Autumn.wmf (12088 bytes) Concepts of Biology (BIOL116) - Dr. S.G. Saupe; Biology Department, College of St. Benedict/St. John's University, Collegeville, MN 56321;;

Gas Exchange - Animals

I.  Importance of gas exchange
            recall the equation for respiration:
  (CH2O)n + O2 CO2 + H2O + ATP (energy)


  1. respiration provides energy source for aerobic organisms = critical importance

  2. two of four major components are gases

  3. requires uptake of O2

  4. requires removal of CO2

  5. understanding gas exchange is important

II.  Primer on molecular movement

A.  Diffusion vs. Bulk Flow


  1. Gases, like oxygen and carbon dioxide, enter and exit organisms, respectively, by diffusion. Insert diagram

  2. Oxygen typically gets to the animal by bulk flow or other active mechanism (i.e., breathing, fish sweep water across gills, frog swallowing.  More on this below)

  3. Carbon dioxide is typically removed by bulk flow

B.  Fick's Law

Take-home-lessons:  This equation tells us that for a given molecule diffusion rate is:

  1. directly proportional to area of absorptive surface (A).  The greater the area for diffusion, then the greater the rate.

  2. directly proportional to the concentration gradient(C1 - C2).  The steeper the gradient, or in other words, the greater the difference in concentration between two areas, the greater the rate of diffusion

  3. indirectly proportional to the distance of travel (L).  The longer the distance of travel

  4. related to the medium of travel (D).  The more viscous and dense the medium, the slower the diffusion rate (which would you rather swim laps in � a pool of water or maple syrup?)

III.  Biological Implications of Fick's Law:  A Large Surface Area (A)  is Required
     There are various solutions to this problem.  The key feature is increase the total surface for gas exchange (oxygen uptake, carbon dioxide loss).  This is another good example of surface-to-volume ratios.  As we learned, to increase surface area for a particular volume, a filament or flattened is the best shape to be � so, how do organisms accomplish this:

Interesting Detour � based on your knowledge of gas exchange and s/v ratios, explain why giant insects are just figments of a film-maker's imagination.

IV.  Biological Implications of Fick's Law:  There must be a short diffusion distance (L)
    There must be a short diffusion distance between the environment and inside the organism. Fick's law tells us that diffusion rate is inversely related to distance � the greater the distance, the slower the rate of diffusion.  In fact, diffusion is painfully slow over long distances.  But, how much slower?  Let's calculate the rate for glucose:           

insert calculation for glucose here

Take Home Lessons:  No individual gas absorbing surface is more than a few cells thick (e.g.. gills, lungs, sea cucumbers, hydra  � tubes with a central cavity bathed in fluid, sponges � lots of chambers; leaves are flat, thin)

V.  Biological Implications of Fick's Law:   A Large Surface Area Provides a Large Area for Desiccation.
A paradox � in order to exchange gases for metabolism, animals (and plants) need to have a large surface area means that the areas through which water is lost is also increased.


  1. put gas absorbing surface in side a humid chamber (i.e., humans � lungs)

  2. live in water (i.e., gills)

  3. plants (waxy cuticle with holes)

VI.  Biological Implications of Fick's Law:  There must be a way to get gases to the absorbing surface

A.  Positive Pressure breathing
    Frogs - push air down throat, lower throat - air enters - raise up to push down throat (= bulk flow)

B.  Negative Pressure Breathing

C.  Concerns

D.  Gills

I.  Biological Implications of Fick's Law:  Mechanism to maintain a large concentration (C) gradient

A.  Ventilation vs. perfusion (insert diagram)

B.  Partial Pressure 

C.  Counter-current mechanisms

VIII.  Hemoglobin 

A.  Oxygen low solubility in water.

B.  Structure

C.  Saturation curve - oxygen saturation (%) vs. PO2 (mm Hg)

C.  Factors that affect oxygen/hemoglobin binding

  1. Hemoglobin composition - fetal hemoglobin 2 alpha, 2 gamma chains (higher affinity for oxygen)
  2. pH (Bohr effect) - pH plasma typically about 7.6; metabolism, etc may lower it some; hemoglobin has a lower affinitiy for oxygen at lower pH, releases more oxygen
  3. 2,3 diphosophoglyceric acid (DPG) - product of glycolysis, high levels under exercise and/or elevation; DPG binds to hemoglobin (allosteric regulator), changes its shape, lowers its affinity for oxygen, releases more oxygen

IX.  Carbon Dioxide Transport

A.  Form

Table 1.  Forms in which carbon dioxide is transported
Form  Percent
Dissolved in plasma (as CO2) 7 - 8
Bound to hemoglobin 20
Bicarbonate in plasma 70

B.  Carbon dioxide and water

CO2 + H2 CO2 (aq)  H2CO3 (aq)  H+ + HCO3-

C.  Carbonic anhydrase - catalyzes formation of bicarbonate, fast enzyme, reversible; changes partial pressure of carbon dioxide to load/unload from blood stream 

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Last updated: January 20, 2004        � Copyright by SG Saupe