How does Buzz Lightyear do it? The battle between microgravity and the cardiovascular system 

By Stefanie Zhang 

Every child once gazed up at the night sky and envisioned themselves that one day, like the Disney toy astronaut Buzz Lightyear, they will reach the stars. Space exploration has seen such advancement in technology and engineering over the centuries that it has allowed us to go from merely orbiting the earth to living in the International Space Station for months and even landing on the moon. Much of space’s extreme conditions such as cosmic radiation, lack of oxygen, and temperature fluctuations have been mitigated by engineering.¹ Yet, one of the major concerns of modern space travel that needs to be addressed is how the astronauts’ bodies change and adapt to prolonged exposure to microgravity of space or partial gravity of destination planets and moons, as the human body is only designed to sustain humans in Earth’s gravitational pull of 9.81 m/s².  

The early human predecessors evolved from quadrupeds to bipeds.² Due to upright walking, the position of our brain relative to the heart has changed from being at similar levels to the brain being at a higher level. As a result, the cardiovascular system, one of human’s most vital systems, had also evolved to counteract Earth’s gravity to allow efficient blood circulation and provide the brain with enough oxygen.² This intricate system is designed for earth’s gravitational force. Therefore, in space, microgravity affects the cardiovascular system by altering the heart, blood, and vessels, which results in an acute syndrome called cardiovascular deconditioning.³

In space, without gravity pooling blood in the legs, a fluid shift in the body occurs that results in an increased fluid abundance in the upper body, causing legs to shrink in size and the face to swell up.⁴ One of the main consequences of this fluid shift is that it disrupts the baroreceptor-heart rate reflex.⁵ Baroreceptors are mechanoreceptors located on the top arch of the aorta that supplies blood from the heart to the rest of the body. These receptors are activated by increased stretch levels of arterial walls when blood pressure increases, thereby signalling the autonomic nervous system to control heart rate and maintain blood pressure within a healthy limit.⁶ However, as blood volume increases in the cardiac compartments due to this fluid shift, the baroreceptors now sense a constant “increase” in blood pressure, which causes the autonomic nervous system to constantly lower heart rate in attempt to alleviate hypertension.⁵ This persistent tension on baroreceptors by this increased blood volume, therefore, results in an alteration in baroreceptor-heart rate reflex sensitivity.⁵ To further counteract this constant increased blood pressure, the kidneys are stimulated to increase fluid excretion to decrease blood plasma volume.⁷ Plasma is the liquid portion of blood that contains more than 90% of water.⁸ Through excretion, overall blood volume decreases, which reduces blood pressure. Therefore, changes in baroreflex sensitivity and fluid level reduction result in diminished physical strain on the heart, as blood volume and heart rate decrease, ultimately resulting in cardiac muscle disuse atrophy as large contractility is no longer required.⁹

Another feature of cardiovascular deconditioning can be observed on a microvascular level through the endothelium.⁵The endothelium, the single-celled inner lining of blood vessels, plays a significant role in maintaining the contractility of blood vessels.¹⁰ As blood flows through the blood vessels, a frictional force called laminar shear stress is generated between the blood and the endothelium.^4,5 The force is important in maintaining the contractile function and overall organisation of the endothelium.¹¹ However, under the influence of microgravity, there is a significant decrease in shear stress force exerted by blood flow on the endothelium.⁵ Researchers have found that the cytoskeletal organisation of endothelial cells is disrupted, as actin filaments, linear polymers essential to cellular integrity, are broken down and reorganised under induced microgravity.¹² This dysfunction is thought to reduce the contractility and elasticity of blood vessels, which poses secondary vascular risks post-space flights.⁵ The combination of these cardiovascular deconditioning features can result in orthostatic intolerance upon re-entry to earth, which causes symptoms like light-headedness, decreased physical capabilities, and even syncope, as not enough blood flows to the brain in an upright position under earth’s gravity. This remains a significant medical problem for astronauts in modern-day space travel.⁵

Though these extreme changes reduce the physical capability of astronauts, countermeasures for space missions were implemented to allow humans to survive in microgravity environment for up to 14 months without severe complications.¹³ Current countermeasures include lower body negative pressure suits that mimic the hydrostatic pressure phenomenon experienced under Earth’s gravity and shift fluid towards the feet to re-sensitize baroreceptors and aid the musculoskeletal system.¹⁴ Also, rigorous pre-flight and in-space exercise regimens are personally tailored to prevent excessive heart muscle atrophy and skeletal muscle atrophy.⁴ Though these methods are proven to slow down hemodynamic changes, there are significant limitations as deconditioning is still observable.¹⁵ Therefore, long-term space flights still pose significant risks and consequences to long-term health.³ As for now, astronauts thus would require an extensive medical check-up and longer recovery period to readapt to earth’s gravity after a mission, but the quality of life may still be affected.¹ Major space agencies have proposed to implement artificial gravity on spaceflights, either through in-flight short-radius centrifuges or a rotating spacecraft to induce Earth-like gravity, but the research in engineering and human physiology is still preliminary.¹⁶

It currently remains unknown whether longer missions will aggravate existing deconditioning or induce previously unknown issues and how different astronauts respond physiologically, or if transitioning between partial gravity, microgravity and gravity on earth can harm our physiology upon re-entry.³ The data from the few numbers of space missions are limited, thus further research is required. But with technological advancement, more experiments and analysis can be done on earth, and such multiscale research using AI will allow us to further understand physiological deconditioning experiences in microgravity. This will not only help improve countermeasures to ensure good health and mental wellbeing of astronauts, but also step us closer to the dream “to infinity and beyond”. 

References:

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