Carriage of oxygen

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  • Carriage of oxygen
    • Haemoglobin
      • Oxygen is transported in the RBC. The cells contain the protein heamoglobin.
      • Complex protein with four subunits. Each subunit consists of a polypetide chain and a haem group
        • The haem group  contains of a single iron ion. The iron ion attracts and holds one oxygen molecule.
        • The haem group has an affinity for oxygen. As each haem group can hold one oxygen molecule, each haemoglobin molecule can carry four oxygen molecules
    • Taking up oxygen
      • Oxygen is absorbed into the blood in the lungs.
      • Oxygen molecules diffusing into the blood plasma enter the RBC.
        • Here they are taken up by the haemoglobin.
      • This takes the oxygen molecules out of solution and so maintains a steep diffusion gradient.
        • This diffusion gradient allows more oxygen to enter cells
    • Releasing oxygen
      • In the body tissues, cells need oxygen for aerobic respiration.
        • Therefore the oxyhaemoglobin must be able to release the oxygen. Called dissociation.
    • Haemoglobin and oxygen transport
      • The ability of haemoglobin to take up and release oxygen depends on the amount of oxygen in the surrounding tissues.
        • The amount of oxygen is measured by the relative  pressure that it contributes to a mixture of gases.
          • Called the partial pressure or pO2. Also called oxygen tension and is measured in units of pressure (kPa)
      • Hemoglobin can take up oxygen in a way that produces an S-shaped curve. This is called the oxyhaemogleobin dissociation curve.
        • At low oxygen tension, the haemoglobin does not readily take up oxygen molecules.
          • This is because the haem groups that attract the oxygen are in the centre of the haemoglobin molecule. This makes it difficult for the oxygen molecule to reach the haem group and associate with it.
        • As the oxygen tension rises, the diffusion gradient into the haemoglobin molecule increases. Eventually one oxygen molecule diffuses into the haemoglobin molecule and associates with one of the haem groups
          • This causes a slight change in the shape of the haemoglobin molecule known as conformational change. It allows more oxygen molecules to diffuse into the haemoglobin molecule and associate with the other haem groups relatively easily
            • This accounts for the steepness of the curve as the oxygen tension rises
        • Once the haemoglobin molecule contains three oxygen molecules, it becomes more difficult for the fourth molecule to diffuse in and associate with last available haem group.
          • This means it is difficult to achieve 100% saturation of all the haemoglobin molecules, even when the oxygen tension is very high.
            • So the curve levels off as saturation approaches 100%, despite an increasing oxygen tension
      • Mammalian haemoglobin is well adapted to transporting oxygen to the tissues of a mammal.
        • The oxygen tension found in the lungs is sufficient to produce almost 100% saturation
          • The oxygen tension in respiring body tissues is sufficiently low to cause oxygen to dissociate readily from the oxyhaemoglobin.
    • Fetal haemoglobin
      • The haemoglobin of a mammalian fetus has a higher affinity for oxygen than that of adult haemoglobin
      • Fetal haemoglobin must be able to pick up oxygen from an environment that makes adult haemoglobin release oxygen.
      • In the placenta, the fetal haemoglobin must absorb oxygen from the fluid in the mothers blood.
        • This reduces the oxygen tension within the blood fluid, which in turn makes the maternal haemoglobin release oxygen.
      • So the oxyhaemoglobin dissociation curve for fetal haemoglobin is to the left of the curve for adult haemoglobin


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