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Teaching Guide
Feeling Pressured
Memory Matters
Getting the Minerals Out
image of diver

Covering 70% of the Earth's surface, the ocean is our planet's largest habitat. Yet we know more about our moon than we do about the deepest parts of the sea -- an intensely dark and cold environment, under tremendous pressure.

But what exactly is pressure? Studying for mid terms? Giving a speech? Asking that special someone to the dance? While these things may cause you stress, pressure is the physical force exerted upon organisms and objects by the environment. Even the Earth's atmosphere exerts pressure on the land and sea. At sea level, the weight of all the air above you creates one "atmosphere" of pressure (one bar) - equivalent to 14.7 pounds pressing on each square inch. In the ocean, pressure increases very rapidly with depth, because water is much denser than air. For every 33 feet (10 meters) of depth, the pressure increases by a further one atmosphere.

In No Limit free diver Loic Leferme performs a 100-meter training dive for the Frontiers cameras. It's possible to calculate (using Boyle's Law) the effect of pressure on Loic's lungs. If his lung capacity is 6.9 liters at the surface, at 100 meters it is just .63 liters - only a little more than a pint! Diving physiology expert Dr. Claes Lungren speculates that divers will one day discover a depth beyond which the human body can not survive. But marine mammals, the world's deepest divers , can actually use pressure to their advantage. As pressure compresses their lungs and ribcage, which are specially adapted to be very flexible, the air space in their bodies is reduced enough to give the animal negative buoyancy. They can dive farther and faster with very little effort.



These activities will offer:

  • exploration of the relationship between pressure and depth
  • an opportunity to construct an experimental apparatus
  • experience in taking measurements with the apparatus
  • an introduction to the impact of pressure on the lungs



  • Clear two-liter plastic bottle with a cap
  • 16 x 150 millimeter or smaller test tube (an eyedropper can be used instead)
  • Water


  1. Fill the bottle with water all the way to the top.
  2. Fill the test tube about halfway.
  3. Carefully and quickly turn the test tube over and push it open-end first into the bottle, trapping an air bubble in the top half of the tube. The tube should bob to the surface. (An eyedropper may be substituted; in which case it should not be inverted. The eyedropper should be inserted about half-full of water.)
  4. Push the tube back into the bottle and fasten the cap. Make sure there is no air space left under the cap.
  5. Squeeze the bottle and observe what happens to the diver.
  6. Record and analyze your observations.


In order for the diver to dive, it may be necessary to adjust the size of the air bubble trapped in the test tube or eyedropper. With the right air bubble size you should be able to control the diver's depth very accurately, by varying the amount of squeeze applied to the bottle -- even getting it to hover at different depths.


  1. What happens to the diver as you squeeze the bottle? Why?
  2. The lung capacity of the average adult is 5.5 liters at the surface and only .37 liters at a depth of 100 meters.
    • What happens to your diver's "lung" (a trapped air bubble) as the diver descends in the bottle?
    • Why?
    • Is this the same as squeezing the bottle with your hands, or different?

ACTIVITY 2 - Three Streams from a Bottle


  • Clear two-liter plastic bottle
  • Nail or awl
  • A ruler or measuring tape
  • Large kitchen measuring cup, or graduated laboratory measure
  • Stopwatch
  • Clear tape, used to seal holes in the bottle. (If the tape becomes too wet to stick, students can simply block the holes with their fingers.)
  • Duct tape
  • A sink or large wash tub
  • Water
  • Graph paper



Work with a partner.

  1. Punch three small holes in a vertical line down the side of the bottle, one a few inches from the top, one roughly in the middle, and close to the bottom. It is important to make the holes as similar in size as possible.
  2. Measure and record the height of each hole above the base of the bottle.
  3. Cover the holes with clear tape and fill the bottle to the top with water. Do not replace the cap.
  4. Partner #1 positions the bottle inside the sink or wash tub. When Partner #2 is ready, Partner #1 quickly removes the tape from all three holes.
  5. Using three pieces of duct tape, Partner #2 quickly marks the point on the sink bottom where each stream initially falls.
  6. Measure and record the horizontal distance from each piece of duct tape to the base of the bottle.

  1. How do the horizontal lengths of the streams compare for the three holes?
  2. What is the relationship between stream length and hole height? Why?
  3. What happens to the length of each stream as the water level in the bottle decreases? Why?
  4. What is the effect, if any, of uncovering one or two holes at a time, rather than all three?


Rates of flow are found by collecting, in a measuring cup or laboratory measure, the water flowing out of each hole over a set time -- 20-30 seconds, for example, depending on the size of the hole. The results can be expressed as "fluid ounces per second" or "ml per second." It is important that your measurements be made under similar conditions. This means that the bottle should be refilled to the exact same point between measurements.


  1. Cover the holes with new pieces of tape and refill the bottle to the top.
  2. Partner #1 prepares to capture the top stream in the measuring cup or laboratory measure, while Partner #2 prepares to time with the stopwatch.
  3. Partner #1 removes the tape from the top hole and begins collecting water, while Partner #2 simultaneously starts the stopwatch. After 20 seconds, Partner #1 stops collecting water.
    NOTE: you may need to adjust this time, depending on the hole size.
  4. Record your data.
  5. Repeat Steps #2-4 for the other two streams of water, measuring each for the same amount of time as the top stream.


  1. How do the flow rates compare for the three holes?
  2. What is the relationship between flow rate and hole height? Why?
  3. What might be the effect, if any, of measuring the flow rate of each of the holes with the other two uncovered?
    Try this out and compare your data.


  1. Within the deep ocean dwell animals that have developed unique adaptations to the harsh environment of the deep sea. Research how different species of marine life have adapted to life at depth. (For more information, visit our Video Database to watch "Hidden Depths" in SAF 405 "Creatures of the Deep")
  2. Conditions faced by the human body in the deep sea are related both to those found in outer space , and those found on high altitude mountain peaks. Compare the conditions in these three environments.
  3. Animal and human lungs work when the oxygen in the air "diffuses" from the many tiny air sacs (alveoli) in the lungs through membranes into the blood. The driving force behind this diffusion is the pressure of the oxygen in the lungs. At the same time, carbon dioxide diffuses out of the blood into the lungs, from which it is then breathed out. Explore the processes of breathing and respiration. Research how fish gills function.


This activity was contributed by Mark Moss. Mark has a Master of Science in Environmental Studies and works as a consultant in Southern California, developing environmental education programs, curriculum, and supplemental materials for children and adults.

Academic Advisors for this guide
Neil Glickstein, Science Departmant, Waring School, Glouchester, MA
Corrine Lowen, Science Department, Wayland Public Schools, Wayland, MA
Suzanne Panico, Science Department, Fenway High School, Boston, MA

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