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Challenge of Microgravity

Challenge of Microgravity

Astronaut Mike Fossum uses ultrasound to scan a crewmate's heart.
Courtesy of NASA.

  • Grades:
  • Length: 60 Minutes


Students use water balloons to simulate and investigate the effects of gravity and microgravity on the distribution of fluid in the body. If completing all activities in The Science of the Heart and Circulation, conduct the post-assessment at the conclusion of this activity.

This activity is from The Science of the Heart and Circulation Teacher's Guide, and was designed for students in grades 6–8. Lessons from the guide may be used with other grade levels as deemed appropriate.

Teacher Background

If students have been reading the Astroblogs, they already know quite a lot about the challenges faced by the circulatory system when humans travel into a microgravity environment. Living in microgravity changes both the heart and the blood.

On Earth, blood is pulled downward by gravity and tends to pool in the lower half of the body. In microgravity, blood is no longer pulled toward the feet, or in any particular direction. Without the effects of gravity, the distribution of blood in the body changes, with less blood than normal in the legs and more blood than normal in the upper body. Therefore, astronauts in space get skinny “chicken legs” and puffy faces, and often feel stuffiness in their ears and noses. Suddenly, the heart is not moving five liters of blood (the amount in most adults) against the strong pull of gravity. Because it does not have to work as hard, the heart becomes slightly smaller and weaker while in space.

In microgravity, the body senses the extra blood in the upper body and interprets it as too much fluid (over-hydration). This signals the kidneys to remove water from the blood and dispose of it as urine. Astronauts lose as much as 20% of their blood volume during a space mission. Other sensors in the body then discern that there are too many red blood cells for the amount of blood circulating, so the body reduces the amount of red blood cells to match the plasma. This reduction in red blood cells is called “space anemia.” After only one day in space or in orbit, astronauts have a lower blood volume. However, their ratio of red blood cells to plasma is similar to that experienced on Earth.

The cardiovascular system adapts well to microgravity, but what happens when an astronaut returns to Earth and “normal” gravity? By the time a spacecraft or orbiter begins reentry into the Earth’s atmosphere, astronauts have fewer red blood cells, their hearts have not been working as hard as they do in normal gravity, and their blood volume is lower than normal. These changes occurred over several days (or even longer), but reentry into normal Earth gravity happens quickly. This abrupt change creates important challenges for the circulatory system. And it can be dangerous for astronauts, because they must function effectively during reentry and landing.

In the activity, "What Is Blood Pressure?" students learned about high and low blood pressure (hypertension and hypotension, respectively). One consequence of low blood pressure is reduced blood flow to the brain. Upon returning to Earth’s gravity, astronauts sometimes experience a specific type of low blood pressure, called orthostatic hypotension, which you also may have experienced if you’ve ever stood up quickly after being seated on a chair. You get a little dizzy because gravity pulls the blood in your body down toward your feet. For a moment, your blood pressure falls slightly, and you feel dizzy. The dizziness goes away as your heart speeds up and stroke volume increases.

This same experience of orthostatic hypotension can happen to astronauts returning from space. Upon reentry, the pull of the gravity increases and blood is pulled back toward the lower body, as it is on Earth. However, since an astronaut’s total blood volume has decreased while in space, the effect is quite a bit stronger than when a person stands up from a chair. Astronauts can become very dizzy, or even lose consciousness during reentry. This condition can last for several days after returning to Earth, until the changes in the astronaut’s circulatory system reverse themselves and the body’s overall blood volume returns to a normal level. Scientists are working to develop short-acting medications to help prevent the effects of orthostatic hypotension and allow astronauts to function normally during landings.

Many biomedical researchers and astronauts also are conducting experiments to determine the impacts of longer-term spaceflight on astronauts’ circulatory systems. For example, would an exercise routine be sufficient to prevent long-term changes in heart strength and size, blood volume, and the number of circulating red blood cells? Researchers are working to answer these questions. Their work also may produce better treatments for people on Earth who are bedridden for long periods of time, or who have diseases of the heart or circulatory system.

Objectives and Standards

Life Science

  • Living systems at all levels of organization demonstrate the complementary nature of structure and function. Important levels of organization for structure and function include cells, organs, tissues, organ systems, whole organisms and ecosystems.

  • Regulation of an organism’s internal environment involves sensing the internal environment and changing physiological activities to keep conditions within the range required to survive.

Science, Health and Math Skills

  • Creating a model

  • Comparing and contrasting

  • Questioning

Materials and Setup

Materials per Student Group

  • Highlighters

  • Oblong-shaped balloon

  • Paper towels

  • Tub half-filled with water (large enough to float a filled balloon)

  • Group concept map

Materials per Student

  • Copy of student sheet (see Lesson pdf)


  1. Depending on time and the ages of your students, you may want to fill water balloons in advance, instead of having students fill the balloons during class.

  2. Have students work in teams of four.

Procedure and Extensions

  1. Give each group of students a long balloon. Instruct students to fill the balloon with water at a faucet and tie a knot at the top. This task is best accomplished by two students working together.

  2. Ask, Do you think the shape of the water balloon will remain constant? Tell students that they will investigate the shape of the balloon under different conditions: lying horizontally on a flat surface, held vertically, and suspended in water. Student groups should discuss and predict the shape of the balloon under each condition.

  3. Have students place their water balloons on a flat surface. Each student should observe and draw the balloon’s shape as it lies on the table.

  4. Next, have one student from each group hold the balloon up by the knot. Instruct students to observe and draw the balloon again.

  5. Finally, have students place their balloons in the tubs of water, and then observe and draw the balloon’s shape again. Under each of their three drawings, have students write a brief description comparing and contrasting each shape, and explaining why the shapes are different.

  6. Ask, Why does the balloon change shape? [Walls of the balloon stretch in response to pressure exerted by the water inside the balloon. The force of gravity pulls the water toward the lowest part of the balloon when it is lying on a table or held by the knot. This does not appear to happen when the balloon is in water.] Discuss the ways in which a water environment might mimic conditions in space, where gravity does not have an effect (microgravity). [The water balloon suspended in water is similar to an astronaut suspended in air within the space shuttle. The density of the water balloon is the same as the density of the water surrounding it, so gravity does not pull on the water balloon any more than it does on the rest of the water in the tub. In a sense, the water surrounding the balloon counteracts the pull of gravity on the balloon itself.]

  7. Ask, In what way could you compare the human body to the balloon? Is there liquid water inside the human body? What percent of a human’s body weight is water? [55–65%]

  8. Hold the balloon vertically. Ask, If this were a human body, where would the blood pool? Does blood flow down in the body like water does in the balloon? Explain that the fluid (blood) in a human body does travel downward through the blood vessels. Under normal conditions, blood is forced back to the heart by: a) continuous flow of blood through the body, b) constriction of the muscles in the walls of the blood vessels, c) contraction of muscles surrounding the veins, and d) “one-way” valves inside many veins that allow blood to move in only one direction—back to the heart.

  9. Ask, How might the body function differently in space? What would happen if the water were not pulled downward by gravity? Remind students of the appearance of the balloon when it floated in the water.

  10. Distribute the student sheet. Have students read and highlight important facts from the article.

  11. Have students individually complete the “3-2-1” activity (see sidebar). When they have finished, let individual students share responses within their groups.

  12. As a class, have students discuss possible answers to the questions generated by the activity. Assign groups of students to research questions that are not resolved, and have them present their findings to the class in any format they choose.

  13. Ask students if they have anything to add to the concept maps they have compiled over the course of the unit.

Related Content


National Space Biomedical Research Institute

National Space Biomedical Research Institute

This work was supported by National Space Biomedical Research Institute through NASA cooperative agreement NCC 9-58.