small in diameter (less than 10 microns) due to gravitational forces. When the cell grows bigger than this size, it is subjected to gravitational forces that require scaffolding or support within the cell nuclei to stabilize the internal components. Their research confirms that larger cells have the extra actin mesh (that acts as an additional support), a key component missing in all other smaller sized cells.
Clearly gravity has played a vital role in the evolution of life on earth over billions of years. Studies such as these recognize the importance of gravity and its phenomenal impact on life on earth. However mankind is now poised to explore the universe beyond our planet and a key concern of this ambitious venture is the effect of zero gravity on human life. NASA embarked on this mission through its Twins study project, a novel experiment in the history of mankind that attempted to study the effects of zero gravity on astronaut twins Mark and Scott Kelly.
Scott spent an extended period on the International Space Station and returned back on Earth on March 2, 2016. NASA scientists intend to compare the results of his body vitals with his twin Mark who remained earthbound. The long-term effects will take some years to be known. But the short-term effects are here for us to see. Astronaut Scott Kelly reported loss of bone mass, atrophied muscles, and redistribution of blood within his body that has strained his heart. Thankfully, most of these are reversible through rehabilitation and exercise. Despite physical setbacks and the increased risk of cancer, due to extended exposure to high levels of radiation, Scott Kelly is proud to be a part of this groundbreaking mission and happy to contribute to scientific discovery.
Ever since our planet was born, it has been subjected to a lot of changes. But there is one force that has remained constant all through these years and that is gravity. There is enough evidence that demonstrates how life on earth has been impacted by gravity. All biological processes have evolved under the ever present force of gravity such that even temporary changes in gravity instantly make a noticeable impact. With our sights set on establishing human life beyond Earth's gravitational pull, it becomes imperative to understand the detrimental effects of zero gravity and devise ways to counter those. The day we resolve those concerns, we will successfully pave the way for mankind to venture into deep space.
References
Azvolinsky, Anna. "Gravity plays a role in keeping cells small." princeton.edu. October 24, 2013.
Feitlinger, Sarah Benton. "Scott Kelly's Historic 'Year in Space' Mission Brings Us One Step Closer to Mars." dogonews.com. April 8, 2016.
Howell, Elizabeth. "Weightlessness and Its Effect on Astronauts." space.com. September 30, 2013.
LE, Freed, and Vunjak-Novakovic G. "Spaceflight bioreactor studies of cells and tissues." ncbi.nlm.nih.gov. 2002.
---. "Spaceflight bioreactor studies of cells and tissues." europepmc.org. 2002.
NASA. "Bones in Space." nasa.gov. August 19, 2004.
---. "Human Vestibular System in Space." nasa.gov. February 26, 2004.
---. "Space Bones." science.nasa.gov. 2001.
Wei-Haas, Maya. "What Happens to the Human Body in Space?" Smithsonian.com. March 1, 2016.
Image Credit: NASA (Wikipedia), Robert Markowitz (Wikipedia)
