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Fact The skin of giraffes is tighter in their legs than the upper

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					Group Names:
                                  Lab T-1
                 What Can Physics Tell Us About Physiology?
                                                                                                     1
Introduction

Physics aims at discovering the most general underlying order behind natural phenomena at the
most fundamental level. The discovered laws of physics can be used to understand a wide range
of natural phenomena from planetary motion to optical diffraction.

In order to understand and appreciate the connection of physics and the real world of birds, lions,
and flowers, we need to understand the specific ways in which certain physics principles apply
to some biological facts through concrete examples.

In order to study the connection between physics and biology most directly, in this lab, we will
engage in ‘model building activity’ of various natural phenomena. We will develop several
‘models’ to explain various natural facts in biology, mostly interesting physiological facts about
various organisms. But, we first need to clarify what we mean by ‘models’ and ‘model building
activity’.

The Aims of this lab

The overall aim of this lab is to understand the connection between physics and biology.
Rather than merely reading or hearing your instructor talk about this connection, you will be
asked to make the connection directly through model-building activities. In particular, you will
be asked to construct models of various interesting physiological facts about animals and humans
using physics principles.

After the completion of this lab, you should be able to make explicit connections between
physics principles and a few interesting biological facts through model building activities.
This means that you are able to:
   1. Identify relevant physics principles
   2. Show explicitly how physics principles explain facts
   3. Use diagrams or pictures to aid explanation
   4. Reflect on simplification of models that one develops

The long-term learning goals of this lab are to learn aspects of thinking like a physicist, in
particular:
    1. Make connections between physics principles and real life in general
    2. Develop competence with more sophisticated model building
    3. Appreciate the usefulness of physics in understanding nature

At the end of the lab, you will be asked to construct a refined model of a particular
biological fact, which reflects a way in which physics and biology are related. You will also
be asked to explain the steps you would take if you were asked to make the connection
between biology and physics. Keep these goals in mind as you work through the lab, as they
are the primary goals of today’s lab.
                                Lab T-1
               What Can Physics Tell Us About Physiology?
                                                                                               2
                     Guidelines for Model Building
   •   Starts out with identifying a relevant physics concepts, principles, and/or equations that
       might not be obvious behind phenomena.
   •   The explanation of phenomena or facts must be explicitly connected to the identified
       physics principles; how the identified physics principles must help explain a phenomenon
       or make a numerical prediction is clearly described without a leap in logic.
   •   Diagrams should be used to aid the explanation and prediction; they enhance the clarity
       and the force of the explanation.
   •   Critically examine and reflect on simplifications made in the construction of a model.


Build a model in your group: “Tightrope Clown”
       Build a model (including a diagram, explanation, and discussion of the
       simplifications you made) of the “tightrope clown” demonstration you saw.
       Your model should address why the tightrope clown stays balanced with the
       poles, but does not balance without the poles.
Write your model so that someone not in your group could read it and understand
what is going on. After you have developed your group’s model, copy it to the
large sheet of paper at your table and put it on the wall for others to see.
                                               Lab T-1
                              What Can Physics Tell Us About Physiology?
                                                                                                                   3

          Fact: The skin of giraffes is tighter in their legs than the upper part of their body.


                                                      a) On a notecard, on your own come up with a possible
                                                         explanation of this rather strange biological fact based
                                                         on some physics principles. Try to make the connection
                                                         between the physics and your explanation as explicit as
                                                         possible. Write it so that someone other than you can
                                                         read it!




                                                      b) Your TA will collect the notecards and redistribute them
                                                         throughout the class randomly. In your group, use at
                                                         least one of the notecards you’re given (or an idea of
                                                         your own, if you can’t do anything with the cards you
                                                         have) to create a model explaining the fact above. A
                                                         good model explains the fact based on some physics
                                                         principles, with the aid of a diagram. It is typically
                                                         helpful to also explicitly list the simplifications you have
Figure 1: Giraffes' skins are tighter in their legs      made in developing your model.

                                                                                Diagram:
      Fact: The skin of giraffes          Physics Principles:
      is tighter in their legs than
      the upper part of their
      body.



    Explanation:




    Simplifications:
                                 Lab T-1
                What Can Physics Tell Us About Physiology?
                                                                                                    4

                                  Thought Experiment

A very tall glass tower has holes in the sides, and the holes are covered
by spring-loaded plungers inside an enclosed tube (all of the springs
have the same spring constant), as in the picture at the left. Water is
then poured into the glass, as in the picture on the right.




Q1: Draw the position of the springs as they are pushed by the water in the picture on the right.
Are there any variations with depth?


Q2: What would you feel if you jumped in and swam to the bottom?


Q3: What are the relevant physics principles involved in your picture?



Q4: Use the physics principles from Q2 to explain the positions of the springs you drew above.
How does this relate to the physics concept embodied in the equation ΔP = ρgΔh?
                                    Lab T-1
                   What Can Physics Tell Us About Physiology?
                                                                                                    5
   Activity: Observe the shape of the water filled glove when you hold it by the tied (knotted) end.

   Question:
     a) Draw a picture of the glove and describe its shape when you hold the it up in the air.




      b) Based on the relevant physics principles that you identified, develop a model that
         explains the shape? In other words, explain why the glove is shaped this particular way.
         A good model explains the fact based on some physics principles, with the aid of a
         diagram. It is typically helpful to also explicitly list the simplifications you have made in
         developing your model.


 Fact: The shape of the    Physics Principles:                           Diagram:
 glove when held by        (Hint: Think back to the previous exercise)
 the tied end.




Explanation:




Simplifications:
                             Lab T-1
            What Can Physics Tell Us About Physiology?
                                                                                                 6

c) Connection to the real world: In the diagram below, the latex glove you investigated
   above is identified with a piece of a vein in the leg of a human (the water in the glove is
   interpreted as blood in the veins).

                                    If the vein really did balloon out and collect water as you
                                    saw above, what could go wrong for the human? (Hint:
                                    What are some symptoms the human would experience?)




                                    What can be done to prevent the vein from ballooning
                                    out?
                                      Lab T-1
                     What Can Physics Tell Us About Physiology?
                                                                                                                      7
                           Testing the model – Measuring human blood pressure

        a) Do you expect blood pressure measured on your arm to be different than on your leg?
           Write your prediction and reasoning below. By the way, nurses almost always measure
           blood pressure on your arm…




        b) Using the blood pressure cuff at your table, measure the blood pressure of one of your
           group members on the arm and on the leg (see inset on how to operate cuff machine).
           Compare this to your qualitative prediction. Were you right?

                            Using the Automatic Blood Pressure Cuff Machine
   1.     Locate this symbol on the blood pressure cuff:


   2.     Slide the cuff on your bare arm (or leg) so that this symbol is furthest away from your heart.
   3.     Rotate this cuff so that this symbol is directly above the brachial artery (arm) or posterior tibial artery
          (leg).
   4.     Press “Start/Stop” button to measure blood pressure automatically.
   5.     Copy systolic and diastolic pressures into your notebook, then press “Start/Stop” button to turn
          device off.
Troubleshooting: If the device turns on but doesn’t work, look for “low battery” signal. If you see it,
ask your TA to replace the battery. The device may not work if you slide the cuff over clothing.

 Blood pressure is typically written as (systolic)/(diastolic), e.g. 120/80, a typical value for
 healthy adults (see below for an explanation of what systolic and diastolic pressures are).

 Arm:                                                   Leg:

                                             Systolic vs Diastolic Pressure
Blood pressure cuffs measure the pressure in your arteries (not your veins) by squeezing your arm. There are two
numbers that are given when your blood pressure is taken: Systolic and Diastolic (e.g. your blood pressure is given as
120/80 is 120 mmHg Systolic and 80 mmHg Diastolic). (The pressures given are relative to atmospheric pressure).

Systolic: Your blood is pumped throughout your body in “squirts”. The systolic pressure is the pressure of the “squirt” of
blood. The blood pressure cuff measures this by squeezing your artery so hard that it blood flow stops, then slowly
releasing the pressure. When the external pressure is small enough that the “squirt” of blood from the heart is able to push
through and un-pinch the artery, this is called the “Systolic” blood pressure.

Diastolic: The blood pressure cuff continues to loosen until your artery is no longer pinched off and snaps back open. The
amount of squeezing at which this happens is your Diastolic blood pressure.
                                    Lab T-1
                   What Can Physics Tell Us About Physiology?
                                                                                                 8
    c) Based on your measurement of the systolic pressure only, identify the fact that you
       observed (e.g. how does blood pressure in the legs compare to the arms?)


 Fact:




    d) Develop a model of this fact. A good model should have a diagram, explanation of the
       diagram and how it helps explain the fact, and a discussion of your simplifications. It
       may help to use some of the physics ideas discussed earlier in the lab in building your
       model.

Fact:                                                            Diagram:
                             Physics Principles:
(written above)




Explanation:




Simplifications:




                     Based on your model, make a numerical prediction of the difference in
                     blood pressure between your arm and your leg in the figure shown at left.


                     Note: The density of blood is ρblood = 1060 kg/m3
                                 Lab T-1
                What Can Physics Tell Us About Physiology?
                                                                                                 9
   e) Measurement Plan:
      In the next step you will make the following measurements to test your model:
          • Blood Pressure of the arm and the leg (both right and left sides)
          • The height difference between the locations where the blood pressure is measured
              on the arm and leg.
          • Blood Pressure of the arm and leg while standing and lying down (or sitting -
              choose whichever you are more comfortable with)
          • Remember to try and repeat the measurement several times when possible


Below are some questions to answer that reveal why the measurements above are a good test of
the model.

Q1: Why does measuring the blood pressure on both the arm and leg help to test your model?




Q2: Why does measuring the blood pressure on both the right and left sides of the body help to
test your model?




Q3: Why does measuring the blood pressure both standing and sitting/laying down help to test
your model?
                                        Lab T-1
                       What Can Physics Tell Us About Physiology?
                                                                                                                      10
         f) Measurement:
            Now you will make quantitative measurements to support your model. Below is a table
            that helps you collect the data that needs to be taken to test your model.

             In order to verify your model you must make many measurements of the difference in
             pressure between your arms and your legs (right and left), standing and lying
             down/sitting, and compare them to the quantitative predictions that your model makes.
              (If you have a condition in which your skin is extremely sensitive, it is not recommended that you use the
                                          blood pressure cuff for extended periods of time.)

             Be sure to make many measurements and take into account errors1!

             After taking measurements, use your model to calculate a predicted difference in pressure
             between arm and leg (ΔP Predicted).

                                                                                             Reminder
         Some useful conversion factors:                                      Your predicted ΔP will be in the SI unit
            The density of blood is ρblood = 1060 kg/m3                      Pascals, 1 Pa = 1 kg/(m s2). To convert to
                                                                                            mmHg, use:
            Δ p = (Leg Pressure - Arm Pressure)
                                                                                    1.013 × 105 Pa = 760mmHg


   Person            Trial #            Arm              Leg             Height          ΔP Measured        ΔP Predicted
                 standing /lying      Pressure         Pressure         Difference         (mmHg)            (mmHg)
                    Right/Left        (mmHg)           (mmHg)                             (+/- error)
SAMPLE          Standing/Right       132/80         207/110           .9 m               75/30 +/- 5       69




1. Based on the accuracy of the equipment and the natural variation of human blood pressure with time, we suggest
using an estimate of +/- 5 mmHg for the error in your measurements. If you have time at the end of this activity,
feel free to investigate this further.
                                Lab T-1
               What Can Physics Tell Us About Physiology?
                                                                                            11

g) Do your measured ΔP agree with your predicted ΔP for both standing and sitting/laying
down? In your answer, take into account your estimated errors from the previous page.

           Standing                                        Sitting/Laying Down




h) What are the simplifications you made in developing this model (from a previous page)?




 Are they valid or acceptable for standing or sitting down/laying? Explain why or why not.

           Standing                                        Sitting/Laying Down
                                     Lab T-1
                    What Can Physics Tell Us About Physiology?
                                                                                                 12
                                          Final Model

   Now, that you have explored model building which connects physics and biology, let's return to
   the original fact about the giraffe’s skin being tighter in their leg than in their upper body.

   By drawing on the examples from this lab, construct a model that explains the fact about the
   giraffe's skin. (Note: If your basic model framework is the same as what you did on page 3, then
   take this opportunity to refine your model in more detail, i.e. articulate and clarify your
   explanation, diagram, and simplifications.)


 Fact:                         Physics Principles:                 Diagram:
 The skin of giraffes is
 tighter in their legs than
 the upper part of their
 body.

Explanation:




Simplifications:




   Suppose you are interested in explaining the fact that some species of cats can
   jump higher than others. Describe the steps you would take in explaining this
   fact using physics (but you don't need to carry out those steps and actually
   build the model).

				
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posted:3/26/2010
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