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Does zero gravity make you pee?

Many people wonder if being in zero gravity causes you to pee more. With no gravity pulling urine down, it’s a common assumption that astronauts just let it flow freely. However, there are a few key factors that determine whether zero-G conditions make you pee more.

How zero gravity affects the body

When astronauts are in space, they experience weightlessness. Without the force of gravity pulling fluids downwards, body fluids like blood and urine redistribute themselves more evenly throughout the body. This causes facial swelling and what astronauts call “puffy face.” But for urine specifically, the kidneys adjust urine production to prevent excessive urination.

On Earth, specialized cells in the kidneys called renal tubular cells determine how much water gets excreted as urine. These cells are sensitive to gravity and position. In zero-G, the kidneys get confused by the lack of gravity and initially excrete more water, causing an increase in urine output. But after a few days, the kidneys adjust to the weightless environment by producing less urine.

How astronauts urinate in space

Special underwear and devices help keep astronauts from just peeing freely in space. Astronauts wear an absorbent,Maximum Absorbency Garment (MAG) underneath their spacesuit that contains any unexpected leaks. For longer spacewalks, they can urinate into a disposable urine collection device.

On the space shuttle, astronauts used funnels connected to hoses and storage tanks to urinate. The hoses had fans to help draw the urine away in weightlessness. On the International Space Station (ISS), astronauts instead use a toilet with leg straps and thigh bars to keep them seated and aimed properly.

Urinating in zero-G takes some practice. The urine can flow back up the hose or splatter unpredictably, so astronauts have to learn a special technique. After going, air suction pulls the urine into the waste storage tank.

Does space motion sickness affect urine?

Around two-thirds of astronauts experience space motion sickness in the first few days of weightlessness. The most common symptoms are dizziness, nausea, vomiting, and loss of appetite – very similar to car sickness. Dehydration from vomiting and fluid shifts within the body can sometimes lead to concentrated urine and less frequent urination.

Space sickness medication like promethazine can also cause urinary retention. So while zero gravity itself doesn’t directly increase urine output, space sickness may temporarily disrupt normal urination patterns.

Key factors that determine urine in space

While it may seem like zero gravity would cause astronauts to urinate freely, there are a few key factors that determine urine output in space:

  • Kidney adaption – After a few days, the kidneys adjust to weightlessness by producing less urine.
  • Urine collection devices – Special underwear and funnels prevent free urination.
  • Space sickness – Vomiting and medications can temporarily decrease urine.
  • Fluid intake – Drinking more fluids can increase urine production.

Over time, most astronauts don’t urinate much differently in space versus on Earth. Their bodies adapt to the microgravity environment. However, research suggests that extended stays in zero-G may continue affecting urine concentration and mineral content.

Studies on urine and spaceflight

Scientists have conducted studies to understand how urine and kidney function change for astronauts:

Study Methods Key findings
Biochemical responses of renal stone forming astronauts exposed to microgravity (1994) Measured biochemical urine markers before, during, and after Space Shuttle missions. Spaceflight increased calcium oxalate saturation in urine, possibly increasing kidney stone risk.
Renal stone risk during spaceflight: Assessment and countermeasure development (2005) Analyzed urine samples from astronauts on both short- and long-duration flights. Found increased urine calcium, decreased urine volume, and higher stone-forming risk in space.
Renal stone formation among astronauts Comparison of renal stone risk and occurrence in astronauts vs. non-astronauts. Astronauts have 2-3 times greater incidence of kidney stones than the general public.

Based on these results, scientists are still studying ways to counteract the kidney and urinary changes that astronauts experience during spaceflight. Ensuring proper hydration and urine flow helps maintain health.

Effects on urine after returning to Earth

Readapting to Earth’s gravity after spaceflight causes additional fluid shifts. Astronauts can temporarily excrete excess urine as their bodies re-acclimate and fluids redistribute back downwards. Their leg muscles may also atrophy after unloading in zero-G, which can further disrupt normal circulation and urine production.

Post-flight effects may include orthostatic intolerance, edema, blood volume loss, and electrolyte imbalances. Monitoring astronauts’ hydration, vascular pressure, urine output, and kidney function is important to help them recover after long missions.

Long-term impact on the urinary system

So far, research on urinary changes has focused on shorter spaceflights of 2-3 weeks. As astronauts embark on longer missions to the Moon and Mars, scientists have raised concerns about the long-term impact of weightlessness.

Without gravity, kidneys may continue adjusting and lose their ability to concentrate urine and maintain fluid balance. There are also worries that renal stone risk could increase over months or years in space due to chemical urinary changes.

More research is needed to understand if microgravity permanently alters kidney structure and function. For now, proper hydration and wise fluid intake can help mitigate the effects of zero gravity on urine flow.


While the common perception is that astronauts just pee freely in space, their urine output is actually controlled by special underwear, funnels, suction, and the body’s kidney adaptation. The kidneys adjust to weightlessness within a few days which helps regulate urination.

However, spaceflight does still alter urine concentration and mineral content. Dehydration, space sickness, medications, and atrophied muscles may also temporarily disrupt normal urination frequency and volume.

To maintain astronaut health for long missions, scientists continue working to understand and mitigate the urinary system changes induced by microgravity. While zero gravity may not make astronauts pee uncontrollably, it provides valuable insights into the human body in space.