1. Why Fluids Shift to the Head and Faces Swell in Space
On Earth, gravity pulls the water in our bodies down toward our feet while we are standing. In space, this pull disappears, and fluids such as blood move from the legs toward the head. The face becomes puffy, and the legs become very thin, described as "bird legs" [1]. NASA explains that about 2 liters of fluid move from the lower body to the upper body when entering weightlessness [2]. This is roughly the amount in a large water bottle, moving from the feet to the chest and head. This is a rough estimate and varies by person and conditions. How does the body react to this shift of water toward the head?
2. How the Body Reduces Its Blood Volume in Space
When fluids shift toward the head in space, the body treats this as an excess of circulating blood and reflexively reduces the total blood volume, according to a NASA report [3]. Since there is no "standing" posture in space, this shift toward the head continues while the astronaut is in orbit. Imagine a bookshelf where all the books move to the top shelf. The body reacts by removing books to match the new arrangement. However, the real body is not as simple as a bookshelf. The reduction in blood is just the first trigger; NASA describes the reduced tolerance to standing as complex and caused by multiple factors [3]. What happens when a body with reduced blood and slowed reactions returns to Earth?
3. Why Blood Pressure Control Fails When Astronauts Stand on Earth
When you stand up on the ground, gravity pulls blood into the lower body. This reduces the blood going to the head. Pressure sensors (baroreceptors) in the aortic arch and the carotid arteries sense this drop in blood pressure. The sympathetic nervous system then releases norepinephrine, which narrows the blood vessels throughout the body and maintains pressure [3]. We repeat this reaction unconsciously every time we stand. If this reaction is too slow, blood flow to the head is insufficient, leading to dizziness or fainting. These symptoms are called orthostatic intolerance. JAXA materials explain that fluid redistribution reduces blood flow to the brain and can cause fainting. Some descriptions state that astronauts cannot stand and walk immediately after return [4]. A body with reduced blood and slowed reactions is suddenly asked to perform this "standing adjustment" the moment gravity returns. How many astronauts actually struggle to stand?
4. How Many Astronauts Could Not Stand After Space Shuttle Flights?
A study published in 1996 examined 14 astronauts before and after three Space Shuttle flights lasting 9 to 14 days. In a 10-minute standing test, all 14 passed before the flight, but 9 of the 14 (64%) failed after the flight [5]. This was the result of short flights. What about those who stayed longer? According to a NASA report, 5 of 6 American astronauts who spent 4 to 5 months on the Russian space station "Mir" failed a 10-minute tilt test on the day of landing. For those on the ISS for about 6 months, 4 of 6 failed [3]. Because the test conditions differ, the numbers cannot be compared directly, but it is clear that this happens after long stays too. The NASA report also notes that historically, the rate of near-fainting in tilt tests after landing was over 80% for women and about 20% for men. The cause is under investigation, and the number of people studied was small, so this does not mean women are weaker [3]. This is not about "weakness" but about what happens when a body adapted to space returns to Earth's gravity. Are there countermeasures?
5. Does Drinking Salt Water Before Landing Help Astronauts?
In 1985, a test was conducted. In the trial, 17 Space Shuttle astronauts took salt tablets and water before landing, while 9 others did not [6]. The results showed that the group taking salt had a 29% smaller heart-rate response when standing, and blood pressure drops were also suppressed [6]. This method became a standard procedure for Shuttle astronauts. Replacing water in the body reduces the burden when standing. However, as seen in the previous section, the cause is not just blood volume. A review article describes a study in which a drug meant to help the body hold on to fluid, given to 7 people, did not prevent the problem, which fits the idea that there are several causes. Salt water is not a magic cure but one effective measure [9]. On the Shuttle, astronauts also wore pressure suits (anti-gravity suits) to prevent blood from falling to the lower body during re-entry and landing. Russia has similar suits [3]. In ground simulations, these compression suits prevented near-fainting in 100% of cases during a 15-minute tilt test. However, this was a ground experiment simulating low blood volume, not the actual results for astronauts. With these measures before and after landing, how normally can astronauts move after a 6-month stay?
6. Why 12 Astronauts Had No Dizziness After 6 Months on the ISS
A study reported in the medical journal Circulation in 2019 measured the blood pressure of 12 astronauts (4 women) who spent about 6 months on the ISS during daily activities, beat by beat, for 24 hours [7]. Measurements were taken before the flight and after landing. The result was that no one experienced orthostatic intolerance, such as dizziness or fainting, before or after the flight. Systolic blood pressure (the top number) dropped from 120 to 106 mmHg during spaceflight and returned to 122 after landing [7]. These 12 people exercised for up to 2 hours a day in space for endurance and muscle strength, and received saline drips after landing. The researchers concluded that with current measures of in-flight exercise training and fluid replacement upon return, no fainting or dizziness occurred during daily activities in the 24 hours after landing [7][8]. Note that this was 24-hour blood pressure during daily activities, which is different from the tilt tests in the previous section. Many people failed difficult tilt tests, but no dizziness occurred during normal movement. These two are not contradictory: one comes from standing and tilt tests just after landing, and the other from blood-pressure monitoring during daily activities in the 24 hours after landing. For these 12 astronauts, with exercise in space and a saline drip after landing, no dizziness or fainting occurred during daily activities in the 24 hours after a 6-month stay. This is the most important point, though the study cannot show how much each measure contributed. What remains unknown?
7. Open Questions About Astronaut Dizziness and Where to Read More
The flow so far has been: fluids shift to the head, the body reacts, and dizziness can occur after return. Then salt water and exercise are used as countermeasures. There are at least two remaining questions. One is why the rate of near-fainting in tilt tests after landing is higher for women. The NASA report states the cause is under investigation [3]. The other is how much exercise and fluid replacement each contributed. The 2019 study did both at the same time, so the separation is unclear [7]. For those who want to read the original sources, there are three entry points. JAXA materials are in Japanese and give an overview of the changes during spaceflight [4]. The NASA report is in English but has many graphs and tables, so differences can be seen just by scanning the numbers [3]. The 2019 study shows blood pressure changes during daily activities at three points: before flight, in space, and after landing [7].