The physiological effects of zero gravity have long been investigated and remain an area of active inquiry ¹. Zero gravity has both short- and long-term effects on the human body, upon entry and following prolonged exposure ² , and is known to affect the entire human body, including the neurovestibular, cardiovascular, musculoskeletal, and hemato-immunological systems. 

 

The vestibular apparatus in the human ear partially operates in response to Earth’s gravity, providing sensory information about motion and spatial orientation. As such, the loss of gravity directly impacts its function ³, leading to loss of balance, dizziness, difficulty walking, and “space motion sickness”, in addition to more severe sensorimotor coordination disorders.  

 

Gravity further ensures an optimum blood pressure level. While standing, the blood pressure in our feet reaches 200 mmHg, while that in the brain remains under 80 mmHg. Without gravity, blood pressure reaches 100 mmHg throughout the entire body, as the capillary blood volume increases by 25%, and the intraocular pressure doubles. The resultant increased cerebral blood pressure may result in strokes, alongside swollen optic nerves, which can impair vision.  

 

As additional results of bodily fluid shifts, zero gravity leads to decreased circulatory blood volume, reduced cardiac size, and reduced aerobic capacity, as well as orthostatic intolerance. These symptoms reflect a state of cardiovascular deconditioning ⁴.  

 

Zero gravity also results in the reduced use of muscles which work to steady the body against gravity. Without a proper diet and exercise routine, astronauts lose significant muscle mass. 

 

Furthermore, prolonged exposure to zero gravity leads to a negative calcium balance in the body, leading to bone loss; weight-bearing bones lose up to 1.5% of mineral density per month during spaceflight. After returning to Earth, while the risk of fracture does not increase, bone density may not always be completely re-established by rehabilitation. NASA has demonstrated that bisphosphonate medications are effective at preventing such bone loss; in parallel, NASA is assessing the potential of potassium citrate to fight the physiological changes that may increase the risk of developing kidney stones.  

 

Prolonged exposure to zero gravity may also lead to anemia linked to a slower production and decreased life span of red blood cells as a result of altered cell membrane composition and increased lipid peroxidation products ⁵,⁶. 

 

Certain data have further demonstrated that zero gravity dysregulates one’s immunological response. In particular, prolonged exposure to zero gravity reduces T lymphocyte counts, attenuating their cytotoxic function and facilitating the reactivation of the latent herpes virus ⁷.   

 

Finally, MRI scans have also revealed that structural changes occur in the brain as a result of zero gravity. In particular, zero gravity results in volumetric gray matter decreases, including large areas around the temporal and frontal poles and around the orbits, alongside bilateral focal gray matter increases in the medial primary somatosensory and motor cortex ⁸. Additional research has identified that the cerebellum and vestibular-related pathways are affected as well ⁹. 

 

In addition to ongoing functional task testing and other forms of monitoring, it is critical for astronauts to engage in regular aerobic and resistive exercise in order to maintain healthy neurovestibular, cardiovascular and musculoskeletal systems. To this end, software-generated workout partners may be used to motivate astronauts to exercise regularly for longer space missions 10.  

 

The effects of zero gravity on the human body remains an area of active research with direct and visibly critical impacts on human health both in space and on Earth.  

 

References 

 

1. Wolfe, J. W. & Rummel, J. D. Long-term effects of microgravity and possible countermeasures. Adv. Sp. Res. (1992). doi:10.1016/0273-1177(92)90296-A
2. Iwase, S., Nishimura, N., Tanaka, K. & Mano, T. Effects of Microgravity on Human Physiology. Beyond LEO – Hum. Heal. Issues Deep Sp. Explor. [Working Title] (2020). doi:10.5772/INTECHOPEN.90700
3. Cosmic Travels Inc.: The effect of zero gravity on the human body | SciBytes | Learn Science at Scitable. Available at: https://www.nature.com/scitable/blog/scibytes/cosmic_travels_inc_the_effect/. (Accessed: 5th June 2022)
4. Grigoriev, A. I., Morukov, B. V. & Vorobiev, D. V. Water and electrolyte studies during long-term missions onboard the space stations SALYUT and MIR. The Clinical Investigator (1994). doi:10.1007/BF00189308
5. Alfrey, C. P., Udden, M. M., Leach-Huntoon, C., Driscoll, T. & Pickett, M. H. Control of red blood cell mass in spaceflight. J. Appl. Physiol. (1996). doi:10.1152/jappl.1996.81.1.98
6. Rizzo, A. M. et al. Effects of long-term space flight on erythrocytes and oxidative stress of rodents. PLoS One (2012). doi:10.1371/journal.pone.0032361
7. Crucian, B. E. et al. Immune system dysregulation during spaceflight: Potential countermeasures for deep space exploration missions. Frontiers in Immunology (2018). doi:10.3389/fimmu.2018.01437
8. Koppelmans, V., Bloomberg, J. J., Mulavara, A. P. & Seidler, R. D. Brain structural plasticity with spaceflight. npj Microgravity (2016). doi:10.1038/s41526-016-0001-9
9. Van Ombergen, A. et al. Spaceflight-induced neuroplasticity in humans as measured by MRI: What do we know so far? npj Microgravity (2017). doi:10.1038/s41526-016-0010-8
10. HRR – Task – Cyber Partners: Harnessing Group Dynamics to Boost Motivation for More Efficient Exercise. Available at: https://humanresearchroadmap.nasa.gov/tasks/task.aspx?i=1639.