From Baikonur to Mars: Our Understanding of Microgravity and the Human Body

Early in the morning of April 12th, 1961, in the top-secret city of Baikonur, Kazakh SSR, Yuri Gagarin’s historical 108-minute spaceflight proved that humans could withstand the conditions of outer space. Later that year, fellow Soviet cosmonaut Gherman Titov spent a full day in orbit, proving that humans could sleep in space and endure prolonged exposure to microgravity (Zak, 2013). A 23-day stay aboard Salyut 1, the first manned space station, in 1971 further confirmed that long-term stays in space were possible for humans, paving the way for longer and more complex missions (Ezel, 2007). The current record for the longest single stay in space is held by Valery Polyakov, who spent an astonishing 438 consecutive days aboard space station MIR in the mid-1990s (Pearlman, 2019). Now in 2020, the research of our predecessors have allowed us to dream of human travel to Mars, yet certain questions persist and new ones abound: how long can the human body function in outer space? How long is too long? And what are the consequences and risks of extended exposure to microgravity?

Figure 1. Cosmonaut Yuri Gagarin received medical testing to determine his eligibility and fitness to become the first man in space. Despite uncertainty as to how the human body would respond in outer space, Soviet cosmonauts were rigorously tested to ensure good health and reduce the chances of harm to the cosmonaut.

In June 2019, NASA embarked on a mission of their own to try to answer these questions, and presented a somewhat disturbing discovery. Space flight can slow down or reverse blood flow in the upper body. This startling discovery came from a NASA study which measured various cardiac and vascular metrics — such as heart rate and internal jugular vein pressure and flow — of astronauts scheduled for prolonged missions to the International Space Station (ISS). Data from pre- and post-flight was gathered in the seated, supine (lying on the back, face-up), and 15-degree head-tilt positions. On the ISS, data was obtained with the astronaut in a 25mmHg lower-body negative pressure (LBNP) simulated environment for around one hour and again without the astronaut exposed to subatmospheric pressure of any kind to determine whether conditions somewhat similar to the gravity-induced gradient existing on Earth which causes blood to flow normally, would result in normal blood flow in space. LBNP was achieved with a chamber sealed at the waist with a pump attached to reduce pressure, exposing the limbs to subatmospheric pressure. The purpose of utilizing LBNP, which drives blood flow to the lower extremities in the absence of gravity, was to have a baseline off of which to compare measurements from participants exposed to microgravity conditions in space and determine whether interruptions in blood flow are due to microgravity or possibly other factors. Use of LBNP on days 50 and 150 of the study on the ISS showed a decrease in cross-sectional area of the inter-jugular vein (IJV) relative to those without LBNP exposure on those days, indicating that microgravity exposure is the cause of increased cross-sectional IJV area. This is associated with an increased risk of thrombosis or stasis (stopping of blood flow) (Marshall-Goebel, 2019).

Figure 2. Mean cross-sectional area and pressure of internal jugular vein (IJV), preflight, in-flight, and postflight. IJV area was greater for day 50 and 150 in absence of LBNP, as it was for supine and HDT for pre-and post-flight.

Preflight data shows no evidence of stagnant or retrograde blood flow in the seated position, which was mostly true for preflight supine and HDT positions as well. The gradient of blood flow on Earth is made possible by gravity, which produces the most normal blood flow in an upright position. During spaceflight, 5 out of 11 astronauts showed stagnant blood flow on Day 50, with 2 out of 10 showing the same stagnation on Day 150. By Day 150, 2 astronauts had developed retrograde (reverse) blood flow, opposing the direction of blood flow produced by gravity on Earth. From the evaluation of ten astronauts from Day 50 to Day 150, five demonstrated no change in blood flow, three unexpectedly demonstrated improved flow, and two demonstrated reduced flow. Blood flow in this group generally worsened from normal to stagnant to retrograde as their time in space increased. In the LBNP group, ten astronauts showed improved flow, two showed worsened flow, and five did not show any change in flow. This suggests that microgravity eliminates the head-to-toe blood flow gradient which maintains normal blood flow on Earth. The use of LBNP seems to counteract the negative effects since the application of greater pressure in the lower body helps to simulate conditions that encourage somewhat normal blood flow and reduce IJV cross-sectional area (Marshall-Goebel, 2019).  LBNP has been used for many years as a way to promote blood flow to the lower limbs and reduce orthostatic intolerance, or discomfort with standing upright, in astronauts (Güell, 1995).

Figure 3. (A) Grades of blood flow during preflight and postflight measurements in seated, supine, and 15-degree head tilt positions, and inflight measurements on days 50 and 150 with and without application of LBNP. Grade 1 = continuous flow; Grade 2 = pulsating flow; Grade 3 = no net flow; Grade 4 = retrograde flow. (B) and (C) Ultrasound images with pulsating flow and thrombosis, respectively. Clot noted with X, and no flow is detected.

The implications of the discovery that reverse or stagnated blood flow may result from long-term visits to outer space raises questions and concerns about whether extensive stays outside of our home planet are safe. Such findings come at a time when NASA is entertaining ideas of how to prepare astronauts for long missions and prevent health complications arising from these missions – such as vitamins, medications to reduce bone loss, and exercise (Abadie, 2020). Knowing that retrograde or stagnated blood flow, in addition to increased IJV size and other findings in this study, is a risk of long-term spaceflight, provides another challenge in preparing for such missions. However, the more risks we are aware of during the planning stages of this mission, the better we can protect astronauts as they endeavour into the dusty red rock of a celestial body no human has ever set foot on.

Edited by Aditya Jhaveri

Posted by Lucy Mangalapalli


Abadie, L.J., Lloyd, C.W., Shelhamer, M.J. (2020, February 6). The Human Body in Space. NASA.

Dunbar, B. (2007, September 5). Cardiovascular System Gets ‘Lazy’ in Space; New Study Gets Blood Flowing on Station. NASA.

Ezell, E.C., Ezell, L.N. (2007, September 20). Soyuz 11: Triumph and Tragedy. NASA.

Güell, A. (1995, February-March). Lower body negative pressure (LBNP) as a countermeasure for long term spaceflight. ScienceDirect.

“How Does Spending Prolonged Time in Microgravity Affect the Bodies of Astronauts?” (2005, August 15). Scientific American.

Marshall-Goebel K, Laurie SS, Alferova IV, et al. (2019, November 13). Assessment of Jugular Venous Blood Flow Stasis and Thrombosis During Spaceflight. JAMA Network Open.

Pearlman, R. (2019, December 29). Astronaut Christina Koch Breaks Record for Longest Space Mission by a Woman.

Perez, M., Dunbar, B. (2020, March 5). Mars Perseverance Mission Overview. NASA.

RIA Novosti. (Circa 1960). Before flying into space, Gagarin underwent numerous examination procedures. Republic.

Zak, A. (2013, June 13). Vostok-2 Mission. NASA.

Image References

Marshall-Goebel K, Laurie SS, Alferova IV, et al. (2019, November 13). Assessment of Jugular Venous Blood Flow Stasis and Thrombosis During Spaceflight. JAMA Network Open.

RIA Novosti. (Circa 1960). Before flying into space, Gagarin underwent numerous examination procedures. Republic.

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