HUMAN SYSTEMS QUESTION 1987:               L. PETERSON/AP BIOLOGY

      Discuss the exchange of oxygen and carbon dioxide that occur at the alveoli and 
	muscle cells of mammals. Include in your answer a description of the transport 
	of these gases in the blood.

  
  
  STANDARDS:
  
  EXCHANGE (8 points max.)
  1 - O2/CO2 diffusion or correct direction of O2/CO2 gas movement
  1 - concentration gradient or explanation
  1 - Alveoli/membrane characteristics
  (large surface area, or thin-walled, or "grape clusters", or sacs, or surrounded 
    by capillaries)
  1 - moist surface necessary for exchange
  1 - interstitial fluid role in transport
  1 - P O2/CO2 description
  1 - Illustrated curve on graph or explanation
                            100 mm Hg lungs
                               40 mm Hg muscles (rest)
                               20 mm Hg muscles (exercise)
  1 - temperature effects
  1 - further explanation as:
     (tissues) higher temperatures -> less affinity
         OR
     (lungs) lower temperatures -> more affinity
         OR
     graph
  1 - CO2 in plasma lowers pH of blood
  1 - Bohr effect - acidity affects affinity of Hb for O2
  1-  example  lower  pH -> less affinity
                  higher pH -> more affinity
          OR 
          graph
  1 - myoglobin in muscle stores O2
         (greater affinity for O2)
  1 -  air is approximately 20% oxygen
  1 - other hemoglobin adaptations, such as: (maternal-fetal or low altitude-high altitude)
  
  TRANSPORT (8 points max.)
     OXYGEN
  1 - carried by hemoglobin or in red blood cell
  1 - hemoglobin found in red blood cell
  1 - hemoglobin characteristics such as:
            protein, or 4 subgroups, or Heme, or iron and porphyrin ring
  1 - Hb cooperativity (allosteric) - one O2 increases affinity for others
            S-shaped curve
  1 - CO can compete with O2 for Hb binding site
     CARBON DIOXIDE                      -
  1 - majority carried as bicarbonate ions (HCO3)
  1 - enzyme carbonic anhydrase speeds reaction
                                        -
  1 - CO2  +  H2O  <--> H2CO3  <--> H+  +  HCO3
  1 -some CO2 carried aqueously (dissolved)
  1- some CO2 carried by Hb (on a different site - does not compete with O2)
  1 - Hb picks up H+ ions to buffer blood
     CIRCULATORY TRANSPORT PATHWAY
  1 - from alveolus to muscle cell
        (pulmonary vein -> left atrium -> left ventricle -> artery -> capillary)
  1 - from muscle cell to alveolus
        (capillary -> vein -> right atrium -> right ventricle -> pulmonary artery)