7.2 Transport of oxygen by haemoglobin

• When haemoglobin is exposed to different partial pressures of oxygen, it does not bind the oxygen evenly
• The oxygen dissociation curve is a graph which shows the relationship between the saturation of haemoglobin with oxygen and the partial pressure of oxygen
• • The shape of the haemoglobin molecule makes it difficult for the first oxygen molecule to bind to one of the sites on its four polypeptide sites because they are closely united
  • Therefore at low oxygen concentrations, little oxygen binds to haemoglobin resulting in the gradient of the curve to be shallow initially
• • However, the binding of this first oxygen molecule changes the quaternary structure of the haemoglobin molecule, causing it to change shape
  • The binding of the first oxygen molecule induces the other subunits to bind to an oxygen molecule
• • 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
  • The gradient of the curve therefore steepens
• Positive cooperativity is used to describe this process because the binding of the first molecule makes binding of the second easier and so on
• Haemoglobin that is 100% saturated with oxygen has the maximum number of oxygen molecules it can bind to
• • After the binding of the third molecule, it is harder for haemoglobin to bind the fourth oxygen molecule
  • This is due to probability - The majority of the binding sites are occupied, it is less likely that a single oxygen molecule will find an empty site to bind to
  • The gradient of the curve reduces and the graph flattens off
• Dissociation curve for adult human haemoglobin

  y axis = saturation of haemoglobin with oxygen / %

  x axis = partial pressure of oxygen / kPa

  
• The further to the left the curve, the greater is the affinity of haemoglobin for oxygen - so it loads oxygen readily but unloads it less easily
• The further to the right the curve, the lower is the affinity of haemoglobin for oxygen - so it loads oxygen less readily but unloads it more easily
• • The partial pressure of the gas is the amount of a gas that is present in a mixture of gases
  • This is measured by the pressure it contributes to the total pressure of the gas mixture
• • In the presence of carbon dioxide, haemoglobin has a reduced affinity for oxygen
  • The Bohr effect states that the greater the carbon dioxide concentration, the more readily the haemoglobin releases its oxygen - explaining why the behaviour of haemoglobin changes in different regions of the body
• At the gas exchange surface, carbon dioxide concentration is low because it diffuses across the exchange surface and is excreted from the organism
• • At the gas exchange surface, the affinity of haemoglobin for oxygen is increased, which coupled with the high concentration of oxygen in the lungs, means that oxygen is readily loaded by haemoglobin
  • The reduced carbon dioxide concentration has shifted the oxygen dissociation curve to the left
• The effect of CO2 concentration on the oxygen dissociation curve

  • 1 = low carbon dioxide concentration
  • 2 = medium carbon dioxide concentration
  • 3 = high carbon dioxide concentration

  
• • In rapidly respiring tissues, the carbon dioxide concentration is high
  • The affinity of haemoglobin of oxygen is reduced, which coupled with the low oxygen concentration in the muscles means that oxygen is readily unloaded from the haemoglobin into the muscle cells
  • The increased carbon dioxide concentration has shifted the oxygen dissociation curve to the right
• • The greater the carbon dioxide concentration, the more readily haemoglobin releases its oxygen
  • This is because dissolved carbon dioxide is acidic and the low pH causes haemoglobin to change shape
• Process of loading of oxygen

  1. At the gas exchange surface, carbon dioxide is constantly being removed
  2. The pH slightly increases due to the low concentration of carbon dioxide
  3. The higher pH changes the shape of haemoglobin to allow it to load oxygen readily
  4. This shape also increases the affinity of haemoglobin for oxygen, so it is not released while being transported in the blood to the tissues
• Process of unloading of oxygen

  1. In the tissues, carbon dioxide is produced by respiring cells
  2. Carbon dioxide is acidic in solution, so the pH of the blood within the tissues is lowered
  3. The lower pH changes the shape of haemoglobin into one with a lower affinity for oxygen
  4. Haemoglobin releases its oxygen into the respiring tissues
• • The process of loading and unloading oxygen ensures that there is always sufficient oxygen for respiring tissues
  • The more active a tissue, the more oxygen is unloaded
• The higher the rate of respiration:

  1. The more carbon dioxide the tissues produce
  2. The lower the pH
  3. The greater the haemoglobin shape change
  4. The more readily oxygen is unloaded
  5. The more oxygen is available for respiration
• • Blue = haemoglobin molecule in an active tissue unloads 75% of its oxygen
  • Green = haemoglobin molecule in a resting tissue unloads only 25% of its oxygen
  • Red = haemoglobin molecule that is fully loaded with oxygen in the lungs

  
• • Not all haemoglobin molecules are loaded with their maximum four oxygen molecules
  • The overall saturation of haemoglobin at atomospheric pressure is normally around 97%
• • When this haemoglobin reaches a resting tissue, only one out of four oxygen molecules are unloaded due to the low respiratory rate
  • The blood returning to the lungs will therefore contain haemoglobin that is still 75% saturated with oxygen
• If a tissue is very active, for example an exercising muscle, then three oxygen molecules will usually be unloaded from each haemoglobin molecule
• In species of animals that live in an environment with a lower partial pressure of oxygen, their haemoglobin has evolved to have a higher affinity for oxygen
• • A lugworm, not very active, is covered by sea water
  • Oxygen diffuses into the lugworm's blood from the water and it uses haemoglobin to transport oxygen to its tissues
• • When the tide goes out, the lugworm can no longer circulate a fresh supply of oxygenated water through its burrow
  • The water contains less oxygen as it is used up
  • The dissociation curve is shifted to the left compared to human - It's haemoglobin is fully loaded with oxygen despite there being little available in its environment
• lugworm vs human haemoglobin

  1 = lugworm

  2 = human

  
• • Llamas live at high altitudes - lower atmospheric pressure and so lower partial pressure of oxygen
  • It is therefore difficult to load haemoglobin with oxygen
  • Their haemoglobin has evolved to have a higher affinity for oxygen
  • The curve has shifted to the left of that of humans