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Haemoglobin and Hydrogen Carbonate Ions carry Respiratory Gases and Control Blood Ph

Haemoglobin Transports Oxygen

  • Lower atm pressure / fewer molecules present / less O2 reaches tissues
  • Body adapts to changes by increasing
    • Heart rate and resting breathing rate
    • Blood plasma
    • Red blood cell production and number of blood capillaries
  • Haemoglobin Hb has 4 subunits, each subunit contains 2 parts
    • Haem → ring of atoms linked to Fe2+
    • Globin → polypeptide chain
    • Sequence of amino acids affects O2 carrying properties
  • Oxyhaemoglobin HbO2 from lungs dissociates in respiring tissues
    • O2 diffuses into body cells while Hb is transported back to lungs
  • Features of red blood cells that allow them to transport O2 more efficiently
    • Biconcave disc → larger surface area to volume ratio for diffusion
    • Absence of nuclei/other organelles → more room for haemoglobin

Hydrogen Carbonate Ions Remove CO2

  • CO2 produced in tissues diffuses into blood plasma
  • There it reacts with H2O in the plasma and in the cytoplasm of red blood cells
    • CO2 + H2O → H2CO3(carbonic acid) → H+ + HCO3- (hydrogen carbonate)
  • In red blood cells, carbonic anhydrase is present
    • Rate of HCO3- production is higher than in blood plasma
    • Establishes a conc gradient → HCO3- diffuses into plasma
    • Cl- diffuses into red blood cells to keep eqm in balance → chlorine shift
  • Thus, most CO2 is transported in blood as HCO3-
  • But, small amount reacts directly with haem to produce carbamino-haem
    • More CO2 binds to haem when O2 conc is low

Control Of Blood pH

  • Dissociation of CO2 produces H+ ions
  • Buffer keeps pH constant
    • Plasma contains phosphate and plasma proteins
    • Red blood cells contain Hb
  • Hb takes up H+ and releases O2 to respiring tissues
    • H+ + HbO2 → Hb + O2
    • The more H+ taken up by Hb the more O2 is released
  • Higher rate of respiration produces more CO2 from respiring tissues
    • More H+ is produced and taken up by Hb
    • More O2 is released, therefore, more CO2 is transported in blood plasma
    • Carbamino-haem travels to gas exchange surface
    • There, more CO2 is removed and more O2 is taken up by haem
    • More O2 is transported and supplied to respiring tissues
    • Thus, CO2 regulates breathing rate

The Oxyhaemoglobin Dissociation Curve

  • % saturation of Hb is plotted against partial pressure of O2 (pO2)
  • pO2 is a measure O2 concentration
    • 100% saturation of Hb → high partial pressure → in lungs
      • Low pCO2 as it is removed from the body → high O2 uptake
      • O2 is transported to and unloaded in tissues
    • 60% saturation (straight line) in muscles → using up O2 carried by Hb
  • S-shaped because each Hb carries 4 molecules of O2
    • First molecule changes shape of Hb → remaining molecules bind more easily
    • It becomes easier for the second and third molecules to bind
    • Curve becomes flat at the end as binding of the last O2 is more difficult again

The Bohr Effect

  • Increased pCO2
    • moves dissociation curve to the right (Bohr shift)
    • causes Hb to give up more O2 to respiring tissues
    • pO2 falls (increase of respiration), more CO2 is released and rises pCO2
  • Curve to the right of the normal for active animals
    • Small animals → low surface area to volume ratio → readily lose heat
    • Higher metabolism increases metabolic rate
    • This requires more O2 for respiration to generate ATP
    • pO2 is high at respiring tissues as it comes directly form lungs
    • pCO2 must have a high value as well (see above)
    • O2 is released at high pO2
  • Curve to the left of the normal (greatest affinity for O2)
    • pO2 is low in deep underground levels and high altitudes
    • Fast release of O2 at low pO2 is required (Hb can adapt)
    • Blood at placenta has low pO2
      • Fetus lives in low pO2
      • Fetal Hb has a high affinity for oxygen/is saturated at low pO2
      • Mother gives up O2 and fetus picks it up
      • Fetus takes up O2 in limited supply
    • Red pigment myoglobin stores O2
      • In muscles/anaerobic/diving animals → when O2 will get very low
      • Fast release of O2 at very low pO2, but storage of O2 at high pO2 In some children fetal Hb is abnormally high / oxygen not released from fetal Hb/ in low pO2/body tissues/muscles / more anaerobic respiration / less ATP available / muscle fatigue