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To the Moon and… the ER? The Physiological Challenges of Space Travel
By Kritika Khati
Everyone’s heard of the phrase “I love you to the moon and back,” commonly used to express infinite love with no bounds or time limit. As it turns out, the reality of traveling to the moon isn’t as smooth or romantic as the phrase makes it out to be. Venturing beyond Earth’s atmosphere presents a multitude of challenges to the human body, changing the physiology we’re used to on Earth.
Microgravity, or colloquially “zero gravity,” is a condition in which an object is nearly weightless. It commonly describes outer space—far away from Earth’s gravitational force. Here on Earth, gravity is a constant force thought to govern the laws of physics. We rarely think about gravity as it is an intrinsic part of our environment. Still, it plays a significant role in many biological processes and factors on Earth, including those within our bodies.
Gravity regulates the body’s circulatory system here on Earth. Upon exposure to microgravity, blood freely flows upwards into the heart and head without gravity’s force pushing down on it. Our bodies are not adapted to this, so many astronauts in microgravity experience swelling in their faces and heads. Moreover, an increase of fluid (blood) in the brain increases pressure in the brain and can have serious health implications. As Dr. Michael Decker from the Center for Aerospace Physiology at the Case Western Reserve University School of Medicine explains, “Some of this increased intracranial pressure can actually impact the eye and lead to visual impairment. Sometimes when astronauts land, that visual impairment does not necessarily resolve.” This condition has been referred to as Spaceflight-Associated Neuro-Ocular Syndrome (SANS) and is common in long-duration spaceflight.
Disruptions in blood flow due to microgravity can also significantly impact the heart. The heart is the organ responsible for pumping blood, oxygen, and nutrients throughout the body. Many studies have explored the impact of microgravity on the cardiovascular system. Research has found that the anatomy itself of the heart changes from an oval shape to a round, spherical shape. Not only does the shape of the heart change, but the heart’s muscles do as well. In a study conducted at Stanford University, researchers aimed to assess cardiac function in space using human-induced pluripotent stem cell-derived cardiomyocytes. This experiment occurred in vitro, or outside of a human body. Induced pluripotent stem cells (iPSCs) are cells derived from the body, usually from the skin or blood, and are programmed to develop into any type of cell within the body. The research team at Stanford differentiated stem cells into cardiomyocytes, the cells that make up the cardiac muscles to contract the heart, and exposed them to microgravity to study how the heart functions in space. This provided insight into the mechanism of physiological change in microgravity, revealing that heart muscle cells actually alter their genes to compensate for the loss of gravity.
In addition to SANs, when returning to Earth, some astronauts face certain long-term effects (lasting longer than 6 months) from their space travels. A study by Dr. Nicholas Vernice indicates that those who travel to space experience approximately a 10%–15% reduction in blood plasma volume, likely due to the fluid and pressure shift from the lower extremities to the upper body. The heart doesn’t have to work as hard due to the reduction of fluid and pressure, which can lead to cardiac atrophy—the breakdown of heart muscle. In addition to these physiological changes, many astronauts face orthostatic intolerance, a condition characterized by presyncopal (fainting) symptoms like nausea, lightheadedness, and dizziness, along with a decreased ability to stand for durations of ten minutes or longer. These symptoms are very similar to patients with a well-known type of orthostatic intolerance—postural orthostatic tachycardia syndrome, commonly known as POTS. The bodies of POTS patients struggle to regulate blood flow when changing positions, often from lying down to standing up. Their bodies fail to properly regulate blood pressure and heartbeat during this shift, leading to common symptoms such as fainting, dizziness, or exhaustion, making daily life difficult.
Due to the similarities between orthostatic intolerance in astronauts and those living with POTS, doctors have been able to extrapolate findings from studies on astronauts and apply them to POTS patients. Dr. Benjamin D. Levine, a cardiologist at Texas Health Presbyterian Hospital Dallas who has led many NASA studies on cardiology, describes their studies: “The work that we did in space really gave me the insights to understand these patients, many of whom end up spending a lot of time in bed and their hearts get small and atrophic. We started utilizing the exercise training programs we developed for astronauts, and it cures them.” The Levine Protocol, created by Dr. Levine originally for astronauts facing orthostatic intolerance, was modified and is commonly used today by patients with POTS. It can improve symptoms of POTS, even curing some people of the condition.
The challenges that the body faces upon exposure to microgravity add depth to the phrase "to the moon and back," while also shedding light on both the adaptability of the human body as well as the creativity of the scientific community here on Earth. The correlation between astronauts and POTS may not have seemed apparent before. However, scientists like Dr. Levine who took an outside-of-the-box—or rather, an outside-of-the-planet—approach to processing astronaut research demonstrate the boundless benefits to humanity that lie ahead as we continue to push the boundaries of space travel!