Carriage of carbon dioxide

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  • Carriage of carbon dioxide
    • How is carbon dioxide transported?
      • Carbon dioxide is released from respiring tissues. It must be removed from these tissues and transported to the lungs
      • Carbon dioxide in the blood is transported in three ways:
        • about 5% is dissolved directly in the blood plasma
        • about 10% is combined directly with haemoglobin to form a compound called carbaminohaemoglobin
        • about 85% is transported in the form of hydrogencarbonate ions
    • How are hydrogencarbonate ions formed?
      • As carbon dioxide diffuses into the blood, some of it enters the RBC.
        • It combines with water to form a weak acid called carbonic acid
          • This is catalysed by the enzyme carbonic anhydrase
      • This carbonic acid dissociates to release hydrogen ions and hydrogencarbonate ions
        • The hydrogencarbonate ions diffuse out of the RBC into the plasma. The charge inside the RBC is maintained by the movement of chloride ions from the plasma into the RBC. Called the chloride shift
      • The hydrogen ions could cause the contents of the RBC to become very acidic.
        • To prevent this, the hydrogen ions are taken up by haemoglobin to produce haemoglobinic acid.
          • The haemoglobin is actin as a buffer
            • A buffer is a compound that can maintain a constant pH
    • Releasing oxygen
      • As the blood enters respiring tissues, the haemoglobin is carrying oxygen in the form of oxyhaemoglobin.
        • The oxygen tension of the respiring tissues is lower than that in the lungs because oxygen has been used in repiration.
          • As a result, the oxyhaemoglobin begins to dissociate and releases to the tissues
    • Releasing more oxygen - the Bohr effect
      • The hydrogen ions released from the dissociation of carbonic acid compete for the space taken up by oxygen on the haemoglobin molecule.
        • So when carbon dioxide is present, the hydrogen ions displace the oxygen on the hameogolobin
          • As a result the oxyhaemoglobin releases more oxygen to the tissues
      • Where tissues (such as contracting muscles) are respiring more, there will be more carbon dioxide.
        • As a result there will be more hydrogen ions produced in the RBC. This makes the oxyhaemolgobin release more oxygen. Called the Bohr Effect.
      • At any particular oxygen tension, the oxyhaemoglobin releases more oxygen when more carbon dioxide is present.
        • So when more carbon dioxide is present, haemoglobin is less saturated with oxygen.
          • So when more carbon dioxide is present, haemoglobin is less saturated with oxygen. This makes the oxyhaemoglobin dissociation curve shift downwards and the to the right (the Bohr shift)
      • The Bohr effect results in oxygen being more readily released where more carbon dioxide is produced from respiration.
        • This is just what the muscles need for aerobic respiration to continue
  • The hydrogen ions released from the dissociation of carbonic acid compete for the space taken up by oxygen on the haemoglobin molecule.
    • So when carbon dioxide is present, the hydrogen ions displace the oxygen on the hameogolobin
      • As a result the oxyhaemoglobin releases more oxygen to the tissues

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