From Bones to Blood Flow: The Science of Surviving Space

Spaceflight, a pioneering technology that has opened revolutionary ideas to the human eye and beyond. From Sputnik, the first artificial satellite to orbit Earth launched by the Union of Soviet Socialist Republics (U.S.S.R) to the prestigious American Space and Aeronautics Agency, NASA. From satellites independent of travel to spacecrafts, able to inhabit humanity within its confines, space travel has been around since the early 1900s. Yuri Gagarin, Soviet pilot, cosmonaut and first human to exit the Earth’s orbit and return alive. Space travel has been a huge part of human advancement. But although we have come this far into the industry, there are still problems that persist, and some of them are biological, and conflict with our very own bodies. The human body is the most complex biological specimen to ever grace the surface of this planet that we inhabit, but that also makes it very difficult for ourselves to understand it.

From the early 1900s to today, space travel has posed extremely problematic with the human body, clashing with its biological methodology of functioning. Space travel affects many bodily functions, some being:

• Space motion sickness
• Fluid shift to the head
• Visual impairment
• Cognitive decline
• Muscle atrophy
• Bone density loss
• Reduced strength and endurance
• Orthostatic intolerance
• Heart shrinkage
• Blood flow changes
• Weakened immune response
• Changes in white blood cell activity
• Increased cancer risk
• DNA damage
• Cataracts
• Air quality risks - Respiratory Problems
• Dust and particles - Respiratory Problems
• Isolation and confinement stress
• Disrupted sleep cycles
• Sensory deprivation or overload
• Altered taste and smell
• Slower wound healing
• Potential reproductive risks

These are all some of the basic problems the body endures during space travel. Although this is only the basic overview of it, there are much more problems that are caused because of space travel. These can be split into categories such as: neurological/cognitive effects, muscular and skeletal effects, cardiovascular effects, immune system effects, radiation exposure, respiratory effects, physiological effects and so on. Let’s look at each of these in a bit more detail.

Space travel has a lot of negative effects on the neurological system of the human body. While private corporations such as SpaceX and Blue Origin have been encouraging space travel as a tourist attraction and agencies such as NASA, JAXA, ISRO, ROSCOSMOS have been encouraging space travel at a level for scientific research, space travel has been having many neurological effects on human beings. While this presents a predicament for humanity, it also encourages scientific research on why and how we can prevent and address this issue. Scientists have conducted studies on the human bodies of astronauts that have flown into space and animal bodies that have endured space travel simulations and have found multiple leads into solving this pressing issue. Astronauts are exposed to hazards that include microgravity, cosmic radiation, hypercapnia, isolation, confinement and disrupted circadian rhythms. Due to these hazardous conditions, astronauts are subjected to neurological conditions and diseases. Due to microgravity in space, which is the absence of a gravitational force, problems such as the vestibular system (a sensory system that creates the sense of balance and spatial orientation), intracranial (within the skull) and intraocular (within the eyes) pressures affect the human neurological system. Microgravity also affects near-field vision, also known as the neuro ocular condition that is widely associated with spaceflight. Cosmic radiation also majorly affects the neurological functioning of the human body by increasing the risk of neurodegenerative conditions (conditions that damage or destroy parts of your nervous system, especially your brain over a period of time) and malignancies (conditions in which cells divide in abnormal patterns and invade tissues, creating a malignant tumor that worsens).

Image of a normal adult’s brain (left), image of an adult’s brain affected by a neurodegenerative condition (right).

Image of a malignant tumor in breast cancer, Ultrasound image.

Hypercapnia affects the amount of oxygen that reaches the brain due to the excess amount of carbon dioxide that can be found in the bloodstream due to an insufficient supply of oxygen in the atmosphere. Hypercapnia can be on an acute or chronic level and can lead to symptoms of shortness of breath, headaches and confusion. Isolation and confinement in space can cause psychological and mental illnesses and conditions that contribute to emotions of anxiety, depression, fluctuating sleep cycles and create hallucinations that can severely affect one’s emotional wellbeing. Space travel also disrupts the circadian rhythm which is a human being’s natural sleep-wake cycle, it is also referred to as the bodily cycle.

Now to look at the muscular and skeletal effects that spaceflight has on the human body. A major issue for the physical functioning of the human body in space is that it is physically undemanding in space due to the microgravity. The body starts to lose its strength due to the lack of work for the muscles and skeletal structure. Because of Earth’s gravitational pull, certain muscles such as the gastrocnemius (calf muscles), back and neck muscles and quadriceps, which are the large muscles at the front of the thigh. These sequences of muscles are specifically called the antigravity muscles, and work and work against the force of gravity to produce basic actions such as keeping us upright or even allowing us to do the most basic of movements. But in environments that are weightless or lack this necessary force of gravity, our antigravity muscles lose their purpose for function. This can lead to problems such as muscle atrophy, loss of bone density, fluid redistribution and orthostatic hypotension. Without the regular use of our muscles, they weaken and deteriorate, which results in a process of muscle damage called atrophy. Research studies show that astronauts lose up to 20% of their muscle mass in a journey of 5 to 11 days in space. Loss of muscle mass equates to the loss of strength, which can be particularly dangerous when the astronaut is on emergency missions that require strength. The cross section image below shows the effects of space travel on a rat’s body. The image to the left depicts its muscle structure on Earth while the image to the right depicts its muscle structure in space. The image clearly highlights the severity that space travel has on human beings if the effects are this severe on a rat’s body.

While muscle atrophy can be prevented, or at least, its damaging effects can be reduced through intensive strength exercises, it’s inefficient for astronauts to be spending long periods of time exercising on missions. There has been research of electrical muscle stimulation that maintains muscle mass regardless of the astronaut exercising or remaining idle.

Image/diagram of healthy muscle and atrophied muscle comparison and differences. 

Bone density loss in weightless environments causes irreversible changes. Solutions to this pressing issue haven’t been discovered/invented yet. With the rapid bone density loss in space, astronauts face changes that weaken skeletal integrity, and the incremental onset of fracture injuries (slow healing processes of bone-related injuries) and renal stone formation (a.k.a kidney stone disease).

Image of increasing bone density

Orthostatic hypotension is a very common malady in space travel. Medically, it is a type of low blood pressure that occurs when standing up after lying down or sitting. It causes dizziness as blood rapidly flows down to the body, leaving the head. Although it is nothing to worry about in Earthly conditions as it is very common and normal, prolonged effects of orthostatic hypotension are rather serious. This can be observed through the very low orthostatic tolerance that is visible in astronauts after their initial return to Earth when conducting a medical examination on them, asking them to lie down and sit back up, or simply stand up. Although this might sound rather serious, with simple and daily workouts, astronauts can increase their orthostatic tolerance while in space. Research conducted has shown that after astronauts regularly exercised in space, they didn’t face any difficulties that were associated with orthostatic hypotension and were fine within the 24 hours of their landing on Earth after a 6 month journey in space.

Now that we have covered the neurological and musculoskeletal effects of space travel on the human body, let’s look at the cardiovascular effects of space travel on the human body. It has been addressed multiple times that space travel imposes significant gravitational and radiation stress on both cellular and systemic physiology, this in turn reflects a myriad of cardiovascular problems that cause significant stress on the human body. While short-distance space travel reflects reversible effects and changes to the body, long-distance space travel that exceeds 6 months, can pose challenging problems. It’s been observed that due to the weightless environment that astronauts travel in, there have been notable fluid changes, with fluid in the body shifting towards the head. This also causes fluctuations in arterial pressure that can erratically fluctuate blood pressure levels and elevated cardiac output. Another problem that is caused by microgravity is the 10%-15% reduction of blood plasma levels, and surprisingly, even though elevated cardiac outputs have been observed, there has been very palpable cardiac atrophy, which has been confusing scientists. “Exposure of the heart to the proton and heavy ion radiation pervasive in deep space contributes to coronary artery degeneration (coronary artery struggles to supply the heart with oxygen, blood and nutrients), augmented aortic stiffness (abnormal stiffening of the aorta), and carotid intima thickening (inner layer of carotid arteries become abnormally thicker) through collagen‐mediated processes (Processes that happen in the body with the mediation of collagen). Moreover, it accelerates the onset of atherosclerosis (disease in which arteries start to clog) and triggers proinflammatory responses (When the immune system creates inflammation to defend against a foreign particle).” (PubMedCentral) Astronauts also face orthostatic intolerance.

Now, to look at how spaceflight can affect the immune system. Due to microgravity in space, the cells in our body face difficulties in communication. Humans are built to resist the force of gravity, but without it, cells grow weaker and struggle to have proper communication. This includes white blood cells and antibodies. Space travel also puts continuous stress on the body, this can in turn, lower the response rate of the immune system to germs. The presence of the human body in space can also affect the way white blood cells work. Sometimes it can cause them to stop multiplying, launch inefficient attacks on viruses and bacteria and also in certain cases, attack healthy body cells by mistake. Because of this, viruses that have already been defeated by the antibodies can reactivate and attack once more, due to the lowered resistance presented by the immune system. 

Now that we have covered the immune system, let’s see how the radiation in space affects our bodies. Radiation is present everywhere in space. The only reason we aren’t fried to death on Earth is because of the ozone layer within our atmosphere that prevents us all from dying instantly. But space doesn’t have a protective ozone layer, so what happens to astronauts? Radiation comes from the Sun, cosmic rays and trapped radiation belts, which astronauts are exposed to. The effects of the exposure to radiation on the human body include: DNA damage, increased risk of cancer, weakened immune system, tissue and organ damage, acute radiation sickness. The damage of our DNA is caused through radiation to manipulate cells into working incorrectly or to cause undesirable mutations. Damaged DNA can later lead to an increased cancer risk. Immune systems that are exposed to DNA can be harmed and damaged, also killing white blood cells in the process. High doses of radiation in short periods of time can damage tissues, organs and cause radiation sickness. 

Space travel can cause respiratory issues because of the way microgravity affects the lungs and airways. In space, fluids in the body shift upwards, which can cause the nasal passages to become blocked and make breathing feel more difficult. Dust and small particles inside the spacecraft can also float around more easily, increasing the chance of them being inhaled and irritating the lungs. Over time, the reduced physical effort needed for breathing in microgravity can cause the respiratory muscles to weaken, making it harder for astronauts to breathe efficiently once they return to Earth's gravity.

In space, astronauts can develop respiratory issues partly because of hypercapnia, which means there’s too much carbon dioxide (CO2) in the blood. Inside a spacecraft, CO2 from breathing doesn’t settle and mix the same way as on Earth because of microgravity. This makes it harder for ventilation systems to clear the air properly, so CO2 can build up in the cabin. Breathing in too much CO2 can cause headaches, dizziness, shortness of breath, and can even affect thinking and decision-making. This is one of the reasons why space agencies carefully monitor air quality on board, as hypercapnia can make respiratory problems worse in space.

With all these physiological issues, come psychological issues as well due to the intense isolation and confinement that astronauts are kept in. They face several psychological challenges during space travel, including isolation, confinement, and separation from family and friends for long periods. Living in a small, enclosed space with the same people and limited privacy can cause stress, tension, and even mood swings. The lack of natural light and normal day-night cycles can also disrupt sleep and affect mental health. Astronauts may experience loneliness, anxiety, or homesickness, especially during long missions, which is why space agencies focus on training, teamwork, and mental health support before, during, and after spaceflights.

In conclusion, spaceflight has a powerful impact on the human body, affecting everything from muscles and bones to the immune system, lungs, and even mental health. The unique conditions of space, like microgravity, increased radiation, and isolation, challenge the body in ways that scientists are still working to fully understand. While astronauts train hard and use advanced technology to stay healthy, long-term space travel will continue to push the limits of human biology. Learning how to protect the body in space is not only important for future missions to the Moon and Mars but also helps improve medical knowledge and treatments here on Earth.