How Does The Human Body React to Being in Space?

· Astronomy, Biology, blog, Health, Space

How Does The Human Body React to Being in Space?


Following on from Issue 19 which looked at the International Space Station (ISS) and Tim Peake’s life and work on it1, this article aims to look into the fundamental ways that an astronaut’s biological systems change in space, down to the level of their individual cells  and metabolism.

Physically and Mentally, Astronauts Adapt Rapidly to Spaceflight

The changes to an astronaut’s cardiovascular, muscular and psychological health are well documented thanks to the work of NASA’s Human Research program looking at astronauts on the ISS, and are already the subject of several articles2-5.

As Tim Peake himself emphasised at a recent interview (with BBC Inside Science6) these changes occur not because the human body struggles to cope with the demands of spaceflight, but because it is brilliant at adapting to micro-gravity environment on board the ISS.

Circulatory adaptation:

An astronaut’s circulatory system experiences significant change when an astronaut first arrives at the ISS. There is a fluid shift of 2 litres of fluid out of their legs towards their head, with leg circumference decreasing by 10-30%. The baroreceptors (blood pressure monitors situated above the heart and in the neck) therefore detect a high blood volume in the upper body. As a result the astronaut’s blood volume decreases, and the red blood cell count also decreases. Furthermore their heart no longer has to work as hard against gravity so cardiac output decreases – the heart muscle atrophies. Cosmic radiation has also been linked to the development of cardiac diseases caused by tissue degeneration.

Skeletal and muscular adaptation:

Their skeletal and muscular systems also undergo obvious change. The lower demand on bones that comes with being in a microgravity environment leads to a reduction in bone density of around 1% per month (this mimics the bone density loss as people age, although astronauts lose bone at a faster rate). This bone density loss is most focused on load bearing bones such as the femur, tibia, pelvic girdle and spine. The spine also stretches (famously Scott Kelly’s spine grew by two inches in during his year in space) whilst the astronaut is in space due to the relaxing of spinal discs between the vertebrae. The muscles also atrophy by as much as a 50% mass loss. Muscular composition changes so there is a greater proportion of weak fast twitch fibre compared to stronger slow twitch fibre.

To counteract these changes astronauts exercise for 2 hours per day, on apparatus such as the Combined Operational Load Bearing External Resistance Treadmill (COLBERT). Compression cuffs have been trialled to counteract fluid migration.  To help research astronauts have been taught to perform ultrasound scans on each other’s hearts.

Psychological Effects:

There are psychological effects of being in an isolated, confined space in a stressful environment under continuous intense scrutiny; these can lead to a decline in mood and enthusiasm. Stress is often exacerbated by sleep deprivation, possibly triggered by unusual sleep posture, an altered light/dark cycle (the ISS orbits once every 90 minutes) and flashes on the eyelid caused by cosmic rays.

Vestibular system:

‘Space Adaptation Syndrome’, related to motion sickness, can trigger nausea and headaches. This is because the fluids in the semi-circular canals in the ear are not pulled down, and the hairs in the cochlea responsible for balance do not react to motion as they would to a similar motion on Earth. Problems are also experienced when readapting to gravity on return. Upwards fluid movement also leads to a greater intracranial pressure which may be responsible for vision problems as the eyeball and optic nerve are squeezed.

Astronauts face many other unexpected difficulties7, from blisters on the top of their feet where their shoes rub, to upwards fluid movement giving them a permanent head cold and stunting their sense of taste. For this reason astronauts prefer foods that contain capsaicin (the substance that makes chillies hot) which triggers pain receptors instead of taste buds.

Changes Also Occur At the Level of Cells and Metabolism

Immune function:

The function of the immune system is to both protect against pathogens whilst fine tuning the body’s microbiome – the ecosystem of microorganisms that live on the body.

Our microbiome is an example of symbiosis (where one organism lives on a host), and humans depend on microorganisms for a wide range of functions so it is often a mutualistic (mutually beneficial) relationship8.  However, perhaps due to the cocktail of cosmic radiation, sleep deprivation, microgravity, stress and a sterile environment, an astronaut’s microbiome has a greater risk of slipping into a state of ‘dysbiosis’, characterised by falling microbial diversity, and changes in the relationships within the microbiome and between microbiome and host. An unhealthy microbiome, has been linked in studies on Earth to conditions such as irritable bowel syndrome.9

Moreover, our body’s immune cells may also be suppressed due to damage from cosmic radiation or stress which increases the likelihood of astronauts becoming ill with bacterial or viral infections, or cause dormant viruses such as the Epstein-Barr virus to be reactivated2. This is exacerbated by easy microorganism transfer between astronauts in the confined spaces on the ISS.

On the flip side of this, increased stress hormone levels might heighten immune alertness, contributing to the increased risk of allergic responses. The immune system is now monitored closely by blood, urine and saliva samples.

Medication, Nutrition and Cancer Risk

Medications may be processed by the body in different ways when the body is under stress. Drugs may be directly damaged by cosmic radiation so resilient packaging is being developed.

The diet on the ISS is not as varied as a normal diet because fresh food will only last a few days after each resupply mission. This is exacerbated by the impact of cosmic radiation on degrading the nutrient content of food over time. Issue 19 of the YSJ reported on the attempts being made to grow fresh fruit and vegetables in space10.

Astronauts require more antioxidants (such as vitamin C, found in fresh fruit) from their diet in space than on Earth to prevent levels of oxidants (produced by cosmic radiation interacting with organic molecules) rising too high. 5 High oxidant levels increase the risk of cell damage. If a cell’s genes controlling cell division are damaged either by an oxidant or directly by cosmic radiation and the cell avoids cell suicide (apoptosis), that cell may go on to propagate into a group of cancerous cells: thus astronauts exposed to high radiation levels with low antioxidant levels in their diet have a higher risk of cancer.

Vitamin D deficiency is another problem principally because of the lack of direct sunlight an astronaut can experience, given the necessity to remain inside a pressurised compartment at all times. Vitamin D deficiency can have consequences such as inhibiting bone mineralisation: this can exacerbate the bone atrophy caused by microgravity.

Metabolic Change

The atrophy of muscles and bones in microgravity results in a shifting of the position of equilibrium in the body’s calcium cycle: the concentration of calcium ions in the blood swells, boosted by calcium released by the breakdown of muscles and bones. One consequence of this is an increased risk of kidney stones. Kidney stones build up as ions such as calcium precipitate out of the newly formed urine in the kidney. They can migrate down the ureter causing pain. In response, drugs such as bisphosphonates are being investigated to slow bone loss and potassium citrate may reduce the risk of kidney stones developing. Iron levels also increase due to the decrease in red blood cells in the blood as blood volume falls. Monitoring the urine of an astronaut is one of the key sources of information about how their body is changing chemically.





In conclusion, astronauts go through obvious physical change due the human body’s brilliance at adapting to the demands (or lack of demand) of microgravity. Cardiovascular and muscular/skeletal changes can be mitigated by regular exercise, although the changes to an astronaut’s psychology and coordination rely more on the astronaut’s character and experience. Yet spaceflight also incurs other serious changes such as immune suppression, microbial dysbiosis and nutrient deficiency; it seems that managing these will be just as important when it comes to keeping astronauts healthy on missions beyond Earth orbit.



  1. YSJ Issue 19 P8-11 ‘ISS Uncovered’; P14-17 ‘How to Become an Astronaut’; and P53 ‘Tim Peake: What is He Doing?’
  2. ‘Gravity, Who Needs It? NASA Studies Your Body in Space’ (Abadie, Lloyd and Shelhamer, NASA Human Research Program)
  3. ‘What Happens to the Human Body in Space?’ (Wei-Haas,, March 1 2016)
  4. ‘The Human Body in Space: Distinguishing Fact From Fiction’ (Springel, Science in the News (Harvard) blog)
  5. ‘The insane ways the human body changes during long-term spaceflight’ (Dickerson, Business Insider UK : March 2015)
  6. BBC Inside Science Podcast ‘What’s Left to Explore’ (broadcast 22/09/16)
  7. ‘An Astronaut’s Guide To Life on Earth’ (Chris Hadfield) is a first-hand account of an astronaut’s career. First Published 2013 by MacMillan; ISBN 978-1-4472-5994-7
  8. ‘I Contain Multitudes’ (Ed Yong) is an interesting tour through examples of symbiosis between animals and bacteria. First Published 2016 by The Bodley Head; 9781847923288
  9. ‘The Challenge of Maintaining a Healthy Microbiome during Long-Duration Space Missions’ (Voorhies and Lorenzi)
  10. YSJ Issue 19 P29 ‘Space Tomatoes’; and P51-52 ‘Food Production in Space’.

Patrick McCubbin, 16, is studying Chemistry, Biology, Further Maths and Physics at Abingdon School in the UK. He hopes to study Biochemistry at university. He was inspired to write this article after completing Mission Discovery at King’s College London in 2016, a course run by the International Space School Education Trust.