Chapter 23: Oxygen and Carbon Dioxide Transport

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Transport of Oxygen and Carbon Dioxide in Body Fluids

  1. Oxygen Transport
    1. Solubility of oxygen in aqueous fluids is low
    2. Metalloproteins (respiratory pigments) reversibly bind to oxygen and increase oxygen carrying capacity by >50-fold
      1. PO2 in the blood remains low and  improves oxygen extraction
  2. Respiratory Pigments
    1. Hemocyanins
      1. Mollusks and some arthropods (horseshoe crabs)
      2. Contain copper
      3. Appears blue when oxygenated
    2. Hemerythrins
      1. Brachiopods, some annelids
    3. Chlorocruorin
      1.  some annelids
    4. Hemoglobin
      1. Most common respiratory pigment
      2. Vertebrates, nematodes, some annelids, crustaceans, and insects
      3. Consist of a protein globin bound to a heme molecule containing iron
      4. Usually carried within Erythrocytes
      5. Myoglobin in muscles
  3. Respiratory pigments bind O2 in a reversible chemical reaction
  4. Figure 23.4 A typical oxygen equilibrium curve for human arterial blood presented in two different ways
    1. % O2 saturation of Hb as a function of Po2 in the plasma
    2.  can show the relationship between partial pressure of oxygen in the plasma and the amount of oxygen in a volume of blood
      1. Varies with hemoglobin content
  5. As partial pressure increases, more pigment molecules bind oxygen, until the saturation point
    1. Saturation tension for PO2 is defined as 95% oxygen saturation.
    2. For humans at rest, this is 90-100 mm Hg (0.95*760*0.14)
    6.
  6. Shapes of Oxygen Equilibrium Curves
    1. Upper part of curve is flat
      1. Little effect for PO2 at loading site.
      2. Hemoglobin is easily saturated in the lungs.
    2. At tissue level,
      1. Figure 23.5 Oxygen delivery by human blood at rest and during vigorous exercise
      2. At Rest
        1. Pu is about 40 mm Hg
        2. 25% of O2 bound to Hb is delivered to the tissues.
        3. PL - Pu is about 5 ml O2/ 100 ml of blood, leaving 15 ml/100ml in venous reserve
        4. Remember: Saturated blood carries 20 ml O2 / 100 ml
      3. During vigorous exercise
        1. Tissue pO2 drops and enters steep part of curve.
        2. Working muscle pO2 is about 20 mm Hg
        3. 10 more ml/100 ml of O2 are delivered to tissues that need it.
  7. Figure 23.6 As the O2 partial pressure of blood falls, less and less of a drop in partial pressure is required to cause unloading of 5 vol % O2
  8. Figure 23.7
    1. Respiratory pigments display hyperbolic or sigmoid oxygen equilibrium curves
    2. Each molecule of myoglobin binds oxygen independently and therefore has a hyperbolic shape
    3. Hemoglobin exhibits a sigmoidal curve because of cooperativity: hemoglobin has a higher affinity for oxygen when more of its heme groups are bound to oxygen
  9. How to measure P50 (Fig. 23.9 )
    1. P50: oxygen partial pressure at which the pigment is 50% saturated
    2. P50 marks the point at which Hb = HbO2
    3. At rest, P50 = 25-30 mm Hg, for humans
  10. P50 is a measure of the affinity of a pigment for oxygen
  11. Fig 23.10 Conditions That Affect Oxygen Affinity
    1. pH and PCO2
      1. BOHR EFFECT or shift
        1. a decrease in pH or increase in PCO2 reduces oxygen affinity; right shift

 

A basic oxyhaemoglobin (oxyhemoglobin) dissociation curve.   This image has been (or is hereby) released into the public domain by its author, Ratznium at the wikipedia project
  1. Fig. 23.10: The Bohr Effect
    1. A right shift in the normal curve due to decrease in pH and associated increase in CO2.
  2. Bohr Effect
    1. As the curve shifts right from A (lungs) to B (tissues) the affinity for O2 decreases
  3. Figure 23.11 The Bohr effect typically enhances O2 delivery in an animal
  4. Other Conditions That Affect Oxygen Affinity
    1. Temperature (Fig 23.13)Increases in temperature decrease oxygen affinity; right shift
      1. Promotes oxygen delivery during exercise
      2.
    2. Organic modulators of hemoglobin (e.g., 2,3-DPG [bisphosphoglycerate])
  5. Fig 23.14 The normal P50 of human hemoglobin within red blood cells depends on a normal intracellular concentration of 2,3-DPG
    1. 2,3-DPG reduces affinity of Hb for O2; right shift
  6. Figure 23.15 A decrease in the O2 affinity of hemoglobin can aid O2 delivery to the systemic tissues when the O2 partial pressure in the breathing organs remains high
    1. DPG levels increase during exercise or at high altitudes when O2 demand is high
    2. People with anemia often have a chronic increase in 2,3-DPG
  7. Box 23.2: Blood Cells and their Production
    1. Erythropoietin (EPO) is secreted by the kidneys in response to low oxygen levels in the blood
    2. EPO stimulates RBC production in the bone marrow
  8. Carbon Dioxide Transport
    1. Carbon dioxide is more soluble than oxygen
    2. little CO2 is transported in the plasma
    3. Some CO2 binds to hemoglobin
    4. Most CO2 is transported as bicarbonate
  9. Carbon Dioxide Transport
    1. CO2  + H2O ↔ H2CO3 ↔  HCO3- + H+
    2. Carbonic anhydrase catalyzes the formation of HCO3-
  10. Fig 23.20a Human carbon dioxide equilibrium curve
    1. Shows the relationship between PCO2 and the total CO2 content of the blood
    2. The shape of the curve depends on the kinetics of HCO3- formation
    3. As the CO2 partial pressure is increased the amount that can be transported in the blood increases
    4. Only a small amount is dissolved CO2. Most is transported as bicarbonate.
  11. Haldane Effect
    1. Deoxygenated blood can carry more CO2 than oxygenated blood (Haldane effect)
  12. Fig 23.21b Carbon dioxide transport in humans at rest
    1. Venous blood (V) has lower O2 content (70%) and higher CO2 (46 mm Hg) partial pressure than arterial blood and can pick up more CO2 in the tissues and release more in the lungs because of the Haldane effect.
  13. Carbon dioxide transport in blood
  14. Carbon Dioxide Transport
  15. Vertebrate Red Blood Cells and CO2 Transport
    1. Carbonic anhydrase is located within RBCs
    2. Reactions to synthesize HCO3- occur in the RBCs even though most of this HCO3- is carried in the plasma
  16. Carbonic anhydrase and velocity of CO2-water interaction

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