Countercurrent Exchange (Brie-anna Rojas)

Title: Countercurrent exchange

Principle(s) Investigated: List all principles that apply to this activity.

• Establishing and maintaining favorable gradients
• Osmosis/diffusion
• Relation of form and function in biological systems
• Applying biological systems to engineering

Standards :

• HS-PS3-4 Plan and conduct an investigation to provide evidence that the transfer of thermal energy when two components of different temperatures are combined within a closed system results in a more uniform energy distribution among the components in the system (second law of thermodynamics)
• HS-LS1-2 Develop and use a model to illustrate the hierarchical organization of interacting systems that provide specific functions within multicellular organisms

Materials: Include a list of materials and sources from which they may be obtained.

• Clear tubing
• Syringes to fit into tubes
• Scissors
• Utility knife/razor (ex. X-Acto knife)
• Duct tape
• Glue gun (optional)
• Six clear cups or beakers per group/pair of students
• Water
• Food coloring

Procedure: Give a detailed explanation of the procedure and include diagrams if possible.

Pre-lab Preparation -

• Cut two equal lengths of clear plastic tubing (equal in diameter).

• In each tube, cut three holes that align and are equidistant apart.
• Line up the holes of both tubes and use hot glue and/or duct tape to connect the tubes.

*The goal is to allow for exchange between fluids passing through both tubes.

• Check for any leaks.

Lab:

1. Split students into pairs or small groups (it takes at least two people to run this properly).
2. Give each group a pre-made set of tubing, six beakers (one with clear water, one with colored water, four empty), and two syringes.
3. Instruct students to fill one syringe with clear water and another syringe with colored water.

1. HYPOTHESIZE (quickwrite): a) What will happen when I inject the colored water and the clear water into the tubes? - no exchange, some exchange; b) Will injecting the water from the same side or different sides give me different results? Injecting from same side - hypothesized results; Injecting from opposite sides - hypothesized results
2. Students will place each syringe into a separate tube. *NOTE: For the first trial, students will inject the fluid concurrently (entering from the same side); trial two will be countercurrent flow (entering from opposite sides).
   1. Trial 1: Setup for CONcurrent flow Trial 2: Setup for COUNTERcurrent flow

1. Have students place an empty beaker at the exit end of each tube - Label exit beaker with trial number or type (ex. "Trial 1" or "Concurrent")
2. Students will then inject the fluids simultaneously and observe any color changes as the fluid exits the tubes and collects in the beakers.
3. KEEP EXIT BEAKERS - DO NOT EMPTY
4. Compare results between the two trials - which resulted in higher exchange (as indicated by color change in the clearwater tube effluent)?

Student prior knowledge: What prior concepts do students need to understand this activity?

-Materials move from area of high concentration to areas of low concentration.

-Semi-permeable membranes allow certain materials to pass through it but not others.

Explanation: Give a thorough explanation of the experiment or demonstration. Your explanation should be written to give your fellow teachers a solid understanding and include greater detail than what you might provide for your secondary students. Make certain to include equations whenever pertinent.

Countercurrent exchange is a highly valuable concept and is an evolutionary advancement that has allowed for great strides in biodiversity. It is so efficient and effective that people have mimicked the process to use in many different engineering systems.

This lesson can be used either as a class experiment in small groups (as written) or as a demonstration depending on the amount of time and/or materials you have. Because countercurrent exchange is difficult to see, this gives a great physical and visual representation of how it works. Diagrams and videos are great to supplement but will not have the same long-lasting effect and performing the test and seeing the results for themselves. I also encourage teachers to have their students write their hypotheses before testing, particularly if this is their first time being exposed to the concept of countercurrent exchange.

Additionally, this lesson can be adapted to apply to many different disciplines. Countercurrent exchange is a physical process but it is found in biological systems, applied to engineering, and can involve chemical transfers as well. So, whether you are teaching physics, biology, chemistry, or an engineering related unit/class, you can present a lesson like this in any context.

If you have access to the equipment, colorimetry tests could be used to better quantify the results from the experiment. Other variations include: high salinity water vs. freshwater (use refractometer to measure change in salinity), hot water and room temp water or slightly cool (use thermometer to measure change). I only use the colored water because it is so visual - everything else is invisible but can be measure more quantitative in other ways. I just like the visual effect of the color. Also, I recommend using highly concentrated colored water (changes can be seen more easily). Quantitative results can give students something to calculate (percent change, difference between input and output) and make it more exact and scientific.

Questions & Answers: Give three thought-provoking questions and provide detailed answers.

1. Why does countercurrent flow allow for more exchange along the system?
2. Countercurrent exchange maintains a favorable gradient along the length of the system. Although concurrent exchange allows for a stronger gradient to begin with, the gradient diminishes more rapidly as equilibrium is reached. In countercurrent exchange, the gradient is weaker but maintains itself for longer (see pictures below for graphical representation)

Applications to Everyday Life: Explain (don't just list) three instances where this principle can be used to explain other phenomenon.

1. Countercurrent heat exchange in most mammals/birds and some fish (laminid sharks, tuna, etc.)
2. Exchange of water and solutes in the vasa recta of the kidneys when attempting to reabsorb water and excrete solutes
3. Heat exchange in Cogeneration Plants

Photographs: Include a photograph of you or students performing the experiment/demonstration, and a close-up, easy to interpret photograph of the activity --these can be included later.

**Photographs have been embedded into activity. Also uploaded to 525S Demonstrations folder on Picasa Web Album**

Videos: This video will help give you an idea about how to lead a discussion about countercurrent gas exchange in fish with your students: http://www.youtube.com/watch?v=cVFqME-NW9s