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Module: Cardiovascularstellar logo

Why Giraffes Don't Faint

Grades: 5-8

(can be adapted for higher levels)

By Lisa Marie Gonzales

Principal Investigator: Dr. Alan Hargens


In this activity, students investigate how human blood flow and blood pressure are affected in gravity and microgravity conditions. Through classroom discussions, they develop theories about why fainting occurs in humans returning from space and how NASA scientists might find ways to prevent fainting by studying the anatomy and physiology of giraffes.

Key Questions

  • How does gravity affect blood flow and blood pressure?
  • What changes occur in blood flow under microgravity conditions?
  • What advantages do giraffes have over humans in preventing fainting?
  • How might NASA scientists use information about the giraffe's physiology to help prevent fainting in returning astronauts?

Time frame:

  • 1 class period


For the class:

For each group of 2­5 students:

Getting Ready

1. Gather materials. Fill each glove about two thirds full of rice or sand and seal the opening with strong tape. You may choose to have students to this in class

2. Organize the room so that students can work on their projects in groups of two or more.

Classroom activity


1. What is fainting? Ask the students if they have ever fainted or felt light-headed and what circumstances led to the episode. Allow them to tell their stories. Make a list of possible reasons for fainting. (Responses include standing up too fast after sitting or laying down for a long period, having the flu or other illness, standing in a hot, stuffy room with low oxygen levels, over-exerting yourself, or holding your breath.) Challenge students to think about why these circumstances can cause fainting. (Lack of oxygen or blood flow to the brain; fainting is the body's way of getting you into a horizontal position so that blood can flow more easily to the head.)

2. Camparing giraffes with humans. Ask them to look at the scale drawings of the giraffe and the man. What physical differences do they notice? Have them make a list of the similarities and differences between the two body types. Allow a brief period of time to discuss their lists.

3. Why giraffes don't faint. Ask the students whether they think humans or giraffes are more prone to fainting and why. (They might logically infer that a giraffe would faint more easily because blood has a much longer distance to travels from its heart to its head.) Point out that giraffes have developed special mechanisms to insure adequate blood flow up their long necks and into their heads. In addition to larger hearts and higher blood pressure than humans have, giraffes also have especially tight skin and strong muscles in their legs. (The tight skin around the legs prevents the blood from pooling and the muscles help pump blood back up, so that enough blood always reaches the head and the giraffes don't faint.)

4. NASA's problem. Tell students that scientists in NASA's Life Sciences Division have a problem they need help with. NASA doctors have noticed that after astronauts have been in space they have a tendency to faint when returning to Earth. In order to study this problem, students have to investigate how blood flow and blood pressure is affected by both gravity and microgravity conditions. To do this, they'll use a simple model of a rubber glove filled with sand as a model of blood distribution in the body. While they're working on the activity, have students think about ways they might counteract fainting, including using information they've learned about giraffes.



1. Hand out gloves. Hand out a rice or sand-filled glove to each group of students or have them fill and seal the gloves themselves.

2. The glove in gravity. Have the students hold the filled gloves by the thumb and the taped end with the other fingers pointed downward. This simulates the body in gravity, the thumb represents the body above the heart. Students should record their observations on the "blood" distribution and the flexibility of the gloves on their worksheets. Have them squeeze one of the downward-pointing fingers near the end and notice what happens to the contents.

3. The glove in microgravity. Have the students lay the filled gloves flat on the table and even out the contents. The rice or sand represents the liquid blood which is evenly distributed when we are horizontal or in microgravity conditions. Have them move the fingers of the gloves and note how easy the fingers are to bend at the joints. The students should record their observations of the blood distribution and the flexibility of the gloves on their worksheets.

4. Finish the worksheet. Have students complete the questions at the bottom of the worksheet based on their observations about what happens to blood flow in gravity and microgravity. The final question can be assigned as homework.

Wrap-up Session

1. Discuss the students observations. Ask them how blood distribution might contribute to fainting. (They should have noticed that in the vertical, or gravity, position the blood drains into the glove fingers and away from the "head, " which can bring on fainting. In the horizontal position, blood is evenly distributed which is why we don't faint when we're lying in bed and why astronauts don't faint in space.)

2. Ask the students whether the horizontal or vertical glove has more pressure at the fingers. Students should conclude that when the glove was in the vertical position there was more pressure on the downward pointing fingers and almost no pressure on the "head."

3. Ask students what would happen if the fingers in the glove were tighter and didn't stretch as easily. (Less blood would flow into them in the vertical position.) When they squeezed the glove fingers what happened to the blood? (It moved back into the upper area of the glove.) How are these same mechanisms used by giraffes to keep from fainting? (The giraffe's tight skin keeps the blood vessels in its legs from expanding and pooling blood. By using its tight skin and leg muscles to squeeze blood out of the lower body, the giraffe raises its blood pressure and increases blood flow to the head.)

4. Ask students what they think happens to the blood distribution of an astronaut is space. (It pools in the upper body around the heart, a condition known as fluid shift. A similar thing happens when you stand on your head or stay in bed for a few days. ) Ask students whether the heart and cardiovascular system has to work as hard to keep blood flowing to the head in microgravity as it does in gravity. (No.) What happens to muscles, such as the heart, when they don't get as much exercise? (The same thing that happens to couch potatoes, they get lazy and out of shape.) Point out that other mechanisms in the body that help maintain blood pressure and flow also don't get used as much in microgravity conditions. After being in space for awhile, an astronaut's cardiovascular system gets "lazy" and can't keep a good supply of blood flowing to the head. The result: a tendency to keep fainting until the cardiovascular system gets used to gravity again.

5. Ask students to brainstorm ways that NASA can learn by studying the cardiovascular systems of giraffes that could help them with astronaut training or equipment.

More Activity Ideas

1. Work with the school nurse, to take actual measurements of blood pressure of different students vs. height and weight. They could use this information to calculate the actual blood pressure in their feet when they are standing. You might want to have the kids input this data on a spread sheet and via e-mail exchange data with students at another school.

2. Follow-up on the glove simulation project, by having each group create a computer Hypercard version of their final report including scanning in their hand drawn diagrams or Quicktime movies of the model being used. You might also want them to research information on blood pressure changes in space and include a background section.

3. Have students simulate "tighter skin" around the fingers of the glove by wrapping rubber bands around the fingers. Would astronauts wearing tight pants (around the legs and feet) help them adapt to gravity? If so, would the end up with the same problem when going back to normal pants?

Background for Teachers:


  • No special prerequisite skills are necessary for this activity.


  • anatomy-the physical structure of a plant or animal.
  • artery-vessel that carries blood away from the heart to the organs.
  • blood pressure-the force exerted by the blood on the walls of the blood vessels.
  • capillary-microscopic blood vessel that delivers oxygen and nutrients and carries away waste products on the cellular level.
  • cardiovascular system-the system of heart, lungs, blood, and blood vessels responsible for delivering oxygen and nutrients to the body tissues and carrying away waste products; the term is sometimes used interchangeably with circulatory system.
  • circulatory system-the system of blood, blood vessels and heart responsible for the circulation of the blood
  • fainting-an abrupt, usually brief, loss of consciousness generally associated with failure of normal blood circulation.
  • fluid shift-the tendency for blood and fluids to pool around the heart and thorax during prolonged bed rest or in microgravity conditions.
  • heart-a muscular organ that pumps blood and helps maintain blood pressure.
  • lungs-a large organ that allows the exchange of oxygen into the blood and carbon dioxide
  • microgravity-the very small gravity force that exists in a space vehicle.
  • mm Hg-millimeter of mercury; the units used to measure blood pressure.
  • physiology-the biological science of life processes, activities and functions.
  • pulse-the rhythmic expansion and contraction of the heart and arteries caused by the pumping action of the heart.
  • sphygmomanometer-a pressure gauge attached to an inflatable cuff that is used to measure blood pressure.
  • vein-vessel that return blood to the heart.


  • Students will collect and analyze data.
  • Students will use a simple model to hypothesize about actual conditions.
  • Students will compare the anatomy and physiology of an animal with that of humans.


During this activity the students will learn:

  • Blood pressure
  • Cardiovascular system

Related STELLAR Activities in the 5-8 Cardiovascular Module:

  • The Basics of Blood Pressure

Science Background:

No matter what position our bodies take, we need to keep a constant flow of blood to the brain. In upright animals, such as humans and giraffes, the heart must pump blood up to the brain against the force of gravity. If you stand too long without moving or make sudden shifts from horizontal to vertical, you could feel faint. This is your body's way of getting you to lower your head and increase blood flow to the brain.

Each type of animal on Earth has a standard blood pressure which is determined by its height, weight, shape, body structure, and amount of activity. The pressure in blood vessels changes throughout the body as a function of the distance between the heart and the tissues through which the blood must be pumped. Body parts that are located below the heart have to work against the pull of gravity to push the blood back to the heart. The veins in our legs have valves that keep the blood from flowing back down, so when the muscles squeeze them and the vessels constrict, the blood has only one direction to go: up. When blood vessels constrict, the pressure on the blood is increased. Since pressure is determined by force/area, a small change in the diameter of the blood vessel can cause a large change in pressure, which in turn increases blood flow through that vessel.

Our bodies have adapted in gravity to keep blood constantly flowing to the brain and other organs and tissues. Special stretch receptors in the carotid (neck artery) and other arteries sense changes in blood pressure and flow. If blood pressure in the arteries leading to the head goes down, these sensors send a signal to the body to increase blood flow to the brain. In microgravity, all those highly tuned mechanisms that our cardiovascular systems use to keep pumping blood against gravity aren't necessary. Hearts no longer have to pump uphill to the head and blood doesn't tend to pool downhill in the legs. Instead blood pools around the heart and thorax, a condition known as fluid shift. Eventually this "extra" fluid is dumped from the body, and blood volume goes down.

When it comes to our cardiovascular systems, the old adage applies: use it or lose it. "Losing it" would be fine if humans went into space and stayed there. But astronauts eventually return, and their under-used blood pressure regulators sometimes fail to work properly when they try to walk around on Earth. They stand up, the blood flows right down to the legs, and the proper signals aren't sent to replenish the brain's blood supply-the returning astronauts faint. NASA scientists hope to remedy this problem by studying animals such as the giraffe, which is able to offset the pull of gravity on its very tall body even when standing still. Giraffes don't faint in part because they have a natural anti-gravity suit. Scientists have found that the giraffe's tight skin and the muscles in its legs keep blood from pooling in its lower body . Additionally, the arterial pressure near the giraffe's heart is about twice that in humans to provide adequate blood pressure and blood flow to the brain.

This activity was edited by: Mary K. Miller



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