Introduction to the Effects of Microgravity on the Human Body

David Talas
May 5, 2019 · 10 min read

What is microgravity?

The surface of earth has a 1G acceleration environment and all life on Earth has developed and evolved in this gravity field. We as humans were born in it, we live in it and most of us will die in it. However there are a few of us who have the chance to experience the microgravity environment.

Microgravity can be defined in different ways. “Micro” derives from the original Greek word “mikros”, meaning “small”, so we can consider any gravity field that has lower gravity than Earth to be microgravity. But micro can be also considered as 10 to the power of -6th to be microgravity based on how it’s used in mathematics. In this meaning, it is only true for low Earth orbit (LEO) and most weightless environments, but not to the Moon or Mars.

In a general sense however, we use microgravity for every situation where gravity is smaller than on Earth, and I will use it in this meaning too.

Difference Between Short and Long-Duration Missions

It is crucial to differentiate between short and long-duration missions, because the human body responds differently based on the time spent in microgravity, so the countermeasures are also different.

In the future, when space tourism becomes available in the same way aviation is available today, space tourists will only spend a short amount of time in weightlessness, whereas astronauts (and cosmonauts) will work in space for prolonged periods.

This means that the selection criteria has to be different. More allowing for shorter flights, and more restrictive on longer stays in space.

I have yet to find a clear definition on what short and long-duration really means, but most of the publications and books I have read defined short-duration space flight as shorter than 2–3 weeks, and long-duration space flight as longer than 3 months (around 100 days).

Acute Responses

So what are the effects of microgravity right after you get exposed to it?

The absence of the gravity vector changes important physical factors such as convection, buoyancy, and sedimentation.

As a result, basic human biology also changes, for example, heat exchange in the absence of convection requires some other means of dissipating heat, such as the use of fans to circulate the air flow. If those fans are not properly muffled, this can affect voice communication and hearing threshold shifts.

In the absence of buoyancy, bubbles do not rise; hence, separating bubbles in an intravenous fluid bag or syringe requires the application of an external acceleration to these objects.

In the absence of sedimentation, the otoliths in your ears do not provide information on the body’s position, which leads to the development of space motion sickness.

As for the direct acute effects of microgravity on the human body, for example, the loss of the gravity vector decreases the hydrostatic pressure, and body fluids are redistributed from the lower body to the thorax and the head, expanding the volume in the upper vascular and interstitial (intercellular) spaces.

This will increase the total amount of fluid inside the skull, but as the skull has a fixed volume, the intracranial pressure (the pressure inside the skull) will increase, with the potential risk of brain herniation and death as a result.

The increased vascular volume will also trigger endocrine responses from the atria of the heart, the adrenal glands, and the kidneys. (Will be discussed in a separate article in the future).

Adaptive Responses

Adaptive responses then occur resulting in a decreased plasma volume and red cell production as well as changes in cardiac output and peripheral vascular resistance.

Decreased tissue oxygenation could occur after short-duration space flight as the astronaut returns to Earth’s gravity pull and must respond quickly to his or her “normal” orthostatic requirements. After returning to Earth, orthostatic intolerance and syncope increases; fatigue and dyspnea with exertion could be manifestations of heart failure in individuals predisposed to coronary artery disease.

Each biological system may adapt at different rates. Some will show major changes as soon as the spacecraft reaches orbit; e.g., neurovestibular adjustments with associated space motion sickness symptoms, which take place during the launch to about 3 days in orbit. In contrast, bone loss is detectable only after a month in space flight. The rate of regional bone loss continues at 1% per month, primarily from the weight-bearing areas, and the cumulative bone loss increases with the length of the space flight.

Other physiological systems undergo gradual and progressive changes over different time frames. Short-term adaptation seems to occur in the cardiovascular as well as the neurosensory and neurovestibular systems.

Chronic Responses + Radiation

Chronic responses happen in longer duration space flight, when the human body tries to adapt to the needs of the new environment for long-term to establish a new homeostasis.

For example, due to the lack of gravity loading, astronauts lose lean body mass, as they don’t have to use their muscles to constantly fight against gravity. And just like any skill, you lose what you don’t use, astronauts’ muscles atrophy in space.

It is not a problem by itself, it is just unnecessary in space to have huge muscle mass, so the body tries to go towards it’s new balance. This is basic adaptation, and it helped us survive through evolution. What the human body doesn’t know this time, is that we eventually want to come back to a gravity field (Earth or Mars), so we will need that muscle mass and muscle coordination later on.

Astronauts also experience mass loss in their weight-bearing bones, due to a very similar effect discussed before. No gravity loading leads to a 1% loss in these bones per month. If we take the quickest trip to Mars, which is 6 months, we can count for approximately 6–10% loss in crucial bone mass, that will increase the risk for fractures significantly, and that is something we want to avoid for our Marstronauts who need to be building their new base on the Red Planet, especially as fractures heal much slower in microgravity, partly because of microgravity itself, and partly because of radiation.

With this rate of bone loss, astronauts are also predisposed to renal calculi formation, and that being a serious emergency event, we really need to develop appropriate countermeasures to these effects.

Talking about radiation, it is crucial to state that we have almost no information about the prolonged effects of space radiation. Ionizing radiation can damage the human DNA and it has been observed to suppress the bone marrow, leading to reduced red blood cell mass and immune function. I will write more about the effects of radiation in a separate article.

Countermeasures

A bike on the ISS to maintain aerobic capacity that is also used for research, pre-breathe protocols in support of EVAs (Extra-Vehicular Activity, a.k.a. space walk), periodic fitness examination (PFE), and pre-landing fitness evaluations.

Commander Tim Kopra exercising on the Cycle Ergometer with Vibration Isolation and Stabilization (CEVIS) in the U.S. Laboratory. Photo taken during Expedition 47. Image courtesy: NASA

A treadmill, designed to maintain muscle and bone mass on the ISS. The running surface of the treadmill is used in much the same way as any conventional treadmill, except the user is held to its surface by the Subject Load Device (SLD) and/or by the Series Bungee Systems (SBS), which uses latex rubber tubes, which attach to the shoulders and waist to counter the microgravity environment.

The Treadmill with Vibration Isolation And Stabilization System. Image courtesy: NASA
Oleg Artemyev exercising on the ISS. Image courtesy: NASA
Alexander Gerst doing bench presses on the ARED on the ISS. Image courtesy: NASA

This device is basically a squat rack and bench press machine in one. It uses piston-driven vacuum cylinders along with a flywheel to impact resistive force in the microgravity environment. The system has been developed to improve antigravity muscle mass and strength. It can be adjusted to different movement types like bench presses, deadlifts or squats.

Chris Cassidy doing deadlifts on the ARED. Image courtesy: NASA
Japan Aerospace Exploration Agency (JAXA) Koichi Wakata doing kneeling split squats. Image courtesy: NASA
Terry Virts wears a ‘Penguin’ suit. Image courtesy: NASA

The “Penguin” Suit provides passive stress over the antigravity muscle groups and across selected joints. Cosmonauts wear this device for 8 hours a day, but quantifying the efficacy of this suit is difficult, as all users customize it to their needs, and the applied pressure is not monitored.

Luca Parmitano in the “Chibis” suit. It is still possible to work while using this device. Image courtesy: NASA

The “Chibis” Suit applies negative pressure to the lower body to counteract cardiovascular deconditioning in space. Tolerance of LBNP (Lower Body Negative Pressure) is useful for predicting the degree of orthostatic intolerance that may be experienced during atmospheric reentry and landing.

Antigravity Suits (G-suits) are useful to minimize post-flight orthostatic intolerance and are actively used in aviation. They put pressure on the legs to prevent fluid shift in high Gz environments, so the blood stays in the head and the pilot remains conscious. This suit does not help with the issues of microgravity, but is used in spaceflight so I found it important to mention here.

Electrostimulation of selected muscles is used actively in sports and rehabilitation medicine to improve muscle strength and regeneration, but used infrequently in space medicine, so its efficacy is questionable. Maybe this will change in the future.

A healthy and balanced diet is crucial for the wellbeing of any person, but it is especially important in space. The ISS gets fresh food supplies from Earth regularly, and astronauts conduct a lot of experiments about growing and preparing food in space. To counteract the effects of microgravity, astronauts are on a high Calcium and Vitamine D diet [4], to reduce bone demineralization, and take potassium supplements to prevent cardiac arrhythmias and ease the post-flight recovery period. Orally taken potassium-citrate could prevent the formation of renal calculi, but this latter one is still under investigation.

Space pharmacology is in its infancy, because the spaceflight environment is so vastly different from Earth, new drug interactions and side effects occur, so more research is definitely needed in this field. Fludrocortisone (Florinef), a synthetic aldosterone used to enhance extracellular fluid volume before returning to Earth was used in the U.S. Extended Duration Orbiter Medical Project, but no dosing regimen could have been established without potential adverse effects, so its use has been halted. Isotonic saline solution ingestion has been shown to increase circulating blood volume that is beneficial when returning to Earth. U.S. investigators have also reported promising results with clodronate disodium, a biphosphonate compound that seems to prevent hypercalciuria (high calcium concentration in the urine) during bed rest , and more recently with alendronate-an amino-bisphosphonate.

The ultimate countermeasure to the effects of microgravity would be to eradicate the cause, microgravity itself. By spinning a spacecraft, one can create centripetal acceleration, which is directed perpendicular to the side of the spaceship. There is an amazing calculator online http://www.artificial-gravity.com/sw/SpinCalc/SpinCalc.htm where you can type in different variables necessary for calculations: Radius (R), Angular Velocity (Ω), Tangential Velocity (V) and Centripetal Acceleration (A). Feel free to play around with these values. The smaller the spacecraft radius, the faster it has to rotate to provide the same amount of centripetal acceleration, which increases Coriolis forces and can induce dizziness, nausea and motion sickness. A huge rotating space station in Low Earth Orbit (LEO) would be the next step for space station building, but it is currently not possible due to high cost and engineering problems.

This is a design by Bryan Versteeg, from spacehabs.com, that can be built up gradually and utilizes levels at many different radius levels effectively giving the occupants, multiple levels of gravity.

Conclusion

To conclude, we can say that the problems we face in microgravity are difficult to manage, and pose a lot of threat for astronauts, especially during long-term missions, particularly Mars missions, where there is no instantaneous help from Earth. The data available is also limited due to the expensive nature of space flight. Hopefully, we will be able to get a better understanding of the effects microgravity poses to humans, and find new things about the diseases that are present on Earth and very similar to the ones that astronauts face: e.g. osteoporosis, muscular atrophy, vision and hearing loss.

Sources

  1. Nicogossian, Arnauld E, et al. “Living and Working in Space: An Overview of Physiological Adaptation, Performance, and Health Risks.” Space Physiology and Medicine: From Evidence to Practice, 4th ed., Springer Science+Business Media LLC, 2016, pp. 111–134.
  2. Cycle Ergometer with Vibration Isolation and Stabilization (CEVIS) System (CEVIS) — 01.16.19, https://www.nasa.gov/mission_pages/station/research/experiments/841.html Accessed 17 Feb 2019
  3. Treadmill with Vibration Isolation and Stabilization System (TVIS) — 05.17.18, https://www.nasa.gov/mission_pages/station/research/experiments/976.html Accessed 17 Feb 2019
  4. Smith S.M., Heer M.A., Shackelford L., Sibonga J.D., Ploutz-Snyder L., Zwart S.R. Benefits for bone from resistance exercise and nutrition in long-duration spaceflight: Evidence from biochemistry and densitometry. J. Bone Miner. Res. 2012;27:1896–1906.

Originally published at https://marstronauts.space.

Marstronauts

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