7.2 Transport of Oxygen by Haemoglobin

Oxygen Dissociation Curves - The shape of the Curve
1. When hgb is exposed to different partial pressures of oxygen, it doesn't bind the oxygen evenly
2. The shape of the hgb = difficult for the first oxygen molecule to bind to one of its sites on its four polypeptide sub units as they're closely united
  1. Therefore a low oxygen concentrations, little oxygen bind to haemoglobin.
    1. Shallow gradient initially
3. The binding of the 1st oxygen molecule changed the quarternary structure of the hgb molecule, causing it to change shape.
  1. This makes it easier for other subunits to bind to an oxygen molecule.
4. It therefore takes a smaller increase in the partial pressure of oxygen to bind the second oxygen molecule than it did to bind the first one. This is known as POSITIVE COOPERATIVITY because binding on the first molecule makes binding on the 2nd one easier.
  1. The gradient of the curve steepens
5. After the binding of the 3rd molecule, the situation changes as it is harder for the hgb to bind to the fourth oxygen molecule. This is simply due to probability as with more sites occupied it is less likely to find a site to bind to.
  1. The gradient of the curve reduces and the graph flattens off.
Oxygen Dissociation Curves - The Placement of the curve with the axes
1. The further to the left the curve is...
  1. The greater the affinity of hgb for oxygen (so it loads oxygen readily but unloads less easily)
2. The further to the right the curve is...
  1. The lower the affinity of hgb for oxygen ( so it loads oxygen less readily, but unloads oxygen more easily)
Effects of CO2 concentration
1. Low carbon dioxide concentration shifts the oxygen dissociation curve to the left.
2. Medium carbon dioxide concentration has an oxygen dissociation curve that is in the middle
3. High carbon dioxide concentration shifts the oxygen dissociation curve to the right
Loading, transport and unloading of oxygen
1. At the gas exchange surface cxarbon dioxide is constantly being removed
2. The pH is slightly raised due to the low concentration of carbon dioxide
3. The higher pH changes the shape of hgb into one that enables it to load oxygen readily
4. This shape also increases the affinity of hgb for oxygen, so it isn't released while being transported in to blood to the tissues
5. In this tissues, carbon dioxide is produced by respiring cells
6. Carbon dioxide is acidic in solution, so the pH of the blood within the tissues is lowered
7. The lower pH changes the shape of hgb into one with a lower affinity for oxygen.
8. Hgb releases its oxygen into the respiring tissues
9. Basically, the more active a tissue, the more oxygen is unloaded.
  1. as higher rate of respiration
    1. tissues produce more CO2
      1. lower pH
        greater hgb shape change
        more readily O2 unloaded
        more O2 available for respiration
Size matters
1. Mice are small and therefore have a large surface area to volume ratio. As a result they tend to loose heat rapidly when the environmental temperature is lower than their body temperature.
  1. So to maintain the body heat, the tissues repire more and oxygen is more easily released from hgb to the tissues

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7.2 Transport of Oxygen by Haemoglobin

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